Heat Treatment of Steel

UNIT 2 ENGINEERING ALLOYS (FERROUS AND NON-FERROUS)
Structure
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 Introduction
Objectives

Engineering Alloys (Ferrous and Non-Ferrous)

Production of Iron and Steel Casting of Ingots Continuous Casting Steels Heat Treatment of Steel Hardenability of Steel Tempering Special Treatments

2.10 Surface Hardening 2.11 Heat Treating Equipment 2.12 Alloy Steels 2.13 Cast Iron 2.14 Non-ferrous Materials 2.15 Aluminium 2.16 Copper and its Production 2.17 Copper Alloys 2.18 Magnesium and its Alloys 2.19 Titanium Alloys 2.20 Bearing Materials 2.21 Alloys for Cutting Tools 2.22 Summary 2.23 Key Words 2.24 Answers to SAQs

2.1 INTRODUCTION
Out of solid materials used in engineering practice metals, plastic and ceramics are very common. Then metals may be used in their elemental forms like aluminium, copper and titanium. When a metallic element has additives much smaller in quantity than base element, the resulting material is called an alloy of base element. Out of all metallic elements it is iron whose alloys are used in largest quantity. All such alloys in which iron forms the base are grouped as ferrous material. The other alloys are grouped as non-ferrous materials. Ferrous materials, both metal and alloys have iron as their base and due to wide range of their properties are most useful for use in engineering machines and structures. Owing to the advents in steel technology and casting technique ferrous metals are cast, shaped and

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Engineering Materials

machined in various shapes and sizes. Several standard shapes of sections are variable commercially which make the job of designer and constructor much easy. They are used for making trusses, bridges, ships and boilers. For such construction standard section and sheets of plats of steel are available. The other machine parts like shafts, gears, bearings, pulleys and bodies of machines can be made in steel through forming, cutting or casting processes or combination thereof. Metal cutting tools, dies, punches, jigs and fixtures are also made in ferrous metal. One of the largest consumer of steel is automobile industry. Despite the modern trend of making light cars nearly 60% of weight of car is still due to steel and an average passenger car contains about 500 kgf of steel in India. Perhaps in countries like USA where cars of bigger size are in use this weight could be as high as 800 kgf/car. The first human effort in the direction of making tools was based upon meteoritic iron obtained from meteorite that had struck the earth. This happened more than 3000 BC. In India the well known Ashoka Column in Delhi was constructed more than 4000 years ago. The blast furnace was invented in 1340 AD and then it became possible to produce large quantities of iron and steel. The future trend is to replace steel by plastics in many machines and equipment. This target has been achieved in a number of home appliances. The demand for steel is level since 1965. Cost fluctuations in most metals have been controlled. The same is true for steel whose cost is increasing at constant rate since early eighties. The comparative price of various metals with piece of gold at 1000 is given in Table 2.1. Table 2.1 : Approximate Comparative Prices of Various Metals with Gold Piece of 100 as Base (per Weight)
Steel Aluminium Copper Magnesium Zinc Gold Lead Nickel Tin Titanium Silver 0.0476 0.2078 0.3140 0.3528 0.1840 1000 0.075 1.5151 1.0823 1.1363 15.1515

Objectives
After studying this unit, you should be able to • • • • • • • • 42 • know how iron and steel are produced, know what are different classifications and applications of steel, understand how different types of steel are formed as alloy, identify different constituents of steel and their effects on properties, know how steel can be treated, understand an alloy steel and effects of alloying on properties, distinguish between steel and cast iron and properties and uses of cast iron, know about alloys of copper, aluminium and their properties and uses, identify bearing materials, and

•

identify creep resistant materials.

Engineering Alloys (Ferrous and Non-Ferrous)

2.2 PRODUCTION OF IRON AND STEEL
Reviewing the principles of the iron and steel making processes, beginning with new materials is taken up first. This knowledge is essential to an understanding of the quality and characteristics of the steels produced by different processes.

2.2.1 Raw Materials
The three basic materials used in iron and steel making are iron or, limestone and coke. Iron does not occur in a free state in nature, yet it is one of the most abundant elements, making up about 5 percent of the earth’s crust in the form of various ores. The principal iron ores are taconite (a black flintlike rock), hematite (an iron oxide mineral) and limonite (an iron oxide containing water). After it is mined, the iron ore is crushed into fine particles, the impurities are removed by various means (such as magnetic separation), and it is formed into pallets, balls, or briquettes using binders and water. Typically, pellets are about 65 percent pure iron and 25 mm in diameter. The concentrated iron ore is referred to as beneficiated. Some iron-rich ores are used directly without palletising. Coke is obtained from special grades of bituminous coal, which are heated in vertical coke ovens to temperatures of 1150oC and cooled with water in quenching towers. Coke has several functions in steel making. One is to generate the high level of heat required for chemical reactions to take place in iron making. Second, it produces carbon monoxide (a reducing gas) which is then used to reduce iron oxide to iron. The chemical by-products of coke are used in making plastics and chemical compounds. Coke oven gases are used as fuel for plant operations, and power generations. The function of limestone (calcium carbonates) is to remove impurities from the molten iron. The limestone reacts chemically with impurities, action as a flux which causes the impurities to melt at a low temperature. The limestone combines with the impurities and forms a slag, which is light and floats over the molten metal. Slag is subsequently removed. Dolmite (an ore of calcium magnesium carbonate) is also used as a flux. The slag is later used for making cement, fertilizers, glass, building materials, rock wool insulation, and road ballast.

2.2.2 Iron Making
The three raw materials are charged into blast furnace by carrying them to the top of and dumping into the furnace. The principle of this furnace was developed in Central Europe, and the first furnace began operating in 1621. The first steel plant in India begins its operation in the early part of twentieth century. The blast furnace is basically a large steel cylinder lined with refractory (heat-resistant) bricks and has the height of about a tenstorey building. The charge mixture is melted in a reaction at 1650oC with air preheated to about 1100oC and blasted into the furnace (hence the term blast furnace) through nozzles (or tuyeres). Although a number of reaction may take place, basically the reaction of iron oxide with carbon produces carbon monoxide, which in turn reacts with the iron oxide, reducing it to iron. Preheating the incoming air is necessary because the burning coke along does not produce sufficiently high temperature for the reactions to occur. The molten metal accumulates at the bottom of the blast furnace, while the impurities float to the top of the metal. At intervals of four or five hours, the molten metal is tapped, into ladle cars. Each ladle car can hold as much as 160 tons of molten iron. The molten metal at this stage has a typical composition of 4 percent carbon, 1.5 percent silicon, 1 percent manganese, 0.04 percent sulphur, and 0.4 percent phosphorous, with the rest being pure iron. The molten metal is referred to as pig iron. Use of the world pig comes from the

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Engineering Materials

early practice of pouring molten iron into small sand molds, arranged like a litter of small pigs around a main channel. The solidified metal is called pig and is used in making iron and steels. The blast furnace is shown in Figure 2.1.
Coke + Ore + Lime Stone

Hopper

Coke Ore Lime Stone 38 C
0

Reduction Zone Gas

480 C Heat Absorption Zone Hot Blast 1205 C 1650 C
0 0

0

Fusion Zone Combustion Zone

Tuyere Slag Ground

Molten Slag Molten Iron Iron

Figure 2.1 : Blast Furnace

2.2.3 Steel Making
Steel was first produced in China and Japan in about 600-800 AD. The process is essentially one of refining the pig iron obtained from the blast furnace. The refining of pig iron consists of reduction of the percentage of manganese, silicon, carbon and other elements, and control of its composition by the addition of various elements. The molten metal from the blast furnace is transported into one of three types of furnace. The steel making furnaces are open hearth, electric, or basic oxygen. The name open hearth derives from the shallow heart shape that is open directly to the flames that melt the metal. Developed in the 1860s, the open-hearth furnace is being replaced by electric furnace and by the basic-oxygen process. These newer methods are more efficient and produce better quality steels. The electric furnace was first introduced in 1906. The source of heat is a continuous electric arc formed between the electrodes and the charged metal (Figure 2.2). Temperature as high as 1925oC are generated in this type of furnace. There are usually three graphites electrodes in direct arc electric furnace, and they can be as large as 750 mm in diameter and 1.5 to 2.5 m in length. Their height in the furnace can be adjusted depending on the amount of metal present and water of the electrodes.
Carbon Electrodes Power Leads

Door

Slag Rammed Hearth Metal

Figure 2.2 : Direct Arc Electric Furnace

44

Steel scrap and a small amount of carbon and limestone are dropped into the electric furnace through the open roof. Electric furnaces can also use 100 percent scrap as its charge. The roof is then closed and the electrodes are lowered. Power is turned on, and within a period of about two hours the metal melts. The current is shut off, the electrodes are raised, the furnace is titled, and the molten metal is poured into a ladle, which is a receptacle used for transferring and pouring molten metal. Electric-furnace capacities range from 60 to 90 tons of steel per day. The quality of steel produced is better than that of open-hearth or basic-oxygen process.
Electrodes

Engineering Alloys (Ferrous and Non-Ferrous)

Trunion Roller

Metal

Figure 2.3 : Indirect Arc Electric Furnace

The induction type electric furnace (Figure 2.4) is used for smaller quantities. The metal is placed in crucible, made of refractory material and surrounded with a copper coil through which alternating current is passed. The induced current in the charge melts the metal. These furnaces are also used for re-melting metal for casting.

Refractory Cement Molten Metal

Copper Induction Coil

Crucible

Figure 2.4 : Induction Type Electric Furnace

The basic-oxygen furnace (BOF) is the newest and fastest steel making process. Typically, 200 tons of molten pig iron and 90 tons of scrap are charged into a refractory lined barrel shaped vessel called converter [Figure 2.3(a)]. Pure oxygen is then blown into the furnace for about 20 minutes under a pressure of about 1250 kPa, through a watercooled lance, as shown in Figure 2.5(b). Fluxing agents, such as burnt lime are added through a chute.

(a) (i) Charging Scrap into Furnace

(ii) Charging Molten Iron

(iii) Addition of Burnt Lime

45
Lance

Engineering Materials

Figure 2.5 : Basic Oxygen Process of Steel Making Illustrated through Various Operations

The vigorous agitation by the oxygen refines the molten metal through an oxidation process in which iron oxide is first produced. The oxide reacts with the carbon in the molten metal, producing carbon monoxide and carbon oxide. The iron oxide is reduced to iron. The lance is retracted and the furnace is tapped by tilting it. The opening in the vessel is so provided that the slag still floats on the top of the molten metal as seen in Figure 2.5(c). The slag is then removed by tilting the furnace in the opposite direction. The BOF process is capable of refining 250 tons of steel in 35 to 50 minutes. Most BOF steels, which are of better quality then open-hearth furnace steels and have low impurity levels, are processes into plates, sheets, and various structural shapes, such as I-beams and channels. Steel may also be melted in induction furnaces from which the air has been removed, similar to the one shown in Figure 2.4. The vacuum melting produces high quality steels because the process removes gaseous impurities from the molten metal.

2.3 CASTING OF INGOTS
After the molten steel has been poured from the steel making furnace it has to be converted in solid shapes called ingot. The ingot is further processed by rolling it into shapes, casting it into semi-finished forms, or forging. For eliminating the need for ingot the shaping process is being rapidly replaced by continuous casting, thus improving efficiency. The molten metal is poured (teemed) from the ladle into ingot moulds in which the metal solidifies. Moulds are usually made of cupola iron or blast-furnace iron, with 3.5 percent carbon, and are tapered in order to facilitate the removal of the solid metal. The bottoms of the moulds may be closed or open; if open, the mould are placed on a flat surface. The taper may be such that the big end is down. The cooled ingots are removed (stripped) from the moulds and lowered into soaking pits, where they are reheated to a uniform temperature of about 1200oC for subsequent processing by rolling. Ingots may be square, rectangular, or round in cross-section, and their weights ranges from a few hundred kgf of 40 tons. Certain reactions take place during the solidification of an ingot, which in turn have important influences on the quality of the steel. For example, significant amounts of oxygen and other gases can dissolve in the molten metal during steel making. However, much of these gases is rejected during solidification of the metal because the solubility limit of gases in the metal decrease sharply as its temperature decreases. The rejected oxygen combines with carbon, forming carbon monoxide, which causes porosity in the solidified ingot. Three types of steel are produced depending on the amount of gas evolved during solidification. These types are : killed, semi-killed, and rimmed. Killed Steel Killed steel is usually deoxidized steel; from which oxygen has been removed and porosity eliminated. In the de-oxidation process the dissolved oxygen in the molten metal is made to react with elements such as aluminium, silicon, manganese, and vanadium that are added to the melt. These elements have an affinity for oxygen and form metallic oxides. If aluminium is used, the product is called aluminium-killed steel. The term killed comes from the fact that the steel lies quietly after being poured into the mould. The oxides in the slag. A fully killed steel is thus free of any porosity and blowholes caused by gases. The chemical and mechanical properties of

46

killed steel are relatively uniform throughout the ingot. However, because of metal shrinkage during solidification, an ingot of this type develops a pipe at the top. It is also called shrinkage cavity and has the appearance of a funnel-like shape. This pipe can comprise a substantial portion of the ingot and has to be removed. Semi-killed Steel Semi-killed steel is partially deoxidized steel. It contains some porosity, generally in the upper section of the ingot, but has little or no pipe; thus scrap is reduced. Although piping in semi-killed steels is less, it is compensated for by the presence of porosity in that region. Semi-killed steels are economical for deoxidation process is quite costly. Rimmed Steel Rimmed steel, generally having a low carbon content (less than 0.15 percent), have the evolved gases only partially killed or controlled by the addition of elements such as aluminium. The gases form blowholes along the outer iron of the ingot – hence the term rimmed. Blowholes are generally not objectionable unless they break through the outer skin of the ingot. Rimmed steels have little or not piping, and have a ductile skin with good surface finish. The blowholes may break through the skin if they are not controlled properly. Impurities and inclusion tend to segregate toward the centre of the ingot. Thus, products made from this steel may be defective and should be inspected. Refining The properties and manufacturing characteristics of ferrous alloys are adversely affected by the amount of impurities, inclusions, and other elements present. The removal of impurities is known as refining, much of which is done in melting furnaces or ladles, with the addition of various elements. The cleaner steels having improved and more uniform properties and consistency of composition are increasingly being demanded. Refining is particularly important in producing highgrade steels and alloys for high-performance and critical applications, such as in aircraft. Moreover, warranty periods on several machine parts such as shafts, camshafts, crankshafts for diesel trucks, and other similar parts can be increased significantly using higher-quality steels. The trend in steel making is for secondary refining in ladles and vacuum chambers. New methods of ladle refining (injection refining) generally consist in melting and processing in a vacuum. Several methods of heating and re-melting have been introduced for their efficiency and cleanliness. These are normally used in controlled atmosphere. Some methods are : electron-beam melting, vacuum-arc re-melting, argon-oxygen decarburisation, and vacuum are double-electrode re-melting.

Engineering Alloys (Ferrous and Non-Ferrous)

2.4 CONTINUOUS CASTING
The traditional method of casting ingots is a batch process. Each ingot is stripped from its mould after solidification and processed individually. Additionally the defects like piping and micro-structural and chemical variations are present throughout the ingot. These problems are alleviated by continuous casting process, which produce better quality steels. Continuous or strand casting was first developed for casting non-ferrous metal strip. The process is now used for steel production, with major efficiency and productivity improvements and significant cost reduction. A system for continuous casting is shown in Figure 2.6. The molten metal in the ladle is cleaned and nitrogen gas through it is blown for five to ten minutes to equlise the temperature. The metal is then poured into a refractory-lines intermediate pouring vessel called tundish where impurities are skimmed off. The tundish can hold as much as three tons of metal. The molten metal vessels through

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Engineering Materials

water-cooled copper moulds and begins to solidify as it travels downward along a part supported by rollers. Before starting the casting process, a solid starter, or dummy, bar is inserted into the bottom of the mould. The molten metal is then poured and freezes onto the dummy bar (Figure 2.6). The bar is withdrawn at the same rate the metal is poured. The cooling rate is such that the metal develops a solidified skin to support itself during its travel downward at speed maintained at 25 mm/s. The shell thickness at the exit end of the mould is about 12 to 18 mm. Additional cooling is provided by water sprays along the travel path of the solidifying metal. The moulds are generally coated with graphite or similar solid lubricants to reduce friction and adhesion at the mould walls. Vibration of moulds may further reduce friction and adhesion tendency. The continuously cast metal may be cut into desired lengths by shearing or touch cutting, or it may be fed directly into a rolling mill for further reduction in thickness. Whether the steel is obtained in form of ingots, stationary moulds or in form of slab from continuous casting process, it is converted into blooms, billets and slabs. The subsequent hot rolled products are described as follows : Blooms Beams and angle sections, rails, bars of different sections. Billets Wire nails and wire mesh, pipes and tubes. Slabs Plates, strips and further cold rolled for reducing thickness.

Electric Furnace Tundish

Oil Cooling Water X – Ray Transmitter Molten Metal Solidified Metal

Argon

Platform

Air Gap Catch Basin

Pinch Rolls

Oxygen Lance (For Cutting) Starting Dummy

Figure 2.6 : Continuous Casting Process of Steel Schematically

SAQ 1
(a) 48 Distinguish between an elemental metal and alloy.

(b) (c) (d) (e) (f)

What are the raw materials used in blast furnace in iron making process? What are electric heating processes for making steel? Describe BOF and its advantages. Distinguish between killed and semi-killed steels. List steel products obtained from blooms, billets and slabs.

Engineering Alloys (Ferrous and Non-Ferrous)

2.5 STEELS
Steel perhaps is the oldest material of construction which has weathered and history and not only maintained its foremost place in industrial application but also enhanced it greatly. The enhancement of its position in industrial world is mainly due to its capacity to be produced in several alloyed varieties and response to various heat treatments. The cutting properties of sharp edges of steel was recognised long back when man began to use swords and knives made in steel. The very same properties of this material have been exploited to create cutting tools which wear very little and such machine parts as gears, shafts, bearing, etc. The steel is now being used in every conceivable engineering structure. Machine bodies, railways and rail road rolling stocks, ships, bridges and boilers are a few examples. Several forms of steel are now available commercially and each is produced to serve some specific purpose. The requirements of steel very largely and more often than one involve consideration of their tensile strength, impact strength and hardness. Table 2.2 describes some applications and requirements of some steels. Table 2.2 : Typical Carbon Steels and their Applications
Application Axles, shafts, small gears Requirements C Availability in form of bar stock. Good strength in bending and torsion. Heat treatable for improvement of surface resistance. Availability in rod form, this is obtained through rolling. Good ductility for coiling. Good response to heat treatment for spring properties. Availability in form of thin sheets which can be pressed into accurate shapes. Good ductility and low yield strength. Availability in thick plate forms. Good strength to withstand stresses during forming and service, particularly good ductility at low temperatures. Good weldability since most ships have welded structure. 0.4 % Composition Mn 0.8 Si 0.1 P 0.05 S 0.05

Helical springs

1.0

0.6

0.3

0.05

0.05

Automative bodies and panels Ship hulls

0.0 8 0.1 8

0.3

−

0.05

0.05

0.8

0.1

0.05

0.05

The carbon content of steel plays an important role in deciding its properties. If no carbon is present in iron, it crystallises in form of ferrite which is bcc soft material and very ductile. Pure iron without impurities is perhaps used only in laboratories and may be as costly as gold. Pure iron is though but not very strong. With addition of carbon, increasing amounts of cementite (Fe3C) crystallise in the structure. Cementite being hard reduces ductility considerably. Table 2.3 would illustrate how ductility (measured as % elongation) of steel is reduced with increasing amount of carbon. When carbon is as much as 6.67% in iron, the entire structure crystallises as cementite and is not at all usable commercially because it neither has ductility nor is machinable.

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Engineering Materials

Table 2.3 : Ductility of Plain Carbon Steel
Carbon Content (%) Pure iron 0.2 0.4 0.6 0.8 1.2 % Elongation 42 37 31 22 17 3

The carbon content in steel serves to classify steel according to its application. Table 2.4 describes the application of different iron-carbon alloys. Figure 2.7 illustrates the effect of carbon percentage on tensile strength of steels mainly. The tensile strength of pure iron (0% C) is around 250 N/mm2 which increases to about 850 N/mm2 at a carbon percentage of 0.8. It has already been pointed out that a very low carbon content the entire metal is made up of ferrite grains which are soft and ductile. As the percentage of carbon increases more and more material is made up of pearlite, a substance that appears to have colour of mother-of-pearl, hence the name. At 0.4% C, pearlite appears to be almost half the area viewed through microscope. At this level of carbon the tensile strength is about 540 N/mm2 . At 0.8% C almost total area consists of pearlite when a strength of 850 N/mm2 is reached. Table 2.4 : Carbon Percentage in Plain Carbon Steels and Application
Range of Carbon (%) 0.1 – 0.8 0.0 – 1.2 1.3 – 2.2 2.4 – 4.2 Application General engineering purposes Wear resistance steel Not used normally Cast iron, casting

2.5.1 Plain Carbon Steel and Applications
Some applications of carbon steels have been described already in Table 2.2. Here some of the applications will again be described after emphasising the manner in which plain carbon steels are classified. Plain carbon steels are those which contain carbon as principal alloying element. These steels may also contain small amounts of such impurities as manganese, sulphur, phosphorous, silicon and nickel. The sulphur and phosphorous are mainly undesirable impurities and attempts are make to keep them at as low level as possible. Their levels beyond 0.05% are not permissible. According to carbon percentage (or microscope structure) the steel is divided into three groups. Eutectoid Steels Such steels contain ideally 0.83% of carbon and have entirely lamellar peralite structure. In practice fully pearlite structure appears in all steels containing about 0.8% carbon. Moreover many alloying elements influence the carbon contents of eutectoid steels. For instance, Mn to the extent of 1% reduces carbon in the eutectoid to 0.7%. Hypo-Eutectoid Steels These steels contain carbon between 0.08% to just below 0.83%. They contain grains of ferrite together with grains of pearlite. The strength increases with increasing carbon content due to increasing proportion of strong pearlite formed but ductility decreases proportionally.

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Hyper-Eutectoid Steels These steels contain carbon significantly in excess of 0.8%. The structure contains pearlite and cementite. Cementite forms along the grain boundaries of pearlite as an inter-granular network and increase brittleness. With respect to range of carbon content the plain carbon steels are divided into following groups. The carbon percentages may overlap in many cases. Dead Mild Steels The carbon range for this steel is between 0.07% to 0.15%. These steels are highly ductile and hence can withstand large amount of plastic deformation through cold working. Solid drawn tubes are made out of deal mild steels. Mild Steels The carbon percentage for mild steels very between 0.15 to 0.25. This steel does not harden appreciably when quenched. It has very good weldability and is commonest of the steels used for structural purposes. Medium Carbon Steels These steels contain carbon between 0.25 and 0.55%. These steels respond to suitable types of heat treatments. High Carbon Steels The carbon content in these steels vary between 0.55% to 0.9%. Very high hardnesses can be achieved through heat treatment. They develop extreme resistance to wear and hence are good for tooling applications. In yet another classification steels containing upto 0.3% C are known as low carbon steels thus including both dead and mild steel. Those containing carbon between 0.3 and 0.6% are medium carbon steels and those containing carbon in excess of 0.6% carbon are high carbon steel thus including carbon tool steels. Table 2.5 describes various applications of plain carbon steels, whereas Table 2.6 describes plain steels for several applications. Table 2.5 : Some Applications of Plain Carbon Steels
Steel Dead mild steel % Carbon 0.07 – 0.15 Applications Rivets, nails, tin plates, stamping, chains, seam welded pipes, automative body sheets, other materials subject to drawing and pressing. Structures, rolled steel sections, drop forgings, screws, case hardening purposes. Machine structures, shafting and forging, gear. Shafting, axles, crane hooks, connecting rods, general purpose forgings. Axles, gear, shafts, rotors, tyres, skip wheels, crank shafts. Rails, loco tyres, wire ropes. Drop hammers, dies, saws, screw

Engineering Alloys (Ferrous and Non-Ferrous)

Mild steel

0.10 – 0.20

0.20 – 0.30 Medium carbon steel 0.30 – 0.40

0.40 – 0.50 0.50 – 0.60 High carbon steel 0.60 – 0.70

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Engineering Materials

drivers. 0.70 – 0.80 Hammers, anvils, wrenches, leaf springs, cable wires, band saws, large dies for cold presses. Shear blades, punches, rock drills, cold chisels. Drills, knives, taps, picks, screwing dies, axes. Razors, files, broaches, boring and finishing tools, such machine parts where wear resistance is required, ball bearings.

0.80 – 0.90 Tool steels 0.90 – 1.10 1.10 – 1.40

Table 2.6 : Plain Carbon Steels for Different Applications
Sl. No. 1 2 Application Nails, rivets, stampings Beams, rolled sections, reinforcing bars, pipes boiler plates, bolts Shafts and gears Properties High ductility, low strength High ductility, low strength, toughness Steel Low C (AISI 1010) Low C (1020)

3

Heat treatable for good strength and ductility Heat treatable for good strength and ductility Toughness High toughness and hardness Retaining sharp edges Hardness, toughness, heat treatable

Med C (1030)

4

Crank shaft, bolts, connecting rod, machine component Lock washers, valve springs Wrenches, dies, anvils Chisels, hammers, shear Cutters, tools, taps, hacksaw blades, springs

Med C (1040)

5 6 7 8

High C (1060) High C (1070) High C (1080) High C (1090) tool steel

SAQ 2
(a) (b) (c) What are distinguish features of eutectoid, hypoeutectoid and hypereutectoid steels? How are plain carbon steels classified as low, medium and high carbon steels? Describe uses of low, medium and high carbon steels.

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2.5.2 Iron-Carbon System Phase Diagram
Iron and carbon make a series of alloys which include a number of steels and cast iron. Steels are the alloys which contain upto 1.2% of carbon while cast irons contain carbon within range of 2.3% to 4.2%. Alloys with carbon greater than 4.6% have poor properties and hence not used.
o o

Engineering Alloys (Ferrous and Non-Ferrous)

The carbon atom is smaller than the iron atom (the diameters being 1.54 A and 2.56 A respectively) and dissolves interstitially in iron. Pure molten iron solidifies at 1539oC in bcc structure. At 1400oC, on further cooling this structure changes into fcc and again at 910oC back to bcc. These three structures in reverse order (i.e. form room temperature) are named as α, γ and δ.

The α-iron is ferromagnetic but loses its ferromagnetism when heated above 770oC (curic temperature). The non-magnetic α-iron was earlier known as β-iron until X-ray studies showed that structures of α − and δ − irons are same. The γ-iron is the structure of closest packing while α − and β − irons are not. There will be abrupt changes of dimensions at transitions α to γ and γ − β. Figure 2.7 depicts the cooling curve of pure iron.
Liquid 1500 1539 1400 γ (fcc) δ (bcc)

1000

910 770

β (Non-magnetic α)

500

α (bcc)

0 Time

Figure 2.7 : Cooling Curve of Pure Iron with Steady Rate of Heat Loss

Alloys in iron-carbon systems also undergo complex structural changes which play an important role in deciding their characteristics. Changes that occur in iron-carbon system are illustrated in iron-carbon phase diagram of Figure 2.8. Strictly speaking the diagram refers to the iron-iron carbide system but still the phase relationship can be expressed in terms of carbon percentage, hence the name. Iron-carbon diagram is divided in various phase fields characterised by the existence of one or mixture of two phases. The liquids above which only molten metal in liquid state can exist is identified as ABCD. Iron and dissolved carbon exist in the liquid state. The solidus below which iron-carbon exist in solid state is identified as AEPGCH. Between these two lies there exist mixtures of solid and liquid.
α and γ phases can dissolve carbon and the solubility changes with temperatures. The solid solution of carbon in α-iron is called ferrite while the solid solution of C in γ-iron is known as austenite. Ferrite and austenite are also designated as α and γ respectively. the solubility of C in austenite is upto 0.2% while ferrite can dissolve C only upto 0.025%. The C solubility in austenite changes along GK in austenite and along LN in ferrite. Beyond point C on solidus, the compound Fe3C which is cementite, separates. Cementite is hard and brittle whereas ferrite on the extreme left of phase diagram is soft and ductile. Austenite also transforms into cementite along the ling GK and into ferrite along IK.

53

Engineering Materials

The line IK and GK are respectively designated as A3 and Acm. Austenite is unstable below the lines A3 and Acm if carbon content is less than 0.8%. Austenite begins to transform into ferrite on cooling and gets enriched in carbon along line A3 until point K is reached. Similarly for carbon content between 0.8 and 2.0% iron carbide will precipitate and carbon in austenite will vary until point K (carbon 0.8%) is reached. At point K austenite will transform into pearlite. Pearlilte is an intimate mixture of ferrite and cementite (Fe3C) having a characteristics lamellar structures composed of alternate platelets of ferrite and cementite. The transformation reaction at K wherein a single solid phase splits into two phases is termed eutectoid. The equation is written as
Solid phase (A) → Solid phase (B ) + Solid phase (C )
(Austenite) (Ferrite) (Cementite)

Apparently at point K three phases exist (P = 3) and three are two components (C = 2), hence using Gibb’s phase rule the degree of freedom, F can be calculated. Gibb’s rule at constant pressure, P+F=C+1 F=2–3+1=0 Thus the eutectoid point like eutectoid point is non-variant. Slightly above 110oC at point C on solidus, the eutectoid transformation occurs. The liquid state at C contains 4.3% C and this liquid begins to transform into two solid phase. One phase is called Ledeburite which is eutectic mixture of austenite and cementite while the other phase is cementite. On further cooling eutectic austenite transforms gradually into cementite, changing composition along GK until it changes into pearlite and cementite at eutectoid point K.

Peritectic
Peritectic transformation occurs in steel having carbon upto 0.55%. This transformation occurs at point P and is characterised by transformation of liquid and solid phase into a single solid phase. For example, the transformation occurring at P is represented by the following equation, Liquid (0.5% C) + δ iron (0.08% C) → γ iron (0.18% C)

Explanation of Reading Iron-Carbon Phase Diagram
Assume that in liquid state mixture of iron and carbon contains 0.4% carbon and is represented by point 1 in Figure 2.8. The line xx shows the path of cooling. As cooling begins the freezing starts in intersection point 2 of xx and liquidus ABC at 1510oC. At a temperature of 1470oC at point 3 the liquid is solidified completely into solid austenite. This phase gradually cools until point 4 on line IK is reached. The precipitation of proeutectoid ferrite begins and the carbon content of austenite varies along line A3. The composition of ferrite varies along the line IL. In the region enclosed between IL and IK he percentage of austenite and ferrite can be determined by using lever rule. The remaining austenite transforms into peralite (88% ferrite and 12% cementite) at eutectoid temperature of 723oC. The % weight of (ferrite + cementite) can be determined by lever rule along line LKM. Further cooling will effect no change in microstructure since carbon percentage in ferrite practically remains constant of 0.008. The microstructure is built by pearlite matrix embedded with ferrite crystals. Taking example of alloys with 3% carbon and considering its cooling from a point above liquids along yy. The composition of course is in the region of cast iron. The austenite separates from liquid and its content will increase along the solidus PG and the amount of liquid reduces with composition varying along liquidus BC. When the temperature of 1130oC is reached, the mixture will consists of austenite containing 2% carbon and liquid of eutectic composition.

54

The liquid will solidify at constant temperature into ledeburite – a mixture of austenite and cementite. On further cooling eutectic austenite decomposes to cementite along the line GK. At temperature of 723oC (eutectoid). The remaining austenite will transform to pearlite. Below the temperature of 723o C all ledeburite will be transformed into a mixture of pearlite and cementite.

Engineering Alloys (Ferrous and Non-Ferrous)

Transformation Reactions
Several transformation were described in above paragraphs while explaining how to read iron-carbon diagram. These transformation are summed up afresh here. Eutectoid Reaction Eutectoid reaction takes place when austenite containing 0.77% C decomposes into ferrite and cementite at 723oC (point K in the phase diagram of Figure 2.8).
0.77% C

Solid austenite (γ ) → Ferrite (α) + Cementite (Fe3C) o
723 C

Solid phase splits into two solids phases. Steel containing carbon between 0.008 and 0.8% is called hypoeutectoid and those containing between 0.8 and 2.0% carbon are hypereutectoid.
Eutectic Reaction

Eutectic reaction occurs when liquid solution containing 4.3% C transformations into austenite (γ) and cementite (Fe3C) at 1130oC. Point C in Figure 2.8 is eutectic point.
4.3% C

Liquid L  o → Austenite ( γ ) + Cementite (Fe3 C)
1130 C

Liquid phase transforms into two solid phases.

Figure 2.8 : Iron-Carbon Phase Diagram

Peritectic Reaction Peritectic reaction occurs when a liquid and a solid phase freeze to form a solid phase. In iron-carbon system peritectic reaction takes place when alloy containing 0.55% C and containing liquid and solid δ iron transforms at 1495oC into solid austenite (γ). Liquid + δ-Iron  o → Solid austenite
(0.55% C) (0.99% C) (1495 C) (0.17% C)

55

Engineering Materials

2.5.3 Time Temperature Transformations
The phase diagram of iron-carbon system evokes much interest for engineers to use the information for useful purposes of increasing strength and eliminating locked in stresses by retaining or avoiding a particular phase. However, important aspect of such a diagram to keep in mind is that it represents equilibrium cooling. Such equilibrium cooling is neither obtained in practice nor it is conducive to develop desired strength and structure in a particular steel. Increasing cooling rates reduces the transformation temperature as was highlighted and may result information of metastable phase. As an example very high cooling rate of iron-carbon system in steel range causes development of a metastable phase called martensite. This phase does not appear in Figure 2.8. The steel and cast iron normally carry some alloying elements, though in varying amounts. These alloying elements have great deal of influence on precipitation of all the phases and this is also not represented by phase diagram of Figure 2.8. The metastable phase marteniste has been introduced here and other phases like pearlite ferrite, and cementite were mentioned earlier. More about their structure and properties will be discussed now but before that transformation curves are described. Experimental determination of isothermal transformation curves goes as under. A number of samples of the size of a 50 paise coin are austenised just above the temperature of 723oC (eutectoid temperature). The samples are rapidly cooled in a salt bath maintained at a temperature slightly below 723oC. After allowing various time intervals the specimens are taken out one by one from the salt both and quenched into water at room temperature. The resulting microstructure is then examined at room temperature. The experiment is repeated with isothermal transformation of eutectoid steel (both regions of hypo- and hypereutectoid steel) at progressively reducing temperatures. If temperature of transformation is plotted as function of time of transformation the resulting curve as shown schematically in Figure 2.9 is called isothermal transformation (IT) curve, or temperature-time-transformation curve (TTT). This is also known as Bain curve after the metallurgist who first introduced the idea of S curve because of its shape.

Eutectoid Temp Temperature ( C)
0

Ps

Pf

Coarse Pearlite Fine Pearlite Bainite

PS Pf Martensite Time

Figure 2.9 : Time Temperature Transformation Diagram for Plain Carbon Steel

The line marked Ps shows the beginning of transformation of austenite into pearlite and the line Pf represents the completion of such transformation. Figures 2.10 and 2.11 show the TTT diagrams respectively for 0.8% C steel and 0.3% steel. The difference between the two can be noted. Even the fastest cooling rate will not be able to mess the nose of the S curve and hence 100% martensite will not be retained at room temperature in structure of steel. The important feature of TTT curve is its bending backwards at nose. Below the nose the austenite transforms into Bainite. Whether it is pearlite or bainite, at a temperature called Ms the transformation into a transition phase Martenstite takes place. Both binite and martensite may be retained in steel by controlling the rate of cooling. If cooling is sufficiently fast so that nose is avoided then austenite transforms into bainite. If the part is now held at this temperature for sufficiently long time the bainite is stabilised. Unlike, pearlite, in bainite the cementite is in particle form, distributed uniformly in the 56

matrix of ferrite. Bainite is harder, stronger and tougher than pearlite. Bainitic steel is more ductile than pearlitic steel for some level of hardness.
727 γ Ps PF Temperature 550 Fine Pearlite Coarse Pearlite

Engineering Alloys (Ferrous and Non-Ferrous)

230

γ Ms

Bs

BF

Bainite

Martensite

110 MF 4sec 0-5min 1min 4min 1 hr 15 hr

Figure 2.10 : TTT Diagrams for 0.8% C Steel with 0.76 Mn

If heated steel is cooled sufficiently fast the nose of Ms temperature, martensite is formed. Its transformation is complete at temperature Mf. Martensite has C dissolved in Fe whereby its bcc structure changes into body centered tetragonal (bct) structure and is marked by high hardness because of : (a) (b) (c) distortion of iron lattice, very fine size of martensite plates, and high density of dislocations associated with twining.
727 Temperature ( C) Ps PF Ferrite+Pearlite

0

Ferrite + Pearlite + Bainite + 340 Bs Ms 210 MF 4sec 0.5min 4min Time 1 hr 15 hr BF

Martensite

Figure 2.11 : TTT Diagrams for 0.3% C Hypoeutectoid Steel

Figure 2.12 schematically shows the cooling pattern to produce different phases in steel. Heat treatments given to steel will be governed by the cooling rates producing final phases with pearlite, bainite or martensite. Many refinements are possible with control of cooling rate and soaking time. 727
Temperature ( C)

0

1 230 110 I 1sec 2sec

2

3

4

Ms MF

II

III 4min Time 2hr

57

Engineering Materials

Figure 2.12 : TTT Curves for Steel and Different Cooling Rates

At this stage, before taking up different methods of heat treatments in any detail, it will be worthwhile to first describe formation and characteristics of different phases.

2.5.4 Pearlite
The tensile strength of pure iron (0.00% C) is around 250 N/mm2 which increases to about 850 N/mm2 at a carbon percentage of 0.8. At very low carbon percentage the entire metal is made up of ferrite grains which are soft and ductile. With increasing carbon percentage more and more material is made up of pearlite, a substance that appears to have colour of mother-of-pearl, hence the name. At 0.4% carbon pearlite occupies almost half the area viewed through microscope. At this level of carbon the tensile strength is about 540 N/mm2. At 0.8% carbon almost total area consists of pearlite with a strength of 850 N/mm2. Examination of pearlite at higher magnification (X 1500) reveals that it has a laminated structure. It is a composite consisting of alternate layers of ferrite and cementite. Cementite provides the strength while ferrite retains ductility of steel. At carbon content above 0.8% the quantity of Fe3C begins to increase and steel begins to lose its ductility. Figure 2.13 shows relationship between pearlite and carbon content. Outside the region of Figure 2.13, within the range of carbon between 0.9 and 1.2% pearlite structure still exists but grains are surrounded by cementite network. These cementite networks not being parts of grains are referred to as free cementite and are the cause of brittleness in steel. In general the strength of steel does not increase due to the presence of free cementite, later it may reduce below the level of 0.8% carbon steel. Since pearlite is an important constituent of steel is control becomes important if properties of steel are to be controlled. It will be worthwhile to understand how it is formed. With reference to Figure 2.7 three forms of iron were identified. These forms are referred to as allotropic forms of iron and at each transformation a thermal arrest is experienced.

100 Area of Micrograph Occupied By Pearlite Ferrite + Pearlite

Fe3 C + Pearlite

50

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Carbon Content (%)

Figure 2.13 : Relationship between Pearlite % and Carbon Content

During the cooling of pure iron the transformation from γ-iron (fcc) to α-iron (bcc) occurs at a unique temperature of 910oC. However, carbon is added to the iron to form steel this transformation takes place over a range of temperatures. The limits of this range are known as the critical points and the higher temperature at which transformation starts during cooling is designated A3. The lower point is designated as A1. It was stated in 58

Section 2.7 that the lower temperature A1 remains constant at 723oC irrespective of the carbon content while the higher temperature A3 gradually reduces as carbon percentage increases (Figure 2.8). At a carbon percentage of 0.8, A3 and A1 are equal to 723oC. If carbon percentage further increases, A3 also increased. The equilibrium diagram will be obtained if A3 and A1 temperatures are plotted against carbon percentage as seen in Figure 2.14. The detailed phase diagram has already been presented in Figure 2.8. This diagram shows how γ-iron which is austenite is transformed into pearlite.
910

Engineering Alloys (Ferrous and Non-Ferrous)

A3 Temperature ( C ) A3 A3 723 A1 A1 A1 A1 A1 A1 A3 A
0

Cementite Cem

Austenite + Cementite

Ferrite + Pearlite

Pearlite + Cementite 0.8

Carbon Content (%)

Figure 2.14 : Critical Points for Carbon Steel

It may be noted that carbon solubility is different in austenite than in ferrite. The fcc (γ-iron) can hold as much as 1.7% carbon in solution at 1130oC. Within the range of carbon under discussion the structure is entirely austenite at this temperature and all the carbon is in solution. The carbon is dissolved interstitially or in other words, a carbon atom does not replace an iron atom to form part of the cube (Figure 2.15(a)). When the lattice is arranged to have a bcc structure the carbon atoms is ejected from its site (Figure 2.15(b)) because space is insufficient. This is the reaction that takes place at A3.
Carbon atom in Space between Iron Atom

Insufficient Space Between Iron Atom to Accommodate Carbon Atom

Figure 2.15 : Interstitial Solution of Carbon

The sequence of events between temperatures A2 and A1 is illustrated in Figure 2.16 during which austenite transforms into pearlite. The bcc ferrite grains nucleate at the boundaries of austenite grains. As the temperature reduces the ferrite covers more and more are in austenite grain which reduces in size. The separation of ferrite from austenite will result in rejection of carbon which will be absorbed by remaining austenite. Thus the remaining austenite gets richer in carbon until the amount of carbon reaches 0.8% causing precipitation of layers of cementite in the austenite region. Thus, the austenite transforms into pearlite consisting of layers of cementite and ferrite. Thus transformation of austenite

59

Engineering Materials

is similar to eutectic reaction but since it occurs in the solid state it is termed eutectoid transformation. This eutectoid reaction (or breaking of solid phase austenite into ferrite and cementite as already described in Section 2.9) takes place at temperature of 723oC which is termed A1. For this reason pearlite is also often referred as eutectoid.

Figure 2.16 : Transformation of Austenite in 0.4% C Steel, as Cooling occurs from Molten State above A 3 Temperature

2.5.5 Martensite
Carbon atoms cannot move fast through the lattice between temperatures of A3 and A1. The formation of pearlite and ferrite depends upon allowing sufficient time for the movement of carbon atoms. This means that such transformation is possible only under equilibrium condition. It may be realised that the edge of fcc cell is 23% greater than that of bcc cell. If a carbon atom is trapped between two iron atoms, they are kept apart and are not able to take up the position in bcc cell. Carbon atoms in martensite occupy the position on edge of unit cell between two iron atoms and thus elongate the edge and cause distortion. Figure 2.17 compares austenite, ferrite and martensite unit cells at (a), (b) and (c) respectively. At (d) the size of edge of martensite unit cell on which the carbon atom size is compared with other edge on which there is no carbon atom. As a result a distorted lattice is obtained. It may be emphasised that such distortions resulting into regions of high strain energy, would impede the movement of dislocations whereby the material would lose its ductility or increase its hardness. This trapping of carbon atom between atoms of Fe occurs when the steel is not allowed to cool in equilibrium condition or in other words it is suddenly cooled from A3 to room temperature or to about 300oC. This process of sudden cooling from A3 is called quenching which may be achieved by plunging a heated steel (A3 temperatures) into water or some other quenching medium. During this process the carbon does not have sufficient time to diffuse or carbon does not occur below a temperature 300oC hence a quench treatment in which temperatures is suddenly dropped from A3 to 300oC is sufficient to create permanent hardening in steel. If the resulting structure is examined under microscope it appears to be needle like, often referred to as acicular, and is called martensite.
Fe Atom C Atom

c C Fe Atom C Atom Fe Fe a

Figure 2.17 : Comparison of Unit Cells of Austenite, Ferrite and Martensite Also Dimension are Compared at (d)

Different carbon contents will naturally result in different hardness. At low carbon levels there is very little change in hardness. The noticeable change in hardness is achieved when carbon content is at least 0.4% and reaches a maximum at eutectoid composition of 0.8%. The hardness that can be achieved through martensite transformation at 0.4 and 0.8% C are respectively 255 VHN and 530 VHN. Also the formation of martensite depends upon the cooling rate. As the carbon content increases the critical cooling rate becomes slower. 60

The critical cooling rates for plain carbon steels vary between 400oC/sec and 500oC/sec. It means that it is easier to harden steel containing 0.8% C than the one containing 0.4% C (Also refer to Figure 2.12). Quenching in water results in fastest cooling rate and corresponds to critical cooling rate for 0.35% carbon. This is not good for hardening. Figure 2.18 shows the critical cooling curves for some plain carbon steels.

Engineering Alloys (Ferrous and Non-Ferrous)

Temperature ( C)

0

C = 0.8% 0.4% 0.6%

Time

Figure 2.18 : Critical Cooling Rate for Different Carbon Content

Example 2.1

A 0.4% C hypoeutectoid plain carbon steel is slowly cooled from 1540oC to (i) slightly above 723oC and (ii) slightly below 723oC. Calculate the weight percent (a) (b) (c) (d)
Solution

austenite present in the steel, ferrite present in the steel in case (i), proeutectoid ferrite prevent in the steel, and eutectoid ferrite and eutectoid cementite % present in the steel in case (ii).

Refer to Figure 2.8. Point 1 above the liquidus represents the state of liquid steel. The cooling occurs along the line xx and an equilibrium cooling is assumed. Freezing begins at point 2 which is intersection of liquidus and line xx. Temperature at 2 is 1510oC. The steel solidifies completely at point 3 where temperature is 1471oC. The whole alloy is now composed of austenite (γ-phase) as indicated by first of Figure 2.19. No change occurs until point 4 on line A3 is reached. At this point the precipitation of ferrite begins out of solid austenite. Further cooling increases the amount of ferrite and austenite decreases. The amount of austenite varies along IK. The composition of ferrite varies along the line IL. Calculation of % content will be made by lever rule. The amount of austenite slightly above 723oC is calculated from the line LK itself. i.e. taking LK as tie line. (a) Weight % of austenite =
=

L5 0.4 − 0.025 = LK 0.8 − 0.025
0.375 = 0.484 or 48.4% 0.775

. . . (i) Weight % of ferrite

= =

5K 0.8 − 0.4 = LK 0.8 − 0.025 0.4 = 0.516 or 51.6% 0.775 .
61

. . (ii)

Engineering Materials

(b)

Weight % of proeutectoid ferrite slightly below 723oC is same as that slightly above, i.e. 48.4%. . . . (iii) For calculating eutectoid ferrite, the weight of carbide will have to be subtracted form total mass of ferrite and cementite. Just below isothermal line LKM ferrite and pearlite are present and lever arm will extend upto ordinate representing 6.67% C. Weight % of total (ferrite + cementite) just below 723oC
= 6.67 − 0.4 6.37 = 6.67 − 0.025 6.645

= 0.96 or 96%
Weight % of Fe3C just below 723oC
= 0.4 − 0.025 0.375 = 6.67 − 0.025 6.645

= 0.0564 or 5.64% Weight % of eutectoid cementite = total ferrite – proeutectoid ferrite = 96 − 51.6 = 44.4% Weight % of eutectoid cementite (by difference)

= 100 − 48.4 − 5.64 − 44.4 = 1.56%
(iv)
Example 2.2

...

A hypoeutetoid steel which was cooled slowly from γ-state to room temperature was found to contain 10% eutectoid ferrite. Assume no change in structure occurred on cooling from just below the eutectoid temperature to room temperature. Calculate the carbon content of steel.
Solution

Refer to phase diagram of Figure 2.8 and let the vertical line xx cross the isotherm at 5 such that 5 is at a distance x′ from temperatures axis. Then by lever rule % total ferrite =
6.67 − x′ 6.67 − x′ = 6.67 − 0.025 6.645 0.80 − x′ 0.80 − x′ = 0.80 − 0.025 0.775

% proeutectoid ferrite =

% eutectoid ferrite = % total ferrite – % proeutectoid ferrite or
10 6.67 − x′ 0.80 − x′ = = 100 6.645 0.775

0.51,498 = 5.169 = 0.775 x′ − 5.316 + 6.645 x′ 0.51,645 = 5.87 x′ ∴
x′ = 0.51645 = 0.088% 3.87

The steel has 0.088% C.
Example 2.3

62

Heat treatments as mentioned below are given to thin steel strips for which TTT diagram is as shown in Figure 2.19. What will be the resulting structure of steel in each case.

Treatments

Engineering Alloys (Ferrous and Non-Ferrous)

(a) (b) (c) (d) (e) (f) (g)

Water quench to room temperature. Hot quench in molten salt to 690oC hold for 2 hours and water quench. Hot quench to 610oC and hold 3 minutes, water quench. Hot quench to 580oC, hold for 25 minutes, water quench. Hot quench to 450oC, hold for 1 hour, water quench. Hot quench to 300oC, hole for 30 minutes, water quench. Hot quench to 300oC, hold for 5 hours, water quench.
TTT Curves 690 C 700 Temperature ( C ) 600 500 400 300 200 Ms (a) (d) (f) 680 C 580 450 C (e) Bainite 300 C (g)
0 0 0 0 0

(c) Pearlite

1

10

10

2

10

3

10

4

Time (sec)

Figure 2.19 : Cooling Scheme of Example 2.3

Solution

From Figure 2.19 the final structures of steel can be determined (a) (b) (c) (d) (e) (f) (g) Martensite Coarse pearlite Fine pearlite 50% fine pearlite and 50% martensite Bainite 50% fine bainite and 50% martensite Fine bainite.

SAQ 3
(a) (b) (c) (d) (e) (f) Describe cooling curve for pure iron. Will this curve change in presence of impurity. Explain eutectoid and peritectic transformation by the help of Fe-C phase diagram. Describe the following phases in iron-carbon phase diagram. Pearlite, ferrite, cementite, austentite and ledeburite. What is an S-curve? What are its other names? What are allotropic forms of iron? Correlates these forms with temperatures of iron cooling form molten state. What is martensite and how is it formed? Explain using unit cell structure.

63

Engineering Materials

2.6 HEAT TREATMENT OF STEEL
A range of properties may be produced in steels because the structure of various phases of microstructure depend upon the rate of cooling. Some aspects in this connection have already been explained. In this section specific treatments will be outlined. All heat treatment processes consist of three main steps. (a) (b) (c) The heating of metal to predetermined heat treating temperature. The soaking of the metal at that temperature until the structure becomes uniform throughout the section. The cooling of the metal at some pre-determined rate such as well cause the formation of desirable structure.

The heat treatments are normally applied to hypo-eutectoid carbon steels. These are : annealing, normalising and hardening. The temperature to which heating is done in all three cases is about 50oC above A3 temperature as indicated in Figure 2.20.
1000 Acm Full Annealing

Normalizing Temperature ( C) 900 Hardening Annealing A3 738 C 700 A1 Stress Relief 600 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Spheroidizing
0 0

800

1.6

Composition, % C

Figure 2.20 : Heat Treatment Range for Carbon Steels

The heat treatment of other steels will be discussed in specific section where these steels are described.

2.6.1 Annealing
It is a heat treatment basically to soften the steels. The heating and cooling both at controlled rate are performed in a furnace. Hypoeutectoid steels are heated above the upper critical temperature (A3 line) while hypereutectoid steels are heated only above the lower critical temperature (A1 line). The cooling is so done that γ-to-α transformation goes to completion to each temperature. The resulting structure consists of large grains of ferrite with coarse pearlite in which thick plates of ferrite and carbide are present. This is softest possible structure and is ideal point for starting mechanical working of steel. Low yield strength and tensile strength are associated with this treatment. Different purpose which are achieved through annealing are listed below : (a) (b) (c) the relief of all internal stresses within the metal. the production of uniform grain structure throughout the metal. the softening of the metal.

The annealing processes are classified either as full annealing or process annealing. Full annealing is essentially the process described above. However, cooling may be done in furnace, in ashes of sand or in specially built cooling pits lines with refractory and covered with refractory lid. If heated to too high a temperature or soaked for too long a time the austenitic phase undergoes grain growth resulting into coarse peralite grains. Such a structure is termed “overheated” and exhibits low mechanical strength. 64

One main problem to overcome during annealing process is the decarburisation and oxidation on the surface. Packing the steel into special boxes are used a neutral atmosphere in the furnace may overcome this problem. For example, low carbon steel parts could be packed into boxes filled with sand, line, ground mica or cast iron swarf while higher carbon components are usually packed into charcoal and other carbonaceous materials. Full annealing is not usually desirables as it results into considerable loss of mechanical strength. Further it is too slow and costly process.
Process annealing, also known as commercial annealing or referred as stress relieving is performed by heating steel to a predetermined temperature which is below the A1 temperature. The metal is air-cooled or quenched in a suitable pickling bath. Mild steels (or hypoeutectoid steel containing less than 0.3% C) after having undergone the mechanical treatment are softened by this process by heating to a temperature between 550oC and 650oC. The distorted grains of ferrites in steel are fully recrystallised by the process. The pearlite grains are not affected by process annealing so that the structure consists of stress free ferrite matrix with distorted pearlite.

Engineering Alloys (Ferrous and Non-Ferrous)

2.6.2 Normalising
Normalising is used as a finishing treatment for carbon steels giving higher strength than annealing. There is no serious loss of ductility also. In this process the heating and soaking is same as in full annealing but part is allowed to cool in air so that cooling are much faster. The finer grains are produced because there is lesser time available for them to grow. The finer grain structure increases the yield and ultimate strengths, hardness and impact strength. The ductility is, however, slightly reduced. The effect of small grain size is more noticeable in impact strength than in tensile strength at low carbon content, say 0.2%. The tensile strength improves only by 5% while impact strength may improve by as much as 20% by normalising than by annealing. At higher carbon content, say 0.4% C it is only the impact strength but also the tensile strength that improves marked by about 15%. Normalising often applied to castings and forgings is stress relieving process. It increases strength of medium carbon steel to some extent. When applied to low carbon steel it improves machinability. Alloy steels in which the austenite is very stable can be normalised to produce hard martensitic structure. Cooling in air produces high rate of cooling which can decompose the austenitic structures in such steels and martensite is produced. This increases the hardness to a great extent. Table 2.7 describes the hardness obtainable in various normalised steels. Table 2.8 describes variation of annealing and normalising temperature and resulting hardness for different carbon contents of plain carbon steels.
Table 2.7 : Hardness of Annealed and Normalised Carbon Steels
Conditions Commercial Iron Annealed Normalised 80 – 100 90 – 100 Hardness BHN Structural Steel Low C 125 140 Medium C 160 190 High C 185 230 220 270 Total Steel

Table 2.8 : Variation of Annealing and Normalising Temperature and Resulting Hardness for Different Carbon Contents
Sl. No. C% Annealing Temperature o C 860 – 900 Normalising Temperature o C 900 – 925 Hardness after Annealing (BHN) 110 – 149 Hardness after Normalising (BHN) 120 – 160

1

0.18 – 0.22

65

Engineering Materials

2 3 4 5 6

0.23 – 0.28 0.29 – 0.38 0.39 – 0.55 0.56 – 0.80 0.81 – 0.99

850 – 890 840 – 880 820 – 870 790 – 840 790 – 830

890 – 910 880 – 900 840 – 870 810 – 840 810 – 840

130 – 180 140 – 206 150 – 217 160 – 230 170 – 230

140 – 190 150 – 220 180 – 230 210 – 270 260 – 300

2.6.3 Hardening
If a steel part is heated 30-50oC above A3 temperature complete austenising is permitted by soaking at that temperature and then cooled suddenly (quenched), the breakdown of austenite is suppressed. The new phase that forms is martensite in which all the dissolved carbon is held in form of body centered tetragonal structure shown in Figure 2.21. Martensite is only metastable phase and may be tempering. It is extremely hard and brittle and has a characteristic acicular appearance when examined under microscope under high magnification. In the steels upto eutectoid composition, the martensite formed by this drastic quenching operation contains all the carbon that was contained in austenite. In higher eutectoid steels, however, some carbon is converted into carbide particles also.
Fe Atom c C - Atom

a a

Figure 2.21 : The Unit Cell of Distorted Body-centered Tetragonal Lattice of Martensite

The hardness of martensite is dependent upon the percentage of carbon present in structure. Figure 2.22 illustrates how this hardness varies with carbon content. It can be seen that hardness of plain carbon steels increases rapidly until the eutectoid composition is reached. After this composition the hardness increases very normally. The fact is that the hardest martensite is formed at eutectoid composition while hardness remains at this level in hypereutectoid steels. The slight increase in hardness of hypereutectoid steel is due to formation of carbide particles which are hard and brittle.
800 Brinell Hardness Hardness as Quenched

600 % Hardness Increase After Quenching 400 Hardness as Annealed

200 0 0.0 0.2

0.4

0.6

0.8 % Carbon

1.0

1.2

1.4

1.6

1.8

Figure 2.22 : Variation of Hardness of Martensite with Carbon Content of Steel

66

2.6.4 Cooling Rate and Quenching Media
Rate of cooling plays an important role in determining the final structure after quenching in case of eutectoid steel. Figure 2.23 schematically illustrates four different structures that are obtained cooling rates when austenised steel is cooled in four different media.
Austenite Austenite and Cementite Ferrite

Engineering Alloys (Ferrous and Non-Ferrous)

Ferrite and

Cementite

Heat 0.83%

Furnace Oil Quench Cool Air Cool Water Quench

Eutectoid

Martensite (Black) Cementite (White)

Very Fine Pearlite

Fine Pearlite

Coarse Pearlite

Figure 2.23 : Mircostructure Resulting from Different Cooling Rates Applied to Austenitised Samples of Eutectoid Steel

Only the drastic water quench produces a fully martensite structure. When quenched in oil the austensite transforms into very fine pearlite. Fine pearlite also results if the austenised eutectoid steel is air-cooled. However, if allowed to cool in furnace coarse pearlite is formed. These effects have already been considered under the headings of annealing and normalising. However, it may be mentioned that very fine pearlite structure that is obtained form oil quenching is named primary troostite. Table 2.9 describes the effects of various cooling media on mechanical properties of eutectoid steels.
Table 2.9 : Mechanical Properties of Eutectoid Steels after Cooling in Different Quenching Media
Cooling Media Water Oil Air Furnace Structure Martensite Troostite Fine pearlite Coarse pearlite UTS (N/mm2 ) 1700 1100 850 520 Y. S. (N/mm2) − 550 270 140 Hardness (Rc) 65 35 25 15 Elongation % (50 mm g. L) Low 5 8 12

Water is the cheapest quench media for plain carbon steels. However, while using water care must be exercised that water is properly agitated during this treatment otherwise air bubbles may be trapped on the surface and insulate the spots from which heat flow will be delayed. Such spots will develop softness. Salt water or brine is more severe quenching media as it removes heat faster. However, in this case the steel must be thoroughly cleaned after quenching otherwise the surface begins to rust. For very low carbon steels hydroxide solutions are often used instead of brine. If slower cooling rates are desired, the steel may be quenched in oil with high flash points. Various grades of quenching oil are available. High carbon steels are invariably quenched in oil since water quenching will develop cracks in such steels.

2.7 HARDENABILITY OF STEEL
When a piece of metal is quenched its loss of heat is determined by several factors but ore face can be easily understood that the loss will not be uniform from total volume. That part of the metal which is in direct contact with quenching media will lost heat faster than

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Engineering Materials

the inner side. This will bring thickness or diameter of the part into focus to play an important role in hardening. Figure 2.24 shows the cooling curves for surface, for material just below the surface and the core. The cooling rate inside the material is governed by thermal conductivity of the steel. It is also the function of the thermal gradient existing within the piece to be hardened. There is always a possibility that at the inner core the rate of cooling is less than the critical rate resulting into unhardened material there.
Hardened Region

Temperature

Temperature

Temperature

Cooling Curves

Figure 2.24 : Depth of Hardening in Steel

The depth of hardened layer is the measure of hardenability. A good hardenability will mean even thicker sections are uniformly hardened. On the other hand a poor hardenability will produce a soft core inside the piece of steel. The hardenability is dependent upon following factors : (a) The steel composition plays an important role. The steels with high carbon content harden to a greater depth than steels with lower carbon for the former have lower critical cooling rate. Alloying element such as chromium improve the hardenability. The quenching medium. The section dimensions.

(b) (c)

The standards describes the steel hardenability by stating the ruling section which is maximum thickness which can be used and still achieve the stated properties throughout the section. Size of thickness also plays an important role in terms of distortion that may occur in the material. Variations in the rate of cooling within the component can lead to different amount of contraction at different points across the section. This differential contraction will result into the distortion of the component.

2.7.1 Measurement of Hardenability
Two testing methods have been developed to measure the hardenability of steels. The first, known as cylinder series test, gives a single value of hardenability. The value is stated in terms of percentage of martensite at the centre when quenched in a certain manner. The second, known as Jominy and quench test results into a curve.

2.7.2 Cylinder Series Test
A series of round bars of different diameters are austenised and quenched in oil or water. The bars are long enough so that the cooling of section at the middle of the length is not affected by the ends. After hardening each bar is cut into half and hardness measured at various points along a diameter. The graph between hardness and distance from the centre is then prepared (Figure 2.25). From this graph the diameter at which 50% of the structure is martensite is determined. When this graph the diameter is plotted against bar diameter, it becomes possible to determine the bar diameter in which 50% martensite would form at the centre. This is called critical diameter for that quenching medium. Since the rate of cooling is less for an oil quench than for water quench, the critical diameter, of any steel

68

will be less for oil quenching than for two water quenching. Severity of quench is an index that quantitatively defines the quenching condition. This index denoted by H is defined as following ratio.
H = Heat transfer coefficient between steel and fluid Thermal conductivity of steel
25 mm 700 600 D.P.N. 500 400 300 75 mm 125 mm D.P.N. 25 mm 700 600 500 400 300 75 mm 125 mm

Engineering Alloys (Ferrous and Non-Ferrous)

200 50 25 0 25 50

200 50 25 0 25 50

Figure 2.25 : Variation of Hardness with Depth in Water-quenched Cylindrical Bars of (a) Plain Carbon Steel, (b) 1% Cr-V Alloy Steel

Naturally, when H → ∞ it represents the severest condition of quench, meaning that surface of steel immediately reaches the temperature of the quenching medium. The critical diameter for such an ideal and unrealisable condition is called ideal critical diameter. For infinite H-value the critical diameter and ideal critical diameter will be same. For other H values the critical diameter will be smaller. Figure 2.26 shows the relationship between critical and ideal critical diameters for various H-values. Table 2.10 describes relative values of H that can be obtained in various quench media under different condition with value of one for still water as base.
150 Critical Diameter (mm) 125 0.3 100 0.1 75 0.02 50 25 0 0 25 50 70 100 125 150 Ideal Critical Diameter (mm) 175 200 Quench Severity ∞ 32 1

Figure 2.26 : Relation between Critical Diameter, Ideal Critical Diameter and Severity of Quench

Table 2.10 : Relative Quench Severities
Agitation of Quenching Medium None None None Violentor spray Movement of Pieces None Moderate Violent − Air 0.02 − − − Severity of Quench Oil Water 0.3 0.4 – 0.6 0.6 – 0.8 1.0 – 1.7 1.0 1.5 – 3.0 3.0 – 6.0 6.0 – 12.0 Brine 2.2 − 7.5 −

2.7.3 Jominy Test
More convenient laboratory test for hardenability is Jominy test. A standard specimen of steel (Figure 2.27) is austenised in normal manner. The lower end of the specimen is then quenched by a standard jet of water, resulting into a varying rate of cooling.
Thin End Cools Slowly

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Engineering Materials

Figure 2.27 : The Jominy Test of Hardenability

The rate of cooling at the jet end is about 300oC/sec while that at the other end is about 3oC/sec. This varying cooling rate produces a wised range of hardness along the length of Jominy specimen. A flat portion is ground along the length and hardness measured at various points. The plot of hardness along the length gives Jominy index of hardenability. The best use of the Jominy curve (Figure 2.28) is made by drawing a horizontal line corresponding to hardness of the semi-martensite zone. The hardness of semi-martensite zone is described in Table 2.11. The point where this line needs the Jominy curve determines the distance from the quenched end which can be inserted in Figure 2.29 to determine the diameter for a particular steel which will be fully hardened in water or oil.
336 105 55 42 28 16.5 13.5 10

60 50 Hardness, RC 40 30

b

a

20 10 0 1.5 3 4.5 6 7.5 9 10.5 12 13.5 15 16.5 18 19.5

Distance From Quenched End (mm)

Figure 2.28 : Hardenability Curves Plotted from End Quench Test Data (a) For Shallow; and (b) For Deep Hardening Steel
140 120 Diameter of Workpiece (mm)

100 Water 80 Oil 60 40 20

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Engineering Alloys (Ferrous and Non-Ferrous)

Figure 2.29 : Determining the Diameter of Fully Hardened Articles according to the Distance from the Quenched End

Table 2.11 : Relationship between the Hardness of the Semi-Martensite Zone and the Carbon Content
Carbon-content % 0.08 – 0.17 0.13 – 0.22 0.23 – 0.27 0.28 – 0.32 0.33 – 0.42 0.43 – 0.52 0.53 – 0.62 Hardness of the Semi-martensite ( Rc) Carbon Steel − 25 30 35 40 44 50 Alloy Steel 25 30 35 40 45 50 55

2.8 TEMPERING
The hardening treatment given to steel increases the hardness but introduces internal stresses because of different cooling rates. The internal stresses are also created because of transformation from austenite to martensite. Tempering treatment aims at reducing these stresses. The treatment consists in heating the hardened component to between 200oC and 600oC and holding it at that temperature for a predetermined period of time and then cooling slowly to room temperature. Since martensite itself is metastable phase, structural changes induced by tempering proceed fairly rapidly. All structures resulting from tempering are termed martensite. The changes occurring during various temperature ranges are described below :
100o – 220oC

Very little change occurs in the micro-structure. However, this heating helps remove considerable amount of internal stresses. The stress relieving treatment is given when maximum hardness is desirable and brittleness is not a problem. The strain is relieved because of removal of carbon atoms from their trapped positions.
240o – 400oC

In this range martensite decomposes rapidly into emulsified form of pearlite known as secondary troostite. This material is very fine in nature and hence provides good shock resistance. The fine edge tools are tempered in this range but more precisely within 270oC-300oC.
400o – 550oC

The precipitate troostite begins to coalesce forming a coarser from of globular pearlite known as sorbite. It may be recalled both troostite and sorbite are now preferably called tempered martensite. This treatment is desirable in such components as beams, springs and axles.
600o – 700oC

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Engineering Materials

Heating hardened steel in this range causes spheroidisation, the structure being known as spheroidite. This structure is formed because of further coalescence of the carbide within the alloy. Spheroidised steels show fairly good machinability since the hard carbide particles are embedded in the soft ferrite matrix and consequently do not have to be cut by the cutting tool. If the spheroidised steel is heated to just above its lower critical temperature the pearlite present will alter to austenite and cooling to room temperature will yield a structure of lamellar pearlite plus proeutectoid ferrite or cementite depending upon carbon content. Judging the temperature of tempering by colour appearance is a tradition which is helpful on shop floors. However, for accuracy the exact temperature measurement are to be preferred. Table 2.12 describes the colour appearance and temperature in connection with several tools.
Table 2.12 : Tempering Temperature and Colours of Tools
Tool Planning tools Milling currents Taps and dies Punches, drill bits Press tools Cold chisels Wood saws, springs Temperature oC 230 240 250 260 270 280 300 Colour Paste straw Dark straw Brown Purplish-brown Purple Dark Purple Blue

Changes in Mechanical Properties with Tempering

Tempering improves the ductility and toughness of quenched steel while decreases hardness. Figure 2.30 illustrates how these properties are influenced by tempering. The tempering temperature is so chosen that it results in the desired combination of the properties. Some steel show drop in impact values in the certain tempering temperatures range. Third drop is an indication of brittleness and such range should be avoided. For mild steel this brittleness (termed blue brittleness) occurs at a tempering temperature of about 300oC.
Izod 1300 1150 Izod, N – m, Brinell Hardness 2 Stress, N/mm , Ductility, % 1000 850 700 550 400 250 U.T.S Y.P. 0.1 % P.S. Elong BHN Reduction Of Area

0

100

200

300

400

500
0

600

700

Tempering Temperature ( C)

Figure 2.30 : Tempering Diagram of Property Chart for Water Quenched 26 mm Diameter Bars of Eu-12 Steel (Scale for Stress Only)

2.9 SPECIAL TREATMENTS
72 In cases of large sections where the water quenching will most likely produces cracks special treatments are used for hardening. The cracks on the surface are produced because

the skin cools faster and changes into martensite while the inner core cools slowly and transforms later accompanied by dilation. This dilation causes outer skin to crack. To avoid this type of cracking special treatments have been developed. Before they are described it will be worth-while to revise Section 2.8 wherein the isothermal transformation was described.

Engineering Alloys (Ferrous and Non-Ferrous)

2.9.1 Isothermal Transformation
Austenite is not usually converted into martensite instantaneously but the process continues for sometime. Different steels take different time for full transformation and the time depends upon the temperatures from where cooling is begun. How to obtain isothermal transformation diagram was discussed in detail in Section 2.8. The selected specimen is austenised and then quenched in liquid bath held at temperature to be investigated. The specimen is held for a different length of time in the bath and then quenched in the water. The resulting structure can be studied under microscope or any other associated property like hardness may be studied. It is seen that definite times are required for the initiation and completion of the transformation and these times vary with the temperature. The progress of transformation, say for 10%, 50% or 90% may also be found. Figure 2.31 which is Figure 2.30, reproduced with more details, illustrates complete transformation diagram of eutectoid steel. As the transformation temperature is lowered from A1 temperature to about 550oC, the nucleation and completion time decreases and pearlte and lamellae become finer. From about 550oC to 250oC, the nucleation and completion time increase. The transformation product here is termed bainie which is composed of two equilibrium phases that are ferrite and cementite. The time of minimum nucleation is identified as the nose or knee. Below about 250oC the transformation product is martensite. This forms almost instantaneously, but the amount formed depends upon the temperature. The upper and lower limits of martensite transformation temperatures are termed MS and MF temperatures. These diagrams are also known as Time-Temperature-Transformation or TTT diagrams and were discussed earlier. They are also referred to as S-curve because of their shape.
Start of Transformation 700 Austenite Austenite + Pearlite Course Pearlite Finish of Transformation 500 50 % Transformation 400 Upper Bainite Lower Bainite Instantaneous Transformation to Martensite MS 200 100 Mf 0 1 10
1

Transformation Temperature ( C)

600

0

Fine Pearlite

300

0 50% 100 %

Martensite + Bainite Martensite + Austenite Martensite 10
3

10

2

10

4

10

5

Time (sec)

Figure 2.31 : Isothermal Transformation Diagram for a Eutectoid Steel. Structures Present after 105 Seconds are given on the Right-Hand Side

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Engineering Materials

2.9.2 Austempering
The component to be hardened is first austenised and then quenched into a lead or salt bath held at just above the martensite transformation temperature. The component is held in the bath until the bainite transformation is completed. It is then removed from the bath and cooled in air to room temperature. The bainite so produced is somewhat softer than martensite of same carbon content and distortion is minimum. Also the austempered steel has improved shock resistance and low notch sensitivity. The process of austempering is depicted in Figure 2.32. Austempering is often limited to section thickness of 20 mm. Austempering is applicable to a few plain carbon steel and requires facility of molten salt bath. This may be regarded a disadvantages over quenching and tempering.
Centre Surface MS M Time (Log Scale) Centre Surface

Figure 2.32 : Austempering Shown on the TTT Curve

2.9.3 Martempering
The piece to be hardened is fully austenised and then quenched into a lead of salt bath held at a temperature just above the at which martensite would begin to form. It is kept at this temperature until its temperature becomes uniform throughout (i.e. outside and inside temperatures do not remain different) and is then water quenched to form complete martensite structure and bainite formation is prevented. This process successfully separates the cooling contraction from the austenite-martensite expansions and thus prevents quench cracking in large articles. The process of martempering is shown in Figure 2.33.
Temperature

Temperature

Time (Log Scale)

Figure 2.33 : Martempering Shown on the TTT Curve

The steel can be tempered to low temperatures to further refine the structure. Table 2.13 describes a few properties obtained from quenchtemper, austemper and martemper treatments.
Table 2.13 : Some Mechanical Properties of 0.95 C, 0.40 Mn Steel at 20oC after Different Treatment
Heat Treatment Water quench, Temper Water quench, Temper Martemper, Temper Martemper, Temper Austemper Hardness (R c ) 53.0 52.5 53.0 52.8 52.0 Impact (J) 8.22 9.60 19.20 16.44 30.83 Elongation % (on 25 min gl) 0 0 0 0 11

74

Austemper

52.5

27.40

8

Engineering Alloys (Ferrous and Non-Ferrous)

2.10 SURFACE HARDENING
In many situations surface hardening instead of through hardening only is sufficient to serve the purpose. Gears are examples. Surface hardening is achieved through case carburising, nitriding or induction heating. Steels containing 0.1 to 0.25% C are best suited for case carburising. Good combination of tough core (lesser hardness) and high surface hardness is achieved by case carburising of nickel steel. Case hardness of 60 RC with a core hardness of 33 to 38 RC gives best results in case of gears. The case hardness is due to residual compressive stress introduced on the surface by penetration of C and N2. Surface hardening is classified into two types : (a) Without addition of any element from outside but only transforming outer layer to martensite. This could be achieved by heating the surface by gas flame or causing magnetic induction so that complete austenite transformation occurs on surface. On quenching martensite and retained austenite form on surface while on the inner side peralite-ferrite is the main phase. The second method is called case hardening in which C and/or N2 are introduced in the surface layer. In carburising the part is surrounded by material or atmosphere rich in C and on heating this C is released and absorbed in steel. Recently case carburising is more effectively performed by heating steel part in the atmosphere or natural gas, coke oven gas, butane or propane or the valatised form of liquid hydrocarbons like terpenes and benzene. Volatilised form of alcohol and glycols or ketones are also used. In these cases the thickness of hardened layer is proportional to root of the time of treatment in hour.

(b)

Liquid carburising consists in dipping the part in fused mixture of chlorides, carbonates and cyanides. Baths maintained at 840oC to 900oC produce a case depth of 0.075 to 0.75 mm. 0.5 to 3.0 mm case depth is attainable if bath is maintained at 900 to 950oC. Plain carbon steel and low alloy steel can be carburised in liquid bath. Nitriding of steel surface is the absorption of N2 in the surface. Nascent N2 for this purpose is obtained from ammonia. Molten cyanide (sodium cyanide) bath maintained at 560oC is quite effective in nitriding particularly if thin case is desired. Plain C steel are not good for nitriding because iron nitride so formed is very brittle. Steels alloyed with Al and Cr and Ni, Cu, Si and Mn are better nitrided than plain C steel. Carbonitiriding, nitrocarburising or gas cyaninding is a process similar to gas carburising in which ammonia is also added to carburising atmosphere. This process produces better hardened case than carburising.

2.11 HEAT TREATING EQUIPMENT
The main equipment for heat treatment is furnace. There are two major types of furnaces – batch and continuous. The furnace selection requires consideration of several points particularly for the reason that they consume a good amount of energy. The efficient use of energy in furnaces and hence their design and use of proper insulation need consideration. The man power requirement for operation, the initial and cost and convenience of maintenance and repair are other important considerations in selecting furnace for heat treatment. Since temperature control and temperature cycle control are very important in heat treatment, electronic and computer controlled furnace are taking precedence over older type. 75

Engineering Materials

Heating in furnace is done by burning oil or gas or by electrical resistance or inductance heating. Gas or oil furnace have distinct disadvantage over electrical furnaces because the former often introduces products of combustion in the heating space thus affecting the part to be treated. The electrical heating, on the other hand, has slower start-up and is not easy to control.

2.11.1 Batch Furnaces
An insulated, chamber for placing the job, the heating system and a door or several doors for placing the job in place are the requirement of these furnaces. The parts to be heat treated are loaded and unloaded in individual batches. A furnace which is easy to use, simple to construct in several sizes and having versatility to accommodate several size is a box furnace which could be horizontal rectangular. Many times a flat platform on wheels is used to carry the parts in the furnace. A pit furnace is made in form of a vertical pit below the ground level. The parts to be treated are lowered in the pit. Long parts like rods, bars, tuning, shafts, etc. can be suspended in the space of the pit furnace. These parts are susceptible to distortion if placed in horizontal position in the box furnace. A bell furnace does not have bottom and is lowered on the stack of parts to be treated. The furnace chamber could be round or rectangular. In elevator furnace the parts to be treated are placed on the rolling platform which is rolled in the proper position and lifted to the heating chamber of the furnace. By placing a quenching tank directly below the surface the savings on space and quenching time are made.

2.11.2 Continuous Furnace
Parts to be treated are placed on some sort of conveyor which move into the furnaces according to programmed heating cooling cycle. The time for loading-unloading is greatly saved and handling of job is reduced. For high production rate and better control of heat treating cycle these furnaces are most suitable.

2.11.3 Salt Bath Furnaces
Salts baths used for heating ensure good control uniform temperature and high heating rates are compared to air or gas. The molten salts or metals have higher conductivity than air or gas. The salt may be heated from outside if it is non-conducting or by passing a low voltage alternating current between electrodes placed in the salt. Direct current is not used because it is likely to cause electrolysis of salt. Among metals, lead is commonly used. Wide range of temperature may be obtained from such baths.

2.11.4 Induction Heating
Alternating current through induction coil surrounding the part to be treated induces eddy current through the part which is thus heated. The advantage of such heating is that no gas or liquid source for heating is used and coil can be shaped to surround the part geometrically. Figure 2.34 shows example of induction coils, which normally are made in copper or copper based alloy which are water cooled. The coil is also designed for quenching the part after heating. For surface hardening requiring local heating induction heating is suitable.
Induction Coils

Cooling Water

Shaped Coils Travel

76
Slideway to be Surface Hardened Parts to be

Engineering Alloys (Ferrous and Non-Ferrous)

Figure 2.34 : Coils for Induction Heating

Furnace Atmospheres

If the heating is not through salt bath, then the job to be treated is subject to varying atmosphere which could be atmospheric air or any one of several gases. Surface oxidation, tarnishing and decarburisation are the problems which the metals face. Oxygen can cause oxidation, rusting and scaling. Carbon dioxide may cause decarburisation depending upon its concentration in furnace atmosphere. Thin blue film, is formed on the surface in the presence of water vapour. Bluing of surface is done for improving surface appearance. Nitrogen provides a neutral atmosphere. Vacuum furnaces often used for small and accurately finished parts provide complete safety from effects of atmosphere.

SAQ 4
(a) (b) (c) (d) (e) (f) (g) What are different heat treatment given to steel? Differentiate between annealing and process annealing. What is quenching? Why should quenched steel be tempered? Do you think the term martempering is misnomer? Suggest a better term. Differentiate between austempering and martempering. Describe different methods of surface hardening. Give examples of surface hardened parts. Describe how heating is done for heat treatment.

2.12 ALLOY STEELS
Carbon steels in their commercial forms always contain certain amounts of other elements. Many of these elements enter the steel from the ores and it is difficult to remove them during the process of steel making. All commercial steels contain varying amounts of Mn, Si, S and P and frequently varying amounts of such elements in Cr, Ni, Mo and V. If alloying elements other than carbon are present only in small amounts (e.g. Mn upto 0.8%, Si upto 0.3%, etc.) then the steel is usually called low alloy steel or plain carbon steel. Sulphur and phosphorus when more than 0.05% of either is present, tend to make steel brittle, so that during steel making these elements are reduced to at least this value. Si has little effect on strength and ductility if less than 0.2% is present. As the content is rasied to 0.4% the strength is raised without effecting ductility, but above 0.4% Si, the ductility is impaired. Si is added as deoxidiser and that part which does not make silicon dioxide remains in steel as impurity. Mn is another alloying element which is present in most steels. If it exists in solid solutin in the ferrite it has a strengthening effect. It may also exist in forms of Mn3C which forms part of the pearlite of MnS. Upto 1% of Mn has strengthening effects on steel and its 77

Engineering Materials

presence in excess of 1.5% induces brittleness in steel. Excess Mn is added to melt during steel making to bring its level to desired value. It also acts as a deoxidiser. Intentional addition of many other elements modifies the structure of steel and hence improves its properties. Steels to which such intentional additions have been made (including those steel which contain Mn in excess of 1% or Si in excess of 0.3%) are known as alloy steels. One particular effect of alloying is that it enables martensite to be produced with low rates of cooling and permits larger sections to be hardened than is possible with plain carbon steel. The important elements that are used to alloy with steel in varying quantities are Ni, Cr, Mo, W, Mn and Si. The bcc metals like Cr, W and Mo when alloyed with steel tend to form carbides which reduce the proportion of Fe3C in the structure. On the other hand the fcc elements like Ni, Al, Cu and Zr do not form carbides. Mn which has three allotropic complex structures also forms carbide. Several advantages in terms of improved mechanical properties and corrosion resistance are obtained by adding one or several alloying elements. The various advantages of alloy steel are : (a) (b) (c) Higher hardness, strength and toughness on surface and over bigger cross-section. Better hardenability and retention of hardness at higher temperature (good for creep and cutting tools). Higher resistance against corrosion and oxidation.

The alloying elements affect the properties of plain C steel in four ways : (a) By strengthening ferrite while forming a solid solution. The strengthening effects of various alloying elements are in this order : Cr, W, V, Mo, Ni, Mn and Si. By forming carbides which are harder and stronger. Carbides of Cr and V are hardest and strongest against wear particularly during tempering. High alloy tool steel use this effect. Ni and Mn lower the austenite formation temperature while other alloying elements raise this temperature. Most elements shift eutectoid composition to lower C percentage. Most elements shift the isothermal transformation curve (TTT) to lower temperature, thus lowering the critical cooling rate. Mn, Ni, Cr and Mo are prominently effective in this respect.

(b)

(c)

(d)

2.12.1 Effect of Individual Alloying Elements
Sulfur

Sulfur is not a desirable element in steel because in interferes with hot rolling and forging resulting in hot-shortness or hot embrittlement. Sulfur however, is helpful in developing free cutting nature. Thus sulfur upto 0.33% is added in free cutting steel. Otherwise, sulfur is restricted to 0.05% in open hearth or BOF steel and to 0.025% in electric furnace steel.
Phosphorous

It produces cold shortness which reduces impact strength at low temperature. So its percentage is generally restricted to level of sulfur. It is helpful in free cutting steels and is added upto 0.12%. It also improves resistance to corrosion.
Silicon

78

Silicon is present in all steels but is added upto 5% in steels used as laminates in transformers, motors and generators. For providing toughness it is an important constituent in steel used for spring, chisels and punches. It has a good effect in steel that it combines with free O2 and form SiO2 and increases strength and soundness of steel casting (upto 0.5%).
Manganese

Engineering Alloys (Ferrous and Non-Ferrous)

1.2 to 1.4% of Mn produces extremely tough, wear resistant and non-magnetic steel called Hadfield steel. It is important ingredient of free cutting steel upto 1.6%. Mn combines with S, forming MnS. For this purpose Mn must be 3 to 8 times the S. Mn is effective in increasing hardness and hardenability.
Nickel

It is good in increasing hardness, strength and toughness while maintaining ductility. 0.5% of Ni is good for parts subjected to impact loads at room and very low temperatures. Higher amounts of Ni help improve the corrosion resistance in presence of Cr as in stainless steel. Nickel in steel results in good mechanical properties after annealing and normalising and hence large forgings, castings and structural parts are made in Ni-steel.
Chromium

Chromium is common alloying element in tool steels, stainless steel, corrosion resistant steel (4% Cr). It forms carbide and generally improves hardness, wear and oxidation resistant at elevated temperature. It improves hardeanbility of thicker sections.
Molybdenum

Molybdenum is commonly present in high speed tool steel, carburising steel and heat resisting steel. It forms carbide having high wear resistance and retaining strength at high temperatures. Mo generally increases hardeability and helps improve the effects of other alloying elements like Mn, Ni and Cr.
Tungsten

It is important ingredient of tool steel and heat resisting steel and generally has same effects as Mo but 2 to 3% W has same effect as 1% Mo.
Vanadium

Like Mo, V has inhibiting influence on grain growth at high temperature. V carbide possesses highest hardness and water resistance. It improves fatigue resistance. It is important constituent of tool steel and may be added to carburising steel. Hardeability is markedly increased due to V.
Titanium

Addition of Ti in stainless steel does not permit precipitation of Cr carbide since Ti is stronger carbide former and fixes are carbon.
Cobalt

It imparts magnetic property to high C steel. In the presence of Cr, Co does not permit scale formation at high temperature by increasing corrosion resistance.
Copper

Atmospheric corrosion resistance of steel is increased by addition of 0.1 to 0.6% copper.
Aluminium

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Engineering Materials

Aluminium in percentage of 1 to 3 in nitriding steels is added to improve the hardness by way of forming Al nitride. 0.01 to 0.06% Al added during solidification produces fine grained steel castings.
Boron

Very small percentage (like 0.001 to 0.005) of B is effective in increasing hardness, particularly in surface hardening boriding treatment.
Lead

Less than 0.35% Pb improves machinability. The effects of alloying element in respect of various desired effects are summarized below : (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) Hardenability – Si, Mn, Ni, Cr, Mo, W, B Toughness – Si, Ni High temperature strength – Cr, Mo, W Corrosion resistance – Cr, Mo, W Wear resistance – Cr, Mo, W, V Low temperature impact strength – Ni Atmospheric corrosion resistance – Cu Machinability – S, P, Pb Fatigue strength – V Surface hardening – Al

2.12.2 Some Important Alloy Steels
Structural Steels

Low alloy steels are used for structural purposes. Such steels are required to possess high yield stress, good ductility and high fatigue resistance. The high yield stresses result in direct weight saving in part of the structure. A typical low alloy structural steel will have following composition : C – 0.12%, Mn – 0.75%m Si – 0.25%, Cu – 0.3% This alloy steel has a yield strength of 350 N/mm2 and about 15% elongation after hot rolling. The presence of Cu improves corrosion resistance while Mn and Si improve weldability by preventing weld embrittlement. Small amounts of Ni, Cr and V added to these steels may improve the yield strength to 625 N/mm2 if used in quenched and tempered conditions. Additions of Ni, Cr and V do not effect the weldability.
Stainless Steels

Stainless steel are particularly known for their resistance to corrosion. This resistance is obtained because of formation of protective oxide layer which spreads all over the surface. This layer does not allow the surrounding atmosphere to further react with the steel which retains its luster and appearance. The oxide layer on the stainless steel surface is formed by the oxide of Cr when it is present in large proportions. This oxide film is impervious to both metal ions and atmospheric oxygen. Improved corrosion resistance is obtained with increasing percentage of Cr, provided that Cr is in solid solution and not combined as carbide. The corrosion resistance is further enhanced by addition of certain amounts of nickel. According to structures obtainable at room temperatures the stainless steels are subdivided into three groups. 80

Ferrite stainless steels contain only chromium as alloying element in addition to small percentage of carbon. The carbon varies between 0.05% to 0.15%, while Cr varies between 13% to 30%. This alloy contains only α-phase at all temperatures. Some Cr precipitates in form of carbides along with ferritic grains at room temperature. This alloy is very ductile and used where outstanding formability in complicated shapes is required. Many deep drawn objects are produced from ferritic stainless steel. This material possesses excellent resistance to corrosion. When alloy steel contain at least 24% Cr and Ni together but not less than 8% of either element, the γ-phase is retained on cooling at normal rates. At very low cooling rates the α-phase may separate fully. Austenitc phase is obtained when quenched from upper critical temperature. The commonest of these steels contain 18% Cr, 8% Ni and 0.1% C. It is called 18 : 8 steel. Austenitic steel is used for construction of chemical plants, decorative purposes and household utensils. Neither of above two groups is heat-treatable. If steel contains Cr and Ni in such proportions that it has a γ-phase at high temperature and an α-phase on cooling at normal rates, it can be quenched to give a martensitic structure. Such heat-treatable steels are known as martensitic steels even when not in heat treated conditions. For developing martensitic these steels are oil-quenched from above upper-critical temperature. Three types of martensite steels are available commercially. These are : (a) (b) (c) 0.07% − 0.1%, C 13% Cr, 0.2% − 0.4% C, 13% Cr, and 0.1% C, 18% Cr, 2% Ni.

Engineering Alloys (Ferrous and Non-Ferrous)

These steels are used for turbine blades, surgical instruments, springs, ball bearings, pump shafts, aircraft fittings, etc. While martensitic steel can be heat treated to obtain high strengths, the strength of ferritic and austenitic steel can be improved only by mechanical working. Various precipitation hardening stainless steels have also been developed. At high temperatures somewhere between 500 and 700oC, the stainless steels lose their resistance to corrosion. This happens mainly because the chromium has a tendency to separate from solid solution and precipitate in form of carbides at grain boundaries. This makes welding of the stainless steels difficult and causes what is known as weld decay. If welded part is reheated to a temperature of about 9001000oC the carbides are re-dissolved and can be converted into stable solid solution on quenching.
High Carbon Tool Steels

Tools are implements that are used to shape, deform or cut other materials. They are largely made in steel, though other alloys have also been developed. The common tool steels contain C, W, Cr, Mo, V, Mn, Si in the range of 0.6 to 1.0%. They have hardness and wear resistance. For shock resistance C is restricted to 0.5%. W and Mo between 2 to 18% provide high temperature strength. V between 0.1 to 2% enhances hardenability while Si adds to toughness. Though the tool and die steels are not produced in as large amount as other steels are, yet they are industrially very important. A variety of steel differing widely in composition and treatment is used for varying purposes. They are used in such operations as cutting, shearing, forming and rolling. These operations require adequate hardness, strength, toughness, wear resistance and heat resistance. For many purposes near-eutecoid and hyper-eutectoid steels have been used for metal cutting but these plain carbon steels have tendency to loose hardness through tempering when rise in temperature occurs during cutting. To overcome this problem high steep tool steel have been developed. The 18.4.1 type of high steel

81

Engineering Materials

contains 18% W, 4% Cr and 1% V. These steels retain sufficient hardness due to carbide formation which is a complex compound Fe4W2C. A tough matrix is provided by Cr. These steel may retain hardness upto a temperature of 500oC. When 5-12% of cobalt is also added, in addition, the hardness through a secondary hardening process is increased at temperature around 600oC.
High Duty Tool and Die Steels

High duty punching tools or dies are made out of many different steels of fairly high alloying contents. High-carbon high-chromium die steel contains 2% C, 0.3% Mn and 12% Cr. Though extremely hard and unmachinable in its hardened form, small and large carbide particles get dispersed in ferritic matrix on annealing from 800oC. In annealed state this steel can be machined with difficulty. This is an important requirement because tools and dies have to be machined before they are hardened for final use.
Magnetic Alloy Steels

These steels are divided into two groups. Those which retain their magnetism and those which do not. The steels that retain their magnetism are termed hard magnetic steels. The other group is magnetically soft. 1% plain carbon steel in its fully hardened condition was the earliest permanent magnetic material. Later developments occurred when W, Cr and cobalt were added as alloying elements. The most useful of permanent magnetic steels contain high proportions of Ni, Co, and Al, with small amount of W. Alnico is a good example which contains 10% Al, 18% Ni, 12% Co, 6% Cu, Rest Fe. Magnetically soft materials are required to demagnetise quickly. In earlier days soft iron was used as a soft magnetic material but later iron-silicon alloys containing upto 4.5% Si were developed. However, modern high duty soft magnetic materials are iron-nickel alloys such as Permalloy which contains 78% Ni. Another soft magnetic material is mumetal containing 75% Ni. These alloys are used for transformer cores and as shield material for submarine cables. Alloy steels find a wide range of application and a few of them are described in Table 2.14.
Table 2.14 : Application of Alloy Steels
Sl. No. 1 2 Application Rail steel Spring steel (tension compression, torsion) Desired Properties Strength, ductility, impact and fatigue strength Good elongation high elastic limit (20 to 30%, 1200-1400 MPa). Good surface finish for fatigue strength Composition (%) C – 0.4 to 0.6 Mn and Cr – upto 1 (a) C – 0.6, Mn – 0.9, Si – 2.0 (b) C – 0.5, Mn – 0.8, Cr – 1.0, V – 0.15 (c) C – 0.5, Ni – 0.9, Cr – 0.5, Ni – 0.6, Mo – 0.2 3 Structural steel (bridges, building, cars, gears, clutches, shafts) Weldable steel for welded structures Concrete reinforcing steel High strength, toughness, high temperature strength, corrosion resistance Weldability, high resistance to atmospheric corrosion, resistance to brittle fracture Bend 90o – 180o , tor-steel with ribs for greater surface area. Elongation = 16%, UTS = 500-650 MPa, Y. S. = 35 MPa. Wide range of alloy steels containing several alloying elements C – 0.15 to 0.3 with some Cu and V C – 0.3 to 0.4, Mn – 0.5 to 0.8 C – 0.45 to 0.6, Mn – 0.7 to 1.1

4

5

82

Elongation = 13%,

UTS = 600 MPa, Y. S. = 350 MPa 6 High speed steel for cutting tools Resist temperature upto 550-600o C. Cutting tools requiring high hardness at working temperature 18 : 4 : 1 steel and 6 : 5 : 4 : 2 steel Application in pipelines upto 400-550o C. Other parts upto 550oC Resistance to abrasion and shock, high toughness strength and ductility C – 0.8, W – 18, Cr – 4, V – 1, C – 0.8, W – 6, Mo – 5, Cr – 4, V–2 Mo – 0.4 to 6 V – 0.25 to 1.0 Cr – upto 6.0, C – low C – 1 to 1.4, Si – 0.3 to 1.0, Mn – 10 to 14

Engineering Alloys (Ferrous and Non-Ferrous)

7

Creep resisting steel

8

Hadfield Mn steel, excavating and crushing m/c. rail road crossing, oil well, cement, mining industries. Used as casting and hot rolled High strength low alloy steel (HSLA) for automotive parts Ball bearing steel

9

High strength/weight ratio. Balanced properties such as toughness, fatigue strength, weldability and formability Rolling element, inner and outer races. High hardness 61-65 Rc high fatigue strength

C – 0.07 to 1.3, Ti, V, Al, Co less than 0.5

10

C – 0.9 to 1.1, Cr – 0.6 to 1.6, Mn – 0.2 to 0.4

2.12.3 Heat Treatment of Stainless Steels
Stainless steels, like other steels, respond to heat treatments over wide range. They are subjected to one or more of following heat treatments.
Stress Relief

Stress relieving treatment removes undesirable residual tensile stresses that are induced in the material as consequence of mechanical working. Stress relieving of stainless steel is carried out by heating it to 370oC and then cooling at a low rate. This treatment also reduces susceptibility to corrosion. However, resistance against corrosion cracking is improved by stress relieving at 770oC.
Annealing

Since ferritic stainless steels are prone to form patches of transformation products particularly after welding it is subjected to annealing treatment. The treatment is carried out by heating ferritic steel to 770oC and then cooling it in furnace or air. This treatment will stress relieve and homogenise the structure.
Solution Annealing

This treatment is given to austenitic stainless steel. When this steel is heated to 1000oC to 1120oC the austenite acts as a powerful solvent for chromium carbide and makes a homogenous structure. This structure is retained by quenching heated steel in water, oil or air depending upon the thickness of the section. Air is best quenching media for thin sections. The process is also referred to as quench annealing.
Hardening and Tempering

This is a treatment very much similar to one for plain carbon steel but only meant for martensitic stainless steel. The steel is heated to a temperature of 950 to 1050oC and quenched in air or oil resulting in formation of hard marteniste. The quenched steel is tempered between 100 and 700oC depending upon the required hardness.
Stabilising Treatment

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Engineering Materials

This is special treatment for stabilised grade of auetenitic steel. The steel is heated to 870 to 900oC and held for 2 to 4 hours. It is then quenched in air, oil or water which causes precipitation of titanium or columbium carbide. These carbides do not permit precipitation of chromium carbide during service life.
Post Weld Treatment

The welding induces undesirable residual stresses and reduces corrosion resistance. Stress relieving, annealing or solution annealing are the treatments recommended for weldments before putting them to use. Weld decay is a common welding defect in stainless steel which is caused by precipitation of carbide of chromium in the weld region and HAZ. The result of this precipitation is great susceptibility to intercycstalline cracking when a corrosive media comes in contacts.

2.12.4 Heat Treatment of Tool Steels
Heat treatment in case of tool steels in an important step before actually using the tool. Most properties are achieved after heat treatment. There are certain special but necessary precautions to be observed in heat treating tool steels. Such precautions may be clear understanding of austenising temperature which is affected very much by addition of alloying elements.
Normalising and Annealing

Normalising becomes necessary in case of forged tools. Forging induces undesirable residual stresses, coarse grains and non-uniform structure. These defects are removed by normalising treatment. The normalised tools are machined to exact dimensions. Such machining increases the hardness because of strain hardening. The intermediate annealing from just below A1 temperature is applied upon the tool to remove the effects of strain hardening. The tools steels have lower thermal conductivity and if the tool is heated at a faster rate distortion and cracking may occur. To avoid such distortion and cracking the tool before any treatment is preheated slowly to temperature between 600 to 800oC. The tool is held at the preheat temperature sufficiently long to allow whole body to come to uniform temperature and then heat to required temperature for any treatment.
Austenising

Following preheat the tool is austenised by heating to correct austenising temperature to obtain fine grained austenite in which carbides are dissolved.
Quenching

The austenised tool is quenched in brine, water, oil, air or molten salt bath depending upon the hardenability and thickness of cross-section. The correct rate of cooling will decide the quenching media and cooling faster than the correct rate will result in under stresses. The distortion of tools in more conveniently avoided by martempering or austempering.
Tempering and Stablisation

84

Tempering of tool steel is again an important step in heat treatment. The quenching of such steels causes the existence of untempered martensite, retained austenite and carbide in the structure. This happens because of finer changes in composition do not permit to establish correct austenising temperature. Tempering besides removing these structural deficiencies also help induce secondary hardness. Figure 2.35 shows the presence of secondary hardness manifested by increased hardness at certain tempering temperature in case of high alloy steels. Both medium and high alloy steel decrease in hardness respectively upto about 460oC and 500oC and thereafter they

increase in hardness. High alloy steels show greatest increase in hardness with increasing tempering temperature.
4 3 2

Engineering Alloys (Ferrous and Non-Ferrous)

Hardness

1

300

400

500

600
0

Tempering Temperature ( C)

Figure 2.35 : Tempering Characteristics of Tool Steels

After tempering initially the treatment is repeated two or three time in multi-tempering practice. Such treatment stabilises the microstructure by reducing the amount of retained austenite. Yet another method of stabilization is to cool the tempered steel to subzero temperature by placing it in solid carbon dioxide (– 75oC) or dipping it in liquid nitrogen (– 196oC). Normalised quenched, tempered and stabilised tool steel possesses maximum hardness, wear resistance and dimensional stability.

2.13 CAST IRON
Cast iron is an important alloy of Fe and C are largely used in industry for its convenience to casting in intricate and good mechanical properties. Steels could also be cast but process is often costlier. Figure 2.8 clearly shows the region of cast iron in equilibrium diagram. The diagram shows that there is a eutectoid at 4.3% (point C) which has solidification temperature of 1135oC. Alloys within carbon range of 2.3% to 4.2% are sometimes referred to as hypo-eutectic irons, since their carbon content is below the eutectic composition. Although, the melting points of cast irons are much higher than several non-ferrous alloys, yet they are within the reach of simple melting furnaces, several of which are commercially available. This is one reason as to why cast irons are so popular as structural material. To understand various phases that are present in a cast iron one may consider cooling of a typical Fe – C mixture from melt, say one having y % C (Figure 2.8). The first material to solidify is austenite which is a solution of carbon in fcc iron. The line PG will give the amount of C in solution in austenite during solidification. This amount is always less than the average percentage in the melt, which means carbon is rejected out of austenite while the liquid phase is enriched in carbon. The last liquid to solidify has the eutectic composition, i.e. 4.3%. The eutectic contains the austenite and carbon. If the liquid is cooled slowly maintaining near equilibrium conditions, the carbon solidified as flakes of graphite in a matrix of soft ferrite, in pearlite or in a mixture of ferrite and pearlite. If liquid is cooled rapidly then ferrite is suppressed and pearlite and cementite precipitate. Casting is a process in which molten is poured in a mould and on solidifying the casting of the shape of mould is obtained. Cast iron, as already stated, in a good material for casting. General properties of cast iron are : (a) (b) Cheap material. Lower melting point (1200oC) as compared to steel (1380-1500oC). 85

Engineering Materials

(c) (d) (e) (f) (g) (h)

Good casting properties, e.g. high fluidity, low shrinkage, sound casting, ease of production in large number. Good in compression but CI with ductility are also available. CI is machinable is most cases. Abrasion resistance is remarkably high. Very important property of CI is its damping characteristics which isolates vibration and makes it good material for foundation and housing. Alloy CI may be good against corrosion.

2.13.1 Contents of Cast Iron
Cast Iron (CI) is prepared form melting pig iron in electric furnace or in cupola furnace. Electric furnace gives better quality. CI contains different elements in addition to Fe. The carbon content of CI is more than 2%. Si varies between 0.5 to 3.0%. It is very important because it controls the form of C in CI. S content in CI varies between 0.06 to 0.12% and is largely present as FeS which tends to melt at comparatively low temperature causing hot shortness. Mn inhibits formation of FeS. Though P increases fluidity of CI – a property helpful for pouring – it has to be restricted to 0.1 to 0.3% because it reduces toughness. P is present in the form of FeP. Mn in CI varies from 0.1 to 1.0% through such a small Mn does not affect properties of CI it certainly helps improve upon hot shortness by taking care of S. Several other alloying elements like Ni, Cr, Mo, Mg, Cu and V may be added to CI to obtain several desirable properties.

2.13.2 Classification of Cast Iron
CI containing C in form of cementite is called white cast iron. Microstructure of such CI consists of pearlite, cementite and ledeburite. If C content is less than 4.3% it is hypoeutectic CI and if C is greater than 4.3% it is hypereutectic CI. White cast iron has high hardness and wear resistance and is very difficult to machine. It can be ground, though. Hardness of white CI varies between 300-500 BHN and UTS between 140-180 MPa. White CI is normally sand cast to produce such parts as pump liners, mill liners, grinding balls, etc. Cast iron containing carbon in form of graphite flakes dispersed in matrix of ferrite or pearlite is classified as gray cast iron. The name is derived from the fact that a fracture surface appears gray. Gray cast iron differs in percentage of Si from white cast iron while C percentage is almost same. The liquid alloy of suitable composition is cooled slowly in sand mould be decompose Fe3C into Fe and C out of which C is precipitated as graphite flakes. Addition of Si, Al or Ni accelerates graphitesation. The graphite flakes vary in length from 0.01 to 1.0 mm. The flakes provide an easy passage to cracks thus not allowing softer microstructure to deform plastically. Larger flakes reduce strength and ductility. The best properties of gray cast iron are obtained with flakes distributed and oriented randomly. Inoculant agents such as metallic Al, Ca, Ti, Zr and SiC and CaSi when added in small amount, cause formation of smaller graphite flakes and random distribution and orientation. Gray cast iron is basically brittle with hardness varying between 149 to 320 BHN and UTS of 150 to 400 MPa. Different properties are obtained by varying cooling rate and quantity of inoculant agents. It has excellent fluidity, high damping capacity and machinability. If gray CI is repeatedly heated in service to about 400oC suffers from permanent expansion called growth. Associated with dimensional changes are less of ductility and strength as a result of growth. When locally heated to about 550oC several times this material develops what are called fire cracks resulting into failure. 86

High strength gray cast iron is obtained by addition of strong inoculating agent like CaSi to liquid metal before casting process. UTS in the range of 250 to 400 MPa is obtained. This cast iron is called Meehanite iron and can be toughened by oil quenching treatment to a UTS of 520 MPa. If graphite in cast iron is present in form of nodules or spheroids in the matrix of pearlite or ferrite the material is called nodular cast iron. This cast iron has marked ductility giving product the advantage of steel, and process the advantage of cast iron. It is basically a gray cast iron in which C varies between 3.2 to 4.1%, Si between 1.0 to 2.8% while S and P are restricted to 0.03 to 0.1% respectively. Ni and Mg are added as alloying elements. Crank shafts, metal working rolls, punch and sheet metal dies and gears are made out of nodular CI. The defects like growth and fire cracks are not found in this class of iron. This makes it suitable for furnace doors, sand casting and steam plants. It also possesses good corrosion resistance making it useful in chemical plants, petroleum industry and marine applications. White CI containing 2.0 to 3.0% C, 0.9 to 1.65% Si, < 0.18% S and P, some Mn and < 0.01% Bi and B can be heat treated for 50 hours to several days to produce temper carbon in the matrix of ferrite or pearlite imparting malleability to CI. This class is known as malleable cast iron and can have as high as 100 MPa of UTS and 14% elongation. Due to such properties as strength, ductility, machinability and wear resistance and convenience of casting in various shapes, malleable CI is largely used for automotive parts such as crank and cam shafts, steering brackets, shaft brackets, brake carriers and also in electrical industry as switch gear parts, fittings for high and low voltage transmissions and distribution system for railway electrification. Addition of alloying elements such as Ni and Cr provide shock and impact resistance along with corrosion and heat resistance of cast iron. These are called alloyed CI 3 to 5% Ni and 1 to 3% Cr produce Ni-hard CI with hardness upto 650 BHN and modified Ni-hard CI with impact and fatigue resistance is produced by adding 4.8% Ni and 4.15% Cr. Ni-resist CI with 14 to 36% Ni and 1 to 5% Cr is alloy CI having good corrosion and heat resistance. Most castings in CI must be stress relieved at 400-500oC because CI has a property to relieve locked in stresses after sometime. CI can be annealed by heating to 800-900oC to improve machinability. Cast iron can be quenched in oil to improve hardness. Such quenching treatment is often followed by heating to 300oC and cooling slowly (Table 2.15).

Engineering Alloys (Ferrous and Non-Ferrous)

2.13.3 Heat Treatment of Cast Iron
Castings in iron are often heat treated for improving mechanical properties and microstructure. The treatments given to cast iron are described briefly here.
Stress Relieving

Internal stress in cast material is very common because every casting undergoes cooling which is non-uniform. After certain time period has passed, these internal stresses tend to be relieved almost spontaneously. Such self-relieving of stresses may cause changes in dimensions which may not be permitted for working of parts of machine. Therefore, it is imperative that cast material (parts) must be stress relieved before bringing such parts in service. Cast iron is stress relieved by heating it to a temperature in the range of 400-500oC and kept at this temperature for a few hours. The cooling is done slowly or as per the rate for a particular structure. Stress relieving of cast iron is referred to as seasoning of casting.
Annealing

Annealing of cast iron is heating it to a temperature between 800 and 900oC and cooling slowly. This process decomposes iron carbide into ferrite and graphite and machinability is improved. This may be necessary for such parts that require machining.
Quenching and Tempering

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Engineering Materials

Cast iron pearlite structure may be heated to lower critical temperature and then quenched to effect very rapid cooling. This treatment causes the precipitation of hard martensite phase and casting is thus capable of providing high wear resistance. The casting after quenching treatment may be further heated to 300oC and cooled slowly to restore original toughness.
Surface Hardening

Many applications of cast iron necessitate high surface hardness. This may be achieved by surface hardening treatments such as nitriding and induction hardening.
Table 2.15 : Typical Mechanical Properties and Applications of Cast Iron
Cast Iron Composition wt. % Condition Structure UTS MPa YS MPa − Elongation (%) − Typical Application

Ferrite

3.4 C, 2.2 Si, 0.7 Mn 3.2 C, 2.0 Si, 0.7 Mn 3.3 C, 2.2 Si, 0.7 Mn 2.2 C, 1.2 Si, 0.75 Mn 2.4 C, 1.4 Si, 0.75 Mn 2.4 C, 1.4 Si, 0.75 Mn 3.5 C, 2.2 Si 3.5 C, 2.2 Si 3.5 C, 2.2 Si,

Annealed

Ferrite matrix

180

Cylinder blocks and head clutch plates Truck and tractor cylinder blocks, gear box Diesel engine castings

Gray Cast Iron

Pearlite

As-cast

Pearlite matrix

250

−

−

Pearlite

As-cast

Pearlite matrix

290

−

−

Ferrite

Annealed

Temper carbon and ferrite Temper carbon and pearlite Tempered martenstie Ferritic Ferritic pearlite Martensitic

345

224

10

General engineering service machinability General service with dimensional tolerance High strength parts, connecting rods, yokes for universal joints Pressure casting as valve and pump bodies Crank shaft, gears and rollers Pinions, gears, rollers and slides

Melleable Cast Iron

Pearlite

Annealed

440

310

8

Martensitic

Quenched and Tempered Annealed As-cast Quenched and Tempered

620

438

2

Ferrite Pearlite Martensitic

415 550 830

275 380 620

10 6 2

Ductile Cast Iron

SAQ 5
(a) (b) (c) What is an alloy? Give the range of composition of alloying elements. State effects of following alloying elements in steel. Tungsten, nickel, chromium, vanadium and cobalt. Which elements will improve the following properties of alloy steels? Hardenability, toughness, machinability, corrosion and wear resistance, fatigue strength. (d) (e) (f) What is stainless steel? Mention those properties which distinguish stainless steel from plain carbon steel. Describe heat treatments for tool steel. Classify cast iron. What are Ni-hard and Ni-resist cast irons?

2.14 NON-FERROUS MATERIALS
Modern technology has been highly dependent upon non-ferrous and alloys for in certain cases they present the advantages of high strength and low weight and for certain other cases they surpass the mechanical strength of ferrous metals. In certain cases the non-ferrous meals like copper and aluminium alloys have no alternative in wide range of steel. Electrical conduction and aircraft bodies are examples. A jet turbine engine is a good example of application of these materials. A typical engine of this type contains 38%

88

titanium, 12% chromium, 37% nickel, 6% cobalt, 5% aluminium, 1% niobium and 0.02% tantalum. Though steel is the largest consumed metal, good amounts of non-ferrous metals are coming into demand for mechanical, electrical, elevated temperature and corrosion resistance. Typically aluminium alloys are used for cooking utensils, aircraft bodies and as building materials, copper is used as electrical conductor in electrical machines and power transmissions, copper alloys are also used at tubing wherever good thermal conductivity is desired. And there are several other examples. In Table 2.16 the prices of several metals was compared with gold price as base at 1000. Comparing these prices one must be careful that the densities of various metals and their alloy will vary widely. The application of material generally can be seen in the machine and structure as its volume and not as weight. Table 2.8 compares the prices on weight basis. Table 2.16 compares the prices on the basis of both weight and volume.
Table 2.16 : Comparison of Prices of Various Non-ferrous Alloys
Metal Per Volume Mo alloys Ti alloys Cu alloys Zn alloys Stainless steel Mg alloys Al alloys Low alloy steel Gray cast iron Carbon steels 3.3 – 4.170 0.33 – 0.660 0.083 – 0.166 0.025 – 0.115 0.055 – 0.150 0.032 – 0.640 0.032 – 0.048 0.023 0.020 0.0167 Price Per Weight 6.24 – 7.8 1.37 – 3.71 0.30 – 0.36 0.67 – 0.158 0.082 – 0.37 0.31 – 0.74 2.65 – 3.97 0.097 0.050 0.041

Engineering Alloys (Ferrous and Non-Ferrous)

Gold – 1000 (per weight and per volume)

2.15 ALUMINIUM
Aluminium was first produced in 1825. Presently it is produced in quantity second only to steel. It is the most abundant metallic element on the crust of the earth easily comprising about 8% of the crust. Bauxite, an hydrous oxide of aluminium and several other oxides, is the principal ore of aluminium. Aluminium is extracted from its ore mainly through electrolytic process. The ore is first washed off to remove clay and dirt, the ore is crushed into powder and treated with hot sodium hydroxide (caustic soda) to remove impurities. Alumina (the oxide of aluminium) extracted from this solution is dissolved in molten sodium fluroride and aluminium fluoride bath at 940-980oC. This mixture is then subjected to direct current electrolysis by passing direct current between carbon anode and cathode. The metallic aluminium forms in liquid state and sinks to bottom of the cell. This liquid aluminium is tapped off from time to time. The aluminium so obtained is 99.5 to 99.9% pure with iron and silicon as the major impurities. Aluminium, then is taken to large refractory lined furnaces for refining before casting. The chlorine gas is used as purging agent to remove the dissolved hydrogen gas, and the liquid metal surface is skimmed off to remove oxidized metal. The molten metal is then cast into ingots for remelting or rolling.

2.15.1 Wrought Aluminium Alloys
Sheets and extrusion ingots are cast through semi-continuous direct chill method. The sheet ingots are scalped wherein about 12 mm of ingot surface is removed. The scappled ingots are preheated to homogenise the structure by heating to a high temperature and

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soaking there for 10-24 hours. The preheating is done at a temperature below the lowest melting point of the constituents. The ingots are then hot rolled to about 75 mm thickness in 4 high reversed rolling stand. Thereafter the rolled sheet is further reheated to the same temperature and further hot rolled to 18 mm to 25 mm thickness. Further thickness reduction may be achieved through cold rolling. The products obtained this way are termed wrought alloys and normally are inform of sheet, plate, rod, wire and extruded sections. The wrought alloys are identified by a four digit code out of which the first digit signifies the aluminium purity (if pure aluminium) or the major alloying element. The second digit indicates the modification of alloy. The third and fourth digits indicate the minimum amount of aluminium in the alloy. The first digit indicates following : (a) (b) (c) (d) (e) (f) (g) Aluminium is pure no alloying element. Alloying element copper but magnesium is also added. Alloying element manganese. Alloying element silicon. Alloying element magnesium. Main alloying elements are magnesium and silicon. Main alloying elements are zinc, magnesium and copper.

2.15.2 Aluminium Cast Alloys
Aluminium alloys the cast by any one of the following processes.
Sand Casting is the simplest and most versatile process small castings, complex castings with intricate cores, large castings and structural castings are produced by sand casting with equal case. In permanent mould casting a metallic mould is used which may be gravity6 filled or rotated for centrifugal action. The castings from permanent mould are fine grained as compared to sand cast products. In die casting maximum rate of production is achieved. The molten metal is forced into die which is split but sufficiently strong to withstand pressure. One important characteristic of die casting is close tolerance in parts. Fine grained structure and automation of process are other advantages.

Aluminium casting alloys need such element for alloying which will not only impart mechanical strength but will also increase fluidity and feeding ability. Therefore their chemical composition must differ from wrought alloys. Silicon is the most preferred alloying element in aluminium cast alloys for its improves fluidity and feeding ability as well as its mechanical strength. Normal silicon content varies between 5 to 12%. Magnesium in the range of 0.3 to 1% provides strength mainly through precipitation. Mg, Zn, Sn, Ti are also added sometimes.

2.15.3 Properties of Aluminium Alloys
Among the various properties of aluminium alloys following are notable : (a) (b) (c) (d) (e) (f) low density (2.7 gm/cc) high electrical and thermal conductivity, only next to Cu good resistance to atmospheric, water and seawater corrosion good machinability, formability and castability maintains good light reflectivity non-toxic, non-magnetic and non-sparking.

Aluminium is a soft but weak material whose strength is increased by strain hardening and several heat treatments. Aluminium is used as a matrix in several fibre reinforced composites. Al2O3 an oxide of Al is very hand and strong and can be dispersed in the matrix of Al by powder metallurgy to produce SAP (sintered aluminium product). Other 90

reinforcing elements used in softer aluminium matrix are boron whiskers, stainless steel fibres and whiskers of Al3Ni. Alminium alloys are divisible in three groups : (a) (b) (c) cast Al alloys wrought Al alloys aluminium composite reinforced with fibres or particles.

Engineering Alloys (Ferrous and Non-Ferrous)

Cast Al Alloys

Low melting temperature, insolubility to gases except H2 and good surface finish are characteristics of these alloys. Important drawback of cast aluminium alloys is their shrinkage after solidification and hence careful mould design is called for. Mechanical properties are inferior to wrought alloys except in creep. Alloys can be sand cast gravity die cast, and cold chamber pressure die cast. Si, Cu, Mg and Sn increase fluidity when casting thin sections. Mechanical properties of cast Al alloys is improved by adding Cu which induces age hardening to impart hardness and stability upto 250oC. Alloys used for die casting are : 380 (Al, Si, 3.5 Cu) and 413 (Al, 11.5 Si). Alloys preferred for permanent mould casting are : 332 (Al, 9 Si, 3 Cu, 1 Mg) and 319 (Al, 6 SI, 4 Cu). Y-alloy containing 4% Cu and 2% Ni retain strength at high temperatures. It is used for piston and cylinders of IC engines.
Wrought Al Alloys

Wrought aluminium alloys are obtained by addition of Mn and Mg. The Al-Mn and Al-Mg alloys cannot be heat treated. Al-Mn alloy combines high ductility with excellent corrosion resistance. Beverage cans, cooking utensils and roofing sheets are made in Al-Mn alloy. Al alloy that responds to heat treatment by age hardening are Al-Cu, Al-Cu-Mg and Al-Mg-Si. Some Al alloys, their composition and applications are described in Table 2.17. Duralumin is one such alloy which contain 4% Cu and small amounts of Mg, Mn and Si. After heat treatment this alloy develops a UTS of 450-550 MPa and finds use in aircraft structures. Apart from cast and wrought alloys the greater tonnage (about 85%) of Al is used in commercially pure form in which impurities are less than 1%. Al extrusions, tube, rods, wire, electrical conductors, chemical process equipment, foils and many architectural fittings are made in commercially pure Al. The properties of aluminium are described in Table 2.18.
Table 2.17 : Some Aluminium Alloys – Properties and Applications
Alloy Designation EC – O 3003 – O 3003 – H16 2024 – T4 5056 – H18 5056 – O Composition (%) 99.5 Al (mini) 98.8 Al, 1.2 Mn 98.8 Al, 1.2 Mn 93 Al, 4.5 Cu, 1.5 Mg, 0.5 Si, 0.5 Mn 94.6 Al, 5.2 Mg, 0.3 Mn 94.6 Al, 5.2 Mg, 0.3 Mn UTS/Elongation (%) N/mm2 75/50 130/140 190/140 500/19 450/10 300/35 Characteristics and Applications Ductile, high electrical conductivity Good formability and corrosion resistance weldable, storage and utensils High strength, aircraft parts, bridges, rivers Good corrosion resistance to sea water good finish when buffed or anodized, marine parts, cooking utensils, bus bodies Good corrosion resistance and formability, general structure, anodized articles, marine and transport parts High strength and corrosion resistance, aircraft parts, bridges

6061 – T6 6061 – O

98 Al, 1 Mg, 0.6 Si, 0.4 Cu 98 Al, 1 Mg, 0.6 Si, 0.4 Cu

320/17

7075 – T6 7075 - O

90 Al, 5.5 Zn, 2.5 Mg, 1.7 Cu, 0.3 Cr 90 Al, 5.5 Zn, 2.5 Mg,

600/11 240/16

91

Engineering Materials

1.7 Cu, 0.3 Cr

Table 2.18 : Typical Properties of Aluminium
Sl. No. 1 2 3 4 Purity % Melting point oC Sp. Gravity Tensile strength, N/mm2 O – Temper H – 18 Temper Elongation % O – Temper H – 18 Temper Hardness BHN O – Temper H – 18 Temper Electrical conductivity* % IACS O – Temper H – 18 Temper Thermal conductivity J/m2/o C/s at 25o C Corrosion resistance Property Value 99.5 Al, 0.25 Si, 0.25 Fe 660 2.70 72 135 60 17 19 35 62 61 234 Very good in rural marine and industrial, atmosphere

5

6

7

8 9

* Compare with copper, 62% of copper electrical conductivity.

In Table 2.17 aluminium alloys have been assigned certain temper like O-Temper and H18 Temper. The temper designation indicates the condition and heat treatment of any given alloy. Generally the temper designation must follow the alloy designation and separated by a dash. For example, the alloys in Table 2.17 must be described as 3003-O, 2004-T4. The temper designations are described below. There are four basic tempers : (a) (b) (c) (d) F – As fabricated O – Annealed H – Strain hardened T – Heat treated

H is always followed by two or more digits. The first digit indicates basic operations while the following digit stands for the final degree of strain hardening. H1 – only strain hardened H2 – strain hardened and partial annealed H3 – strain hardened followed by stabilization The second digit stands for amount for cold work. The digit 8 represents fully cold worked or full hard. The digit of 4 means half hard and 2 means quarter hard. Thus, H18 means full hard by strain hardening only. T designation is followed by numbers 2 to 9. Their meanings are : T2 – Annealed (only for castings) T3 – Solution heat treated and then cold worked T4 – Solution heat treated and naturally aged to stable condition T5 – Artificial ageing after any one of the following : Elevated temperature, rapid cool fabrication such as casting or extrusion T6 – Solution heat treated and fabricated T7 – Solution heat treated and stabillised T8 – Solution heat treated, cold worked and then artificially aged 92 T9 – Solution heat treated, artificially aged and then cold worked.

2.15.4 Age-hardening of Aluminium Alloys
In certain alloys precipitation from a single phase may occur. The precipitate phase may be in form of find sub-microscopic particles distributed both around the grain boundaries and throughout the grains. In certain alloys of Al-Cu, Mg-Si and Be-Cu such phases precipitate after suitable heat treatment. These precipitated phases have strengthening effects of the alloys. This hardening of alloys is termed age hardening or precipitation hardening. Here the process of age-hardening will be described with particular reference to aluminium alloys containing 4% Cu. Figure 2.36 depicts the equilibrium diagram of Al-Cu system. It is seen that the solubility of Cu in α-phase solid solution decreases steadily and quite considerably with decrease in temperature. At temperature corresponding to point 3, copper forms copper aluminide (CuAl2) which is deposited as coarse particles in and around the grains of α-solid solution. CuAl2 is extremely hard and brittle. If the alloy is now reheated to about 550oC, between the points 2 and 3, CuAl2 is reabsorbed in α-solid solution resulting into single-phase alloy. If alloy from this state is quenched to room temperature, there is insufficient time for CuAl2 to form and Cu atoms are now held in a super-saturated solid solution within the aluminium.
Slow Cool Temperature ( C) 66 60 0 50 40 30 20 Age 10 As Cost 0 2 4 6 % Copper 8 10 1 a + Liquid 2

Engineering Alloys (Ferrous and Non-Ferrous)

0

3 Cu Al2

Slow Cool Overage Re-Heat Quench

Figure 2.36 : The Aluminium-rich Portion of the Copper Aluminium Equilibrium Diagram Showing the Mechanism of Precipitation Hardening for a 4% Copper Alloy, Over Aging Causes a Coalescence of the CuAl2 Particles and Consequent Loss of Strength in the Alloy

When this alloy is allowed to stay at room temperature for five to seven days, the strength improves significantly because of slow precipitation of find submicroscopic particles. These particles are almost uniformly distributed around the grains. The time of this precipitation may be reduced to a few hours by heating the quenched alloy to 120oC. This is known as artificial age-hardening. Closed control of both time and temperature is essential in precipitation hardening for this purpose. Salt baths at constant temperatures are used 4% Cu aluminium alloy is most suitable for this type of treatment. However, this alloy loses its corrosion resistance in hardened state and must be protected by cladding. Age-hardening alloys containing Si and Mg behave in a similar manner. However, the submicroscopic particles that provide strengthening are made of magnesium silicide (Mg2Si). Thus the age-hardening effect of CuAl2 is reinforced by Mg2Si.

2.16 COPPER AND ITS PRODUCTION
Copper is marked by a host of good engineering properties. The foremost is its good electrical conductivity and bulk of copper is used as electrical conductor. It also has a high thermal conductivity and coupled with its resistance to corrosion it is largely used as heat exchanger tubes particularly under circumstances when corrosive atmosphere exists. Its medium tensile strength and ease of fabrication are added advantages in its industrial application. 93

Engineering Materials

Copper is extracted from its sulfide ore. Such ores also contain sulfides of iron. Low grade ore is converted into sulfide concentrate which is smelted in reverberatory furnace to produce a mixture of sulfides of iron and copper, called mate. The slag is separated from matte. The copper sulfides is then chemically converted into impure or blister copper of 98% purity, by blowing air through the matte. The iron sulfides is oxidized and converted in slag. The blister copper is then transferred to refining furnace where most of impurities are converted into slag and removed. This fire refined cooper is called tough pitch copper and is further refined electrolytically to produce 99.95% pure copper called electrolytic tough pitch (ETP) copper. ETP copper is used for production of wire, rod plate and strip. These products serve several industrial purposes. But ETP copper contains 0.04% oxygen which forms interdendritic Cu2O when copper is cast. If copper is heated to a temperature of 400oC in the atmosphere of hydrogen, then hydrogen reacts with densritic Cu2O and produces steam. These H2O molecules being large in size do not diffuse readily and cluster around grain boundaries thus causing internal holes. This phenomenon is called hydrogen embrittlement. The methods of avoiding hydrogen embrittlement are adding phosphorous in the alloy copper and thus allowing P2O5 to form. The other method is to cast ETP copper under a controlled reducing atmosphere to produce copper which is oxygen free high conductivity (OFHC) copper.

2.17 COPPER ALLOYS
Several alloys of copper are used in industry for varying purposes. Copper forms alloys with zinc (the brasses), tin (the bronzes), with tin and phosphorous (the phospher bronzes), aluminium (the aluminium bronzes) and with nickel (the cupronickels).

2.17.1 The Brasses
70/30 brass also known as cartridge brass contains 70% Cu and 30% Zn. It is used for cartridge cases, condenser tubes, sheet fabrication and for general purposes. Its ultimate tensile strength varies between 350 and 600 N/mm2. It is soft and ductile in and annealed form can withstand severe cold working. 60/40 brass of Muntz metal contains 60% Cu and 40% Zn. Its UTS varies between 400 and 850 N/mm2. It is suitable for hot working operations as well as for casting. Many cast valves and marine fittings are made out of this brass. Addition of 2% Pb improves its machinability. Small additions of Fe, Al, Sn, Mn and Ni to 60/40 brass improves its strength considerably. Marine propellers and shafts, pump rods, autoclaves, switch gears and high strength fittings are made out of these brasses. Brazing alloys are essentially the brasses of 50/50 composition with small additions of Sn, Mn and Al. These brasses are hard and brittle.

2.17.2 The Bronzes
The coinage bronze used for making coins in earlier days contains 95% Cu, 4% Sn and 1 % Zn. The Zn acts as a deoxidiser. This alloy is soft and ductile. Admiralty gun metal contains 88% Cu, 10% Sn and 2% Zn. This bronze is normally cast to produce steam and water fittings and bearings. The addition of Pb improves the pressure tightness of the alloy.
Phosphor bronzes are commonly used in manufacture of bearings, hard drawn wires and bronze springs. In addition to tin they contain small percentage of phosphorous as alloying element. 0.2% P forms Cu3P which is a hard compound. It acts as deoxidiser and improves fluidity.

Copper aluminium alloys posses high strength with good resistance to fatigue, corrosion and abrasion and are golden in colour. Aluminium can dissolve in copper to the extent of 9% and greater content than this induces brittleness. Wrought alloys which are good for 94

hot and cold working applications contain 5 to 7% Al. Casting alloys contain 10% Al. Small percentage of Fe, Ni and Mn are added to casting alloys to make them more easily heat treatable. Aluminium bronze is well known for its colour and often called Imitation gold. Al bronze compares well with the strength of steel. Bronzes in general are known for the following characteristics : (a) (b) (c) (d) costlier than brass, better corrosion resistance, stronger than brass, and bearing material.

Engineering Alloys (Ferrous and Non-Ferrous)

2.17.3 Copper-Nickel Alloys
Complete solubility occurs between copper and nickel. All alloys have similar microstructure and can be cold or hot worked. Cupro-nickel also known as German silver is extremely malleable and ductile. It is good against corrosion due to salt water. Condenser tubes are main parts made out of this alloy. It is also used for coinage. 70/30, 80/20 and 75/25 alloys are very common.
Monel metal is essentially 70% Ni and 30% Cu with small amounts of iron and other elements. Alloy is well known for its high strength and corrosion resistance. This alloy is largely used for chemical and food processing plants. It also finds great use as turbine blades, valves corrosion resistance bolts, screws and nails. It is known for its characteristics silver luster.

Tables 2.19 and 2.20 respectively describe Brasses and Bronzes with their applications.
Table 2.19 : Composition, Properties and Applications of Brasses
Gliding metal (95 Cu, 5 Zn) Red brass (85 Cu, 15 Zn) Cartridge brass (70 Cu, 30 Zn) Yellow brass (65 Cu, 35 Zn) Muntz metal (60 Cu, 40 Zn) Leaded red brass (85 Cu, 5 Zn, 5 Sn, 5 Pb) Leaded commercial bronze (89 Cu, 9.25 Zn, 1.75 Pb) Admiralty brass (71 Cu, 28 Zn, 1 Sn) High leaded brass (64 Cu, 33 Zn, 2 Pb) High ductility and corrosion resistance, coins, medals, gold platings Good corrosion resistance, workability, heat exchanger tube, radiator cores Good strength and ductility, rivets, springs, automotive radiator cores Screws, rivets reflectors, plumbing accessories, automotive radiator cores Soundness and good machinability, condenser tubes, architectural work Fair strength, soundness and good machining in cast state, pressure valves, pipe fittings, pump fittings Screws, screw machine parts, electrical connectors, builder’s applications Condenser, evaporator and heat exchanger tubes, marine applications Flat products, gears, wheels

Table 2.20 : Composition and Applications of a Few Bronzes
Phosphor bronze (94.8 Cu, 5 Sn, 0.2 P) Phosphor bronze (89.8 Cu, 10 Sn, 0.2 P) Gun metal (88 Cu, 10 Sn, 2 Zn) Aluminium bronze (88 Cu, 10.5 Al, 3.5 Fe ) Beryllium bronze (98 Cu, 1.7 Be, 0.3 Co) Bolts, electric contracts, spring, bearing Such applications where high strength and resistance to salt water is desired, bushing and gears Sand cast, sued under heavy pressure such as gears and bearings High UTS Very high mechanical strength, springs, used against fatigue, wear and corrosion (UTS – 1200 MPa)

95

Engineering Materials

2.17.4 Copper-Beryllium Alloys
Copper-beryllium alloys contain between 0.6 to 2% Be and 0.2 to 2.5% Co. These alloys can be precipitation hardened and cold worked to develop a tensile strength as high as 1460 MPa. This is the highest strength among the copper alloys. Cu-Be alloys are used as tools requiring high hardness and non-sparking characteristics for the chemical industry. These alloys are very useful for making springs, gears, valves and diaphragms for their excellent corrosion resistance, fatigue properties and strength. These alloys, however, are costlier.

2.18 MAGNESIUM AND ITS ALLOYS
Magnesium is a light metal with density of 1.74 g/cm3. Magnesium is much costlier than aluminium (density 2.74 g/cm3) with which it compares for lightness. Magnesium in its molten state burns readily, hence it is difficult to cast the alloys of magnesium. Magnesium alloys have low corrosion resistance and show poor fatigue and creep behaviour. Their h. c. p. structure does not permit to deform readily at room temperature since only three slip systems exist in h. c. p. at room temperature. The best advantage that magnesium alloys offer is that of low density and many aircraft parts are made in these alloys. Al when added to Mg in the range of 3 to 10% with small amounts of Zn and Mn increases strength, hardness and castability. Addition of Mn (1.2%) with small amount of C does not increase strength but improves corrosion resistance. Mg-Al-Zn alloys have good mechanical strength and corrosion resistance. These alloys are good casting material and generally used at high temperature like 250oC. Extrusions and forgings for general purpose are made in these alloys are used in aircraft, automotive, radio and instrument industries. Some magnesium alloys are described in Table 2.21 along with their applications.
Table 2.21 : Magnesium Alloys
Composition (%) Wrought alloys Mg, 3 Al, 1 Zn, 0.2 Mn Mg, 2 Th, 0.8 Mn Condition Annealed T8 UTS N/mm2 228 228 YS N/mm2 − 198 Elongation % 11 6 Application Air borne cargo equipment Missile and aircraft sheets upto 427oC Highly stressed aerospace uses, extrusions, forgings Sand casting requiring good room temperature strength Pressure tight and permanent mould castings used at 150-260o C

Mg, 6 Zn, 0.5 Zr

T5

310

235

5

Cast alloys

MG, 6 Zn, 3 Al, 0.15 Mn

As-cast

179

76

4

Mg, 3 Re, 3 Zn, 0.7 Zr

T6 T5

235 138

110 97

3 2

2.19 TITANIUM ALLOYS
Pure titanium is a strong ductile and light weight metal. It is very strong, highly resistant to corrosion of all types but has the drawback that it readily reacts with common gases at around 300oC. It reacts readily with C, O2, N2 and theses elements cause embrittlement of Ti. It melts at 1725oC, has a UTS of 600-800 MPa and precent elongation of 25%. Ti 6 Al 4 V alloy develops a UTS of 1300 MPa and had good creep, fatigue and oxdiaton resistance. Aero engines gas turbine blades and other parts of engine and components of 96

air frame are made of this alloy. Ti 5 Al 2.5 Sn is also a strong alloy (900 MPa UTS) which is used in aircraft engine components at 470 to 500oC.

Engineering Alloys (Ferrous and Non-Ferrous)

2.20 BEARING MATERIALS
In general it can be said that a good bearing material should posses following characteristics : (a) (b) (c) (d) (e) it should be strong enough to sustain bearing load. it should not heat rapidly. it should show a small coefficient of friction. it should wear less, having long service life. it should work in foundary.

Generally it is expected that the journal and bearing would be made of dissimilar materials although there are examples where same materials for journals and bearings have been used. When the two parts are made in the same material the friction and hence the wear are high. Cast iron has been used as bearing material with steel shafts in several solutions. However, the various non-ferrous bearing alloys are now being used largely as bearing material because they satisfy the conditions outlined above more satisfactory. Bronzes, babbitts and copper-lead alloys are the important bearing materials that are widely used in service. Certain copper zinc alloys, that is brasses, have been used as bearing materials, but only to limited extent. Since brass in general is chapter, it has replaced bronze in several light duty bearings.

2.20.1 Bearing Bronzes
Bearing bronzes are the copper-tin alloys with small additions of other constitutions. Under conditions of heavy load and severe service conditions, bronzes are especially of great advantages. They possess a high resistance to impact loading and, therefore, are particularly used in locomotive and rolling mills bearings. However, they get heated up fast as compared to other bearing materials, such as babbitts. Bronze lined bearings are easily removed and finished bushings are generally available in stocks. A few of the bronzes that are widely used are described in Table 2.22.
Table 2.22 : Bearing Bronzes
Bronze and SAE Number Leaded gun metal, 63 Phosphor bronze, 64 Bronze backing for lined bearings 66 Semi-plastic bronze, 67 Composition (%) Cu 86-89, Sn 9-11, Pb 1-2.5, P 0.25 max. impurities 0.5 max. Cu 78.5-81.5, Sn 9-11, Pb 0.05-0.25, Zn 0.75 max. impurities 0.25 max. Cu 83-86, Sn 4.5-6.0, Pb 8-10, Zn 2.0, impurities 0.25 max. Cu, 76.5-99.5, Sn 5-7, Pb 14.5-17.5, Zn 4.0 max., Sb 0.4 max., Fe 0.4 max., impurities 10.0 max. Mechanical Properties UTS YS Elongation N/mm2 N/mm2 % 200 80 10 Heavy loads 167 80 8 Bronze backed bearings Soft and good antifriction properties Application Bushing

167

80

8

133

−

10

2.20.2 Babbitts
The alloys of tin, copper, lead and antimony are called babbitts. The tin provides the hardness and compressive strength of babbitts, copper makes them tough, antimony prevents shrinkage while lead contributes to ductility. Bearing liners are extensively made in babbitts for their better antifriction properties than bronzes.

97

Engineering Materials

When Babbitt is backed up a solid metal of high compressive strength it gives good service under high speeds, heavy pressure, impact loads and vibrations. The backing material could be bronze or steel. A thin layer of high-tin Babbitt thoroughly fused to a tinned bronze or a steel shell has exceptional load carrying capacity and impact strength. In case of cast iron bearings the Babbitt in anchored in place by dovetail slots or drilled holes, because Babbitt does not fuse with cast iron. Babbitt bearing linings of dependable strength and life are made by pouring molten material into bearing, allowing to solidify and fuse thoroughly and then machining to finished sizes. While the melting point of Babbitt varies between 180 to 245oC, depending upon composition, the pouring should be done when metal is in fully fluid state. For example, SAE 10 babbitt has a melting point of 223oC, it should not be poured below 440oC. Some Babbitt materials are described in Table 2.23.
Table 2.23 : Babbitts (White Bearing Metals)
SAE No. 10 Composition (%) Sn 90; Cu 4-5; Pb 0.35 max.; Fe 0.08 max.; As 0.1 max.; Bi 0.08 max. Sn 86; Cu 5-56; Sb 6-7.5; Pb 0.35 max.; Fe 0.08 max.; As 0.1 max.; Bi 0.08 max. Sn 59.5; Cu 2.25-3.75; Sb 9.5-11.5; Pb 26.0 max.; Fe 0.08, Bi 0.08 max. Sn 4.5-5.5; Cu 0.5 max.; Sb 9.25-10.75 max.; Pb 86.0 max.; As 0.2 max. Applications Thin liner on bronze backing

11

Hard Babbitt good for heavy pressures Cheap Babbitt good for large bearings under moderate loads Cheap Babbitt for large bearing under light load

12

13

2.20.3 Copper-Lead Alloys
Copper-lead alloys, containing a large percentage of lead have found a considerable use as bearing material lately. Straight, copper-lead alloys of this type have only half the strength of regular bearing bronzes. They are particularly advantageous over Babbitt at high temperature as they can retain their tensile strength at such temperature. Most babbitts have low melting point and lose particularly all tensile strength at about 200oC. Typical copper-lead alloys contain about 75% copper and 25% lead and melt at 980oC. The room temperature tensile strength of copper-lead alloy is about 73 MPa and reduces to about 33 MPa at about 200oC.

2.20.4 Other Bearing Materials
An extensively hard wood of great density, known as lignum vitae, has been used for bearing applications. With water as lubricant and cooling medium its antifriction properties and wear are comparable with those of bearing metals. Lignum vitae has been used with satisfactory results particularly in cases of step brings of vertical water turbine; paper mill machinery, marine service and even roll neck bearings of rolling mills. More recently, in such cases where use of water as lubricant is necessary, especially if sand and grit are present soft vulcanised rubber bearings have been used. A soft, tough, resilient rubber acts as a yielding support, permitting grit to pass through the bearing without scoring the shaft or the rubber. Longitudinal grooves in the rubber lining allow free passage of the cooling water with any foreign matter present. With feathered edges these grooves are also very effective in forming passages in the front of which the supporting pressure is built up in the fluid film. These bearings have coefficient of friction which compares well with roller bearings and pressure of 4.0 to 5.5 MPa may be carried if journal is very smooth and load is applied after it has attained a peripheral speed of 150 m/min. The cooling water temperature in case of rubber bearings must always be below boiling point. In some cases rubber bearings have been found to give as much as ten times the service as bearings of lignum vitae or metals.

98

Rubber bearings have been successfully used in centrifugal and deep well pumps, and washers and several other applications where water must be used as lubricant. The resilience and cushioning properties of rubber may be exploited in reducing vibrations of high speed shafts. Synthetic and neutral composite materials, plastic and reinforced plastic are also being used as bearing materials now-a-days. However, their characteristics are not well established as yet. Powder metallurgy bushing permits oil to penetrate into the materials because of its porosity and is good for its antifriction properties. Bearings are frequently ball-indented in order to provide small basins for the storage of lubricant while the journal is at rest. This supplies some lubricant during starting. The bearing walls may some time be indented and filled with graphite to provide lubricating effect at the start.

Engineering Alloys (Ferrous and Non-Ferrous)

2.21 ALLOYS FOR CUTTING TOOLS
Apart from tool steels described in Unit 11, may alloys which contain wholly non-ferrous elements have been developed. Such alloys behave better than tool steel in many respects and are widely used in industry. These alloys are mainly divided into two groups : stellites and cemented carbides.
Stellite

Stellite is an alloy of Co (40-60%), Cr (25-35%), W (4-25%) and C (1-3%). It is a cast alloy containing C, Cr, and W in Cobalt matrix. Its main characteristic is low coefficient of friction and it possesses high hardness, red hardness, high wear and corrosion resistance. Desired size and shape is achieved by casting and no heat treatment is required. They are mainly used for cutting tools and can cut steel at twice the cutting speed of HSS stellite can be used to cut all types of materials like steels, as high speed steels because they are cast but perform better than HSS with higher life. Satellites are used for cutting hard die faces, can surfaces wear plates and crushes. The hardness varies between 40 to 60 RC and they retain their hardness upto high temperature because they do not undergo phase changes.
Cemented Carbide

These are small pieces with cutting edges and mechanically jointed or brazed to tool shank. Cemented Carbide tool tips are produced by process of powder metallurgy by sintering the powder carbides of W, Ta, Ti in Co powder. The contents are 4095% WC, 3-30% Co, 0-30% TaC and TiC and hardness of tips is in excess of 65 RC compared to 60 RC of stellite. High hardness, high compressive strength at high temperatures are the main characteristics.
Cermets

Cermets are the variation of cemented carbides when the carbides of W and Ti are solidified in the softer matrix of Co and Ni to obtain high hardness, resistance to oxidation and thermal shock and resistance to high temperature abrasion.
Ceramic Tools

Aluminium oxide (Al2O3) is pressed and sintered in a powder metallurgy technique in various shapes of cutting edges which are fastened to mechanical shanks. The hardness of this ceramic tool is above 65 RC and has chemical inertness and high resistance to wear. Ceramic tools are made in small pieces of various geometrical shapes and can be disposed off when not usable.

99

Engineering Materials

2.22 SUMMARY
Engineering alloys can conveniently be subdivided into two types : ferrous and non-ferrous. Ferrous alloys have iron as their principal base metal, whereas non-ferrous alloys have a principal metal other than iron. The steels, which are ferrous alloys, are by far the most important metal alloys mainly because of their relatively low cost and wide range of mechanical properties. The mechanical properties of carbon steels can be varied considerably by cold working and annealing. When the carbon content of steels is increased to about 0.3%, they can be heat-treated by quenching and tempering to produce high strength with reasonable ductility. Alloying elements such as nickel, chromium, and molybdenum are added to plain-carbon steels to produce low-alloy steels. Low-alloy steels have good combination of high strength and toughness and are used extensively in the automotive industry for uses such as gears, shafts, and axles. Aluminium alloys are the most important of the non-ferrous alloys mainly because of their lightness, workability, corrosion resistance, and relatively low cost. Unalloyed copper is used extensively because of its high electrical conductivity, corrosion resistance, workability, and relatively low cost. Copper is alloyed with zinc to from a series of brass alloys which have higher strength than unalloyed copper. Bronzes are other series of alloys when Cu is alloyed with tin or aluminium. Stainless steels are important ferrous alloys because of their corrosion resistance in oxidising environments. To make a stainless steel “stainless”, it must contain at least 12% Cr. Cast irons are other industrially important family of ferrous alloys. They are low in cost and have special properties such as good castability, wear resistance, and durability. Grey cast iron has high machinability and vibration damping capacity due to the graphite flakes in its structure. White and iron, yet another variety having carbon in cementite form and is harder. Other non-ferrous alloys briefly discussed in this unit are magnesium, titanium, and nickel alloys. Magnesium alloys are exceptionally light and have aerospace applications and are used in radio and instrument industry. Titanium alloys are expensive but have a combination of strength and lightness not available from any other metal alloy system and so are used extensively for aircraft structural parts. Nickel alloys have high corrosion and oxidation resistance and are therefore commonly used in the oil and chemical process industries. Nickel when alloyed with chromium and cobalt forms the basis for the nickelbase superalloys which are necessary for gas turbines in jet aircraft and some electricpower generating equipment. In this unit, we have discussed to a limited extent the structure, properties, and applications of some of the important engineering alloys. However, it must be pointed out that may important alloys have been left out due to the limited scope of this unit.

2.23 KEY WORDS
Austenite (γ Phase in Fe-Fe3C Phase Diagram) Austenitising : An intersitial solid solution of carbon in FCC iron; the maximum solid solubility of carbon in austenite is 2.0%. : Heating a steel into the austenite temperature range so that its structure becomes austenite. The austenitising temperature will vary depending on the composition of the steel. : An intersitial solid solution of carbon in BCC iron; maximum solid solubility of carbon in BCC iron is 0.02%.

α Ferrite (α Phase in the Fe-Fe3C Phase diagram) 100

Pearlite

: A mixtue of a ferrite and cementite (Fe3O) phases in parallel plates (lamellar stsructure) produced by the eutectoid decomposition of austenite. : A ferrite which forms during the eutectoid decomposition of austenite. : Cementite which forms during the eutectoid decomposition of austenite; the cementite in pearlite. : A steel with 0.8% C. : A steel with less than 0.8% C. : A steel with 0.8 to 2.0% C. : A ferrite which forms by the decomposition of austenite at temperature above the eutectoid temperature. : Cementite which forms by decomposition of austenite at temperature above the eutectoid temperature. : A supersaturated interstitial solid solution of carbon in body-centered tetragonal iron. : A mixture of a ferrite and very small particles of Fe3C particles produced by the decomposition of austenite; a non-lamellar eutectoid decomposition product of austenite. : A mixture of particles of cementite (Fe3C) in an a ferrite matrix. : A time-temperature-transformation diagram which indicates the time for a phase to decompose into other phases isothermally at different temperatures.

Engineering Alloys (Ferrous and Non-Ferrous)

Eutectoid Ferrite Eutectoid Cementite (Fe3C)

Eutetoid (Plain-carbon Steel) Hypoeutectoid (Plain-carbon steel) Hypereutectoid (Plain-carbon Steel) Proeutectoid Ferrite

Proeutectoid Cementite (Fe3C)

Maenstie Bainite

Spheroidite Isothermal Transformation (IT) Diagram

Continuous-cooling : A time-temperature-transformation diagram which Transformation (CCT) Diagram indicates the time for a phase to decompose into other phases continuously at different rates of cooling. Martempering (Marquenching) : A quenching process whereby a steel in the austenitic condition is hot-quenched in a liquid (salt) bath at above the Ms temperature, held for a time interval short enough to prevent the austenite from transforming, and then allowed to cool slowly to room temperature. After this treatment the steel will be in the martensitic condition, but the interrupted quench allows stresses in the steel to be relieved. : A quenching process whereby a steel in the austenitic condition is quenched in a hot liquid (salt) bath at a temperature just above the Ms of the steel, held in the bath until the austenite of the

Austempering

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Engineering Materials

steel is fully transformed, and then cooled to room temperature. With this process a plain-carbon eutectoid steel can be produced in the fully bainitic condition.
Ms Mf Tempering (of a Steel) : The temperature at which the austenite in a steel starts to transform to martensite. : The temperature at which the austenite in a steel finishes transforming to martensite. : The process of reheating a quenched steel to increase its toughness and ductility. In this process martensite is transformed into tempered martensite. : An iron-carbon alloy with 0.02 to 2% C. All commercial plain-carbon steels contain about 0.3 to 0.9% manganese along with sulfur, phosphorus, and silicon impurities. : The ease of forming martensity in a steel upon quenching from the austentic condition. A highly hardenable steel is one which will form martensite throughout in thick sections. Hardenability should not be confused with hardness. Hardness is the resistance of a material to penetration. The hardenability of a steel is mainly a function of its composition and grain size. : A test in which a 1 inch (2.54 cm) – diameter bar by 4 inch (10.2 cm) line is austenitised and then water-quenched at one end. Hardness is measured along the side of the bar up to about 2.5 inch (6.35 cm) from the quenched end. A plot called the Jominy hardeability curve is made by plotting the hardness of the bar against the distance from the quenched end. : Iron-carbon-silicon alloys with 1.8-3.6% C and 0.5-1.9% Si. White cast irons contain large amounts of iron carbide which make them hard and brittle. : Iron-carbon-silicon alloys with 2.5-4.0% C and 1.0-3.0% Si. Grey cast irons contain large amounts of carbon in the form of graphite flakes. They are easy to machine and have good wear resistance. : Iron-carbon-silicon alloys with 3.0-4.0% C and 1.8-2.8% Si. Ductile cast irons contain large amounts of carbon in the form of graphite nodules (spheres) instead of flakes as (about 0.05%) before the liquid cast iron is poured enables the nodules to form. Ductile irons are in general more ductile than gray cast irons. : Iron-carbon-silicon alloys with 2.0-2.6% C and 1.1-1.6% Si. Melleable cast irons are first cast as white cast irons and then are heat-treated at about 940oC (1720oF) and held about 3 to 20- h. The iron

Plain-carbon Steel

Hardenability

Jominy Hardenability Test

White Cast Irons

Gray Cast Irons

Ductile Cast Irons

Malleable Cast Irons

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carbide in the white iron is decomposed into irregularly shaped nodules or graphite.

Engineering Alloys (Ferrous and Non-Ferrous)

2.24 ANSWERS TO SAQs
Please refer preceding text for answers of all the SAQs.

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