What is Heat Treatment Purpose of Heat Treatment Steps in Heat Treatment Types of Heat Treatment (Annealing, Tempering, Hardening, Case Hardening) Heat Treatment of Polymer and Ceramics
2) Selection of Heat Treatment Processes for Different Mechanical Components (Gears, Bearings, Springs, Shafts, Screws, Flexible Machine Elements) 3) Case Studies 4) References
What is Heat Treatment
y Heat Treatment is the time and temperature
controlled heating and cooling of metals to alter their physical, chemical and mechanical properties without changing the product shape.
y It is a very enabling manufacturing process that can
not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics.
Purpose of Heat Treatment
y To increase the strength of material. y To improve hardness. y To improve machining. y To improve formability. y To restore ductility after a cold working operation. y To relieve internal stresses. y Grain size refinement.
Steps in Heat Treatment
Uniform temperature is of primary importance in heating cycle and it is obtained by slow heating. the
Once the metal is heated to the proper temperature, it must be held at that temperature until the metal is heated throughout and the changes have time to take place.
After being heated to the proper temperature, the metal must be returned to room temperature to complete the heat-treating process. The rate at which the metal should be cooled depends on both the metal and the properties desired.
Types of Heat Treatment Processes
Full Annealing Normalizing Process Annealing Spheroidizing Stress Relieving
y Tempering y Hardening y Case Hardening
Carburizing Nitriding Cyaniding etc.
y The term annealing refers to a heat treatment in
which a material is exposed to an elevated temperature for an extended time period and then slowly cooled.
y Ordinarily, annealing is carried out to
(1) relieve stresses; (2) increase softness, ductility, and toughness; (3) produce a specific microstructure.
y Full Annealing
1. 2. Utilized in low- and medium carbon steels The alloy is treated by heating to a temperature of about 50 C above the upper critical line for hypoeutectoid, or, 50 C above the lower critical line for hypereutectoid. The alloy is then furnace cooled The microstructure produced is coarse pearlite that is relatively soft and ductile. Used to refine the grains i.e., to decrease the average grain size. It is accomplished by heating at least 55 c above the upper critical line for both hypo and hyper eutectoid composition. The treatment is terminated by cooling in still air.
1. 2. 3.
Annealing Processes (Contd.)
y Process Annealing
1. 2. 3. Used to negate the effects of cold work that is, to soften and increase the ductility of a previously strain-hardened metal Process annealing takes place at temperatures just below the eutectoid temperature of 727 C. This treatment is applied to low-carbon, cold-rolled sheet steels to restore ductility. Heating the alloy at a temperature just below the eutectoid at about 700 C in the + Fe3C region of the phase diagram. It gives maximum softness, ductility and machinability by producing spheroid microstructure which is even softer than coarse pearlite. Use to relieve residual stresses The piece is heated to the temperatures approaching the eutectoid temperature of 727 C , held there long enough to attain a uniform temperature, and finally cooled to room temperature in air.
y Stress Relieving
Annealing Processes (Contd.)
y Fully harden material is very hard, brittle
and have residual stresses.
y Tempering is applied to that material to
reduce brittleness, increase ductility, and toughness and relieve stresses in martensite structure.
y It consists of heating the hardened steel to
a temperature below the critical range, holding this temperature for a sufficient period, and then cooling in still air. degrees of strength, hardness and ductility obtained depend directly upon the temperature to which the steel is heated.
y High tempering
temperature improves ductility at the sacrifice of tensile, yield strength and hardness.
y Hardening is accomplished by heating the metal slightly in excess of the critical temperature, and then rapidly cooling by quenching in oil, water, or brine. y This treatment produces a fine grain structure, extreme hardness, maximum tensile strength, and minimum ductility. y Generally, material in this condition is too brittle for most practical uses, although this treatment is the first step in the production of high-strength steel.
y The objective in casehardening is to produce a hard case over a
tough core. Casehardening is ideal for parts that require a wear resistant surface and, at the same time, must be tough enough internally to withstand the applied loads.
y The steels best suited to case hardening are the low-carbon and
low-alloy steels. If high-carbon steel is case-hardened, the hardness penetrates the core and causes brittleness.
y In case hardening, the surface of the metal is changed chemically
by inducing a high carbide or nitride content. The core is unaffected chemically. When heat treated, the surface responds to hardening while the core toughens.
Case Hardening (Contd.)
CARBURIZING 1. Consists of holding the metal at an elevated temperature while it is in contact with a solid or gaseous material rich in carbon. 2. Time must be allowed for the surface metal to absorb enough carbon to become high carbon steel. 3. The material is then quenched and tempered to the desired hardness. NITRIDING 1. Consists of holding special alloy steel, at temperatures below the critical point, in an hydrous ammonia. 2. Absorption of nitrogen as iron nitride into the surface of the steel produces a greater hardness than carburizing, but the hardened area extends to a lesser depth. CYANIDING 1. Rapid method of producing surface hardness on an low-carbon steel. 2. Immersion of the steel in a molten bath of cyanide salt, or by applying powdered cyanide to the surface of the heated steel. 3. The temperature of the steel during this process should range from 760° to 899°C depending upon the type of steel, depth of case desired, type of cyanide compound, and time exposed to the cyanide. 4. The material is dumped directly from the cyanide pot into the quenching bath.
Annealing of Ceramics
y When a ceramic material is cooled from an elevated temperature,
internal stresses, called thermal stresses, may be introduced as a result of the difference in cooling rate and since they may weaken the material or, in extreme cases, lead to fracture, is termed as thermal shock
y Once such stresses have been introduced, however, elimination, or at
least a reduction in their magnitude, is possible by an annealing heat treatment in which the ceramic is heated to the annealing point, then cooled the piece at a sufficiently slow rate to the room temperature.
Thermal Tempering of Ceramics
y The strength of a glass piece may be enhanced by intentionally inducing compressive residual surface stresses. y This can be accomplished by a heat treatment procedure called thermal tempering. With this technique, the glassware is heated to a temperature above the glass transition region yet below the softening point. It is then cooled to room temperature in a jet of air or, in some cases, an oil bath. y The residual stresses arise from differences in cooling rates for surface and interior regions. After the glass piece has cooled to room temperature, it sustains compressive stresses on the surface, with tensile stresses at interior regions.
Heat Treating of Polymers
y Heat treating (or annealing) of semicrystalline polymers can
lead to an increase in the percent crystallinity, and crystallite size and perfection, as well as modifications of the spherulite structure.
y Increasing the annealing temperature leads to the following:
(1) an increase in tensile modulus, (2) an increase in yield strength, and (3) a reduction in ductility.
y Note that these annealing effects are opposite to those typically
observed for metallic materials enhanced ductility.
i.e. weakening, softening, and
Materials & Heat treatment Processes for Different Mechanical Components
Some of the mechanical components which are discussed herey Gears y Bearings y Springs y Shafts y Screws y Flexible Machine Elements
y Material of gear:
Cast iron low cost, moderate power rating. Carbon steel power gear, medium power. Alloy steel gear highest strength and durability Aluminium alloy gear light weight, noncorrosive, light duty instrument gears. Plastic (PBT, LCP) gear low power drive (toys, clocks, etc.), long term dimensional stability, excellent lubricity y Properties needed in Gear: tough enough to withstand load, hardness in surface to resist wear y Heat treatment performed in gears: Cast iron gears heat treatment is generally not done Carbon steel gears normalizing, then hardening and tempering Alloy steel gears hardening and tempering is done Gears in agriculture, construction or in road building case hardening (carburizing, nitriding, flame hardening, spin hardening)
y Material of bearing: Journal bearing Babbitt material [lead based/tin based] (excellent conformability and embeddability but has low compressive and fatigue strength, HB 20-30, useful for medium load), leaded bronze (HB 40-60, strength is high, useful or larger unit load), aluminium alloys (low cost, low weight) copper alloys.
stainless steel (corrosion resistive)
Roller contact bearing
SAE 52100 (high carbon chromium steel, 440c
y Properties needed in bearings: hardness to resist wear y Heat treatment for bearings: Speroidizing of racers, hardening and tempering of each components like balls, rollers, racers etc. to achieve uniform hardness with tempered martensite and dispersion of carbide.
Material for springs: Music wire C85 steel, toughest, used for small springs Oil-tempered wire C65 steel, general purpose springs Hard drawn wire C66 steel, cheapest general purpose springs Chrome vanadium steel good for higher stress conditions, shock and impact load, also used in where fatigue resistance and long endurance are needed. Chrome silicon steel excellent for highly stressed conditions, long life, good for shock load Phosphor bronze corrosion resistive Beryllium copper electrical springs. Properties needed in spring: tough and ductile Heat treatment for springs: y A wound spring can lose its spring tension due to anelastic behavior, which causes the spring to unwind or change its shape over time. y To avoid this springs are placed in an oven at 315 - 375 ºC (600 - 707 ºF) for 2 hours for spring aging. This will allow the spring to change shape or unwind. This unwinding or changing shape can be accommodated during the design of the spring and be compensated. Once the springs are treated to spring aging, they do not usually change shape.
y Material for shaft:
Low carbon steel mostly used Medium carbon steel also mostly used Hot rolled plain carbon steel least expensive, since hot rolled scaling is always present on the surface Cold drawn plain carbon steel better yield strength, used for transmission shafts, smooth surface finish. Alloy steels expensive, used in severe service conditions, used in great stress conditions, residual stress is less.
y Properties needed for shafts: tough and ductile y Heat treatment for shafts:
Annealing to relieve residual stresses and to make it ductile, surface hardening when shaft needed to be wear resistive.
y Material for screws: 4140 alloy steel tough, good torsional strength, good fatigue strength Nitralloy 135m material is first machined and then nitrided, shorter life, low cost H-13 tool steel high strength, tough, can handle high pressure and torque D-2 tool steel good wear resistance, but low on torque strength Cru-wear ® high wear resistance(>D-2), compressive strength and exceptional toughness 17-4 PH stainless excellent combination of strength and wear resistance, useful for small screw CPM®-9V tool steel high % of vanadium carbide so useful for high wear applications CPM®-420V Tool steel useful for corrosion resistance y Properties needed in screw: it should be hard on the surfaces to resist wear and tough enough to withstand the load. y Heat treatment for screw: Annealing to reduce residual stresses Hardening to increase strength Tempering to reduced brittleness Case hardening to increase wear resistance.
Flexible Machine Elements
y Belts y Flat belt urethane, rubber-impregnated fabric reinforced with steel wire or nylon cord y V belt fabric or cord of cotton, rayon or nylon and impregnated with rubber. y Timing belt rubberized fabric and steel wire. y Chains y 306 stainless steel and 316 stainless steel y Wire ropes y Several strands of metal wire laid into helix.
CASE STUDY 1: Heat-treatment of Phosphate Glass Fibers and Its Effect on Composite Property Retention
y Degradable phosphate glass fibers were produced using a melt-drawing
process and then subject to heat-treatment in order to assess the change in their properties and in the properties of polycaprolactone composites reinforced with the fibers. degradation of the fibers that is attributed to the relaxation of the glass structure. The heat-treatment also increases the fiber stiffness but causes an initial loss in strength that recovers as the fiber degrades. of properties but still suffered an initial loss in properties comparable to composites produced using non heat-treated fibers. Early stage losses in composite properties would appear to be the result of another process that is not related to the fiber degradation.
y It was found that the heat-treatment causes a change in the mode of
y Composites made using the fibers showed a better long-term retention
CASE STUDY 2: Study of a Compound Reverted Gear Train
Objective: Transmission of power from a source, such as an engine or motor, through a machine to an output actuation. Solution: A two-stage, compound reverted gear train such as shown in Fig. will be designed. We will see through the material selection and heat treatment process required for the elements of the design. The initial specifications are as follows:y y y y y y y y
Power to be delivered: 20 hp Input speed: 1750 rpm Output speed: 82 88 rev/min Usually low shock levels, occasional moderate shock Input and output shafts extend 4 in outside gearbox Maximum gearbox size: 22 in * 14 in * 14 in Output shaft and input shaft in-line Gear and bearing life -12 000 hours; infinite shaft life
CASE STUDY 2 (Contd.)
We will calculate the gear teeth from the input and output speed values given. The train value is known. From here we find the suitable pairs of gears for the required gear train value. Then from the diameters of the gear they would be checked for the gearbox size constraint. For gear 4: It comes out to be the smallest gear with 16 teeth and transmitting the maximum load. We will first check it for wear. From these we find the strength to be around Sc=215,000 kpsi. So the material which satisfies this value is Grade 2 steel with Sc=225,000 kpsi. The steel is carburized and hardened to obtain the required value of surface hardness. Using the same material now we check it for the bending stress and find out the factor of safety. For gear 5: We perform wear and bending stress design. The values of the stresses in wear and bending are found to be 76280 psi and 25400 psi respectively. The material that satisfies these strength requirements is chosen as Grade 1 Steel through hardened to 250 HB. For gear 2: Bending stress is calculated to 13040 psi. For this the material selected is Grade 1 steel flame hardened. For Gear 3: Wear and bending stress is calculated as 8584 psi and 44340 psi. Material selected is Grade 1 steel Through hardened to 200 HB. Shaft Material selection: Initially an inexpensive steel 1020 CD is selected.
y Joseph E. Shigley, Charles R. Mischke, K.J. Nisbett, Mechanical Engineering design , 8th Edition, TATA McGraw hill publications. y Boris Klebanov, David Barlam, Frederic Nystrom, Machine elements Life & Design , 2008 edition, CRC Press. y William D. Callister, Jr. Materials Science and Engineering, 7th Edition, John Wiley & Sons, Inc. y www.nptel.iitm.ac.in y www.wikipedia.com y y
www.gearshub.com Heat-treatment of Phosphate Glass Fibers and Its Effect On Composite Property Retention by Andrew J. Parsons, Ifty Ahmed, Jing Yang, Sophie Cozien-Cazuc, Chris D. Rudd School of M3, University of Nottingham, UK