Thermal Cutting

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Introduction to Thermal cutting from ASM Handbook

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Thermal Cutting
Revised by Ed Craig, AGA Gas, Inc.

Introduction
THERMAL CUTTING processes differ from mechanical cutting (machining) in that the cutting action is initiated either by chemical reaction (oxidation) or melting (heat from arc). All cutting processes result in the severing or removal of metals. Additional information on cutting processes used in metal-forming operations can be found in the articles "Laser Cutting" and "Abrasive Waterjet Cutting" in this Volume.
Oxygen cutting is accomplished through a chemical reaction in which preheated metal is cut, or removed, by rapid

oxidation in a stream of pure oxygen. Typical oxygen cutting processes are oxyfuel gas, oxygen lance, chemical flux, and metal powder cutting. Oxyfuel gas cutting and its modifications, chemical flux cutting and metal powder cutting, which are used to cut oxidation-resistant materials, are discussed in this article.
Arc cutting melts metal by heat generated from an electric arc. Because extremely high temperatures are developed, arc

cutting can be used to cut almost any metal. Modifications of the process include the use of compressed gases to cause rapid oxidation (or to prevent oxidation) of the workpiece, thus incorporating aspects of the gas cutting process. Arc cutting methods include air carbon arc, gas metal arc, gas tungsten arc, shielded metal arc, plasma arc, and oxygen arc cutting. The methods of industrial importance that are covered in this article include plasma arc cutting, air carbon arc cutting, electric arc cutting using consumable tubular electrodes (Exo-Process), and oxygen arc cutting.
Thermal Cutting
Revised by Ed Craig, AGA Gas, Inc.

Oxyfuel Gas Cutting
Oxyfuel gas cutting includes a group of cutting processes that use controlled chemical reactions to remove preheated metal by rapid oxidation in a stream of pure oxygen. A fuel gas/oxygen flame heats the workpiece to ignition temperature, and a stream of pure oxygen feeds the cutting (oxidizing) action. The oxyfuel process, which is also referred to as burning or flame cutting, can cut carbon and low-alloy plate of virtually any thickness. Castings more than 750 mm (30 in.) thick commonly are cut by the oxyfuel process. With oxidation-resistant materials, such as stainless steels, either a chemical flux or metal powder is added to the oxygen stream to promote the exothermic reaction. Equipment for such cutting is somewhat awkward, however, and speeds and cut quality are lower than those obtained with plasma arc cutting. The simplest oxyfuel gas cutting equipment consists of two cylinders (one for oxygen and one for the fuel gas), gas flow regulators and gages, gas supply hoses, and a cutting torch with a set of exchangeable cutting tips. Such manually operated equipment is portable and inexpensive. Cutting machines, employing one or several cutting torches guided by solid template pantographs, optical line tracers, numerical controls, or computers, improve production rates and provide superior cut quality. Machine cutting is important for profile cutting, that is, the cutting of regular and irregular shapes from flat stock. Principles of Operation Oxyfuel gas cutting begins by heating a small area on the surface of the metal to the ignition temperature of 760 to 870 °C (1400 to 1600 °F) with an oxyfuel gas flame. Upon reaching this temperature, the surface of the metal appears bright red. A cutting oxygen stream is then directed at the preheated spot, causing rapid oxidation of the heated metal and generating large amounts of heat. This heat supports the continued oxidation of the metal as the cut progresses. Combusted gas and the pressurized oxygen jet flush the molten oxide away, exposing fresh surfaces for cutting. The metal in the path of the oxygen jet burns. The cut progresses, making a narrow slot, or kerf, through the metal.

To start a cut at the edge of a plate, the edge of the preheat flame is placed just over the plate edge to heat the material. When the plate heats to red, the cutting oxygen is turned on, and the torch moves over the plate to start the cut. During cutting, oxygen and fuel gas flow through separate lines to the cutting torch at pressures controlled by pressure regulators, adjusted by the operator. The cutting torch contains ducts, a mixing chamber, and valves to supply an oxyfuel gas mixture of the proper ratio for preheat and a pure oxygen stream for cutting to the torch tip. By adjusting the control valves on the torch handle or at the cutting machine controller, the operator sets the precise oxyfuel gas mixture desired. Depressing the cutting oxygen lever on the torch during manual operation initiates the cutting oxygen flow. For machine cutting, oxygen is normally controlled by the operator at a remote station or by numerical control. Cutting tips have a single cutting oxygen orifice centered within a ring of smaller oxyfuel gas exit ports. The operator changes the cutting capacity of the torch by changing the cutting tip size and by resetting pressure regulators and control valves. Because different fuel gases have different combustion and flow characteristics, the construction of cutting tips, and sometimes of mixing chambers, varies according to the type of gas. Oxyfuel gas flames initiate the oxidation action and sustain the reaction by continuously heating the metal at the line of the cut. The flame also removes scale and dirt that may impede or distort the cut. The rate of heat transfer in the workpiece influences the heat balance for cutting. As the thickness of the metal to be cut increases, more heat is needed to keep the metal at its ignition temperature. Increasing the preheat gas flow and reducing the cutting speed maintain the necessary heat balance. Oxygen flow must also be increased as the thickness of the metal to be cut increases. To maintain a steady-state reaction at a satisfactory cutting speed, the velocity and volume, as well as the shape of the oxygen jet, must be closely controlled. Because the cutting-oxygen jet is surrounded by preheating flames, it is affected by these gases and the surrounding atmosphere. The jet must have sufficient volume and velocity to penetrate the depth of the cut and still maintain its shape, force, and effective oxygen content. There is also a relationship between the purity of the cutting oxygen and the time required for oxidation. This invariably has an influence on the ultimate cutting speed.
Quality of Cut. The limits within which the cutting reaction can effectively operate are determined by many factors

besides those mentioned. Oxyfuel gas cutting involves control of more than twenty variables. Suppliers of cutting equipment provide tables that give approximate gas pressures for various sizes and styles of cutting torches and tips, along with recommended cutting speeds; these are the variables that the operator can control. Other variables include type and condition (scale, oil, dirt, flatness) of material, thickness of cut, type of fuel gas, and quality and angle of cut. (When not otherwise defined, a cut is usually taken to mean a through or "drop" cut, made in horizontal plates with the cutting tip in the vertical position.) Higher cutting speeds with good cut quality are obtained during the oxyfuel process using a special tip and torch configuration that provides a curtain of oxygen around the cutting oxygen. The protective curtain maintains a higher level of cutting oxygen purity, which speeds up the oxidation process. Cutting speeds can be increased by approximately 25% for thicknesses up to 25 mm (1 in.). When dimensional accuracy and squareness of the cut edge are important, the operator must adjust the process to minimize the kerf (the width of metal removed by cutting) and to increase the smoothness of the cut edge. Careful balancing of all cutting variables helps attain a narrow kerf and smooth edge. The thicker the work material, the greater the oxygen volume required and, therefore, the wider the cutting nozzle and kerf. Process Capabilities Oxyfuel gas cutting processes are primarily used for severing carbon and low-alloy steels. Other iron-base alloys and some nonferrous metals can be oxyfuel gas cut, although process modification may be required and cut quality may not be as high as is obtained in cutting the more widely used grades of steel. High-alloy steels, stainless steels, cast iron, and nickel alloys do not readily oxidize and therefore do not provide enough heat for a continuous reaction. As the carbon and alloy contents of the steel to be cut increase, preheating or postheating, or both, often are necessary to overcome the effect of the heat cycle, particularly the quench effect of cooling. Some of the high-alloy steels, such as stainless steel, and cast iron can be cut successfully by injecting metal powder (usually iron) or a chemical additive into the oxygen jet. The metal powder supplies combustion heat and breaks up oxide films. Chemical additives combine with oxides to form lower temperature melting products that flush away.

Applications. Large-scale applications of oxyfuel cutting are found in shipbuilding, structural fabrication, manufacture

of earth-moving equipment, machinery construction, and the fabrication of pressure vessels and storage tanks. Many machine structures originally made from forgings and castings can be made at less cost by redesigning them for oxyfuel gas cutting and welding, with the advantages of quick delivery of plate material from steel suppliers, low cost of oxyfuel gas cutting equipment, and flexibility of design. Structural shapes, pipe, rod, and similar materials can be cut to length for construction or cut up in scrap and salvage operations. In steel mills and foundries, projections such as caps, gates, and risers can be severed from billets and castings. Mechanical fasteners can be quickly cut for disassembly using oxyfuel gas cutting. Holes can be made in steel components by piercing and cutting. Machine oxyfuel gas cutting is used to cut steel plate to size, to cut various shapes from plate, and to prepare plate edges (bevel cutting) for welding. Gears, sprockets, handwheels, clevises and frames, and tools such as wrenches can be cut out by oxyfuel gas torches. Often, these oxyfuel cut products can be used without further finishing. However, when cutting medium- or high-carbon steel or other metal that hardens by rapid cooling, the hardening effect must be considered, especially if the workpiece is to be subsequently machined.
Thickness Limits. Steel less than 3 mm (

in.) thick to over 1.5 in (60 in.) thick can be cut by oxyfuel gas cutting, though some sacrifice in quality occurs near both ends of this range. With very thin material, operators may have some difficulty in keeping heat input low to avoid melting the kerf edges and to minimize distortion. Steel under 6 mm ( thick often is stacked for cutting of several parts in a single torch pass. Procedures for light cutting (<9.5 mm, or in.) in.

thick), medium cutting (9.5 to 250 mm, or to 10 in. thick), heavy cutting (>250 mm, or 10 in. thick), and stack cutting are discussed in "Oxyfuel Gas Cutting" in Welding, Brazing, and Soldering, Volume 6 of the ASM Handbook.
Advantages and Disadvantages. A number of advantages and disadvantages are apparent when oxyfuel gas cutting

is compared to other cutting operations such as arc cutting, milling, shearing, or sawing. The advantages of the oxyfuel process are:
• • • • • • •

Metal can be cut faster. Setup is generally simpler and faster than is the case for machining and about equal to that of mechanical severing (sawing and shearing) Oxyfuel gas cutting patterns are not confined to straight lines as in sawing and shearing, or to fixed patterns as in die-cutting processes. Cutting direction can be changed rapidly on a small radius during operation Manual oxyfuel gas cutting equipment costs are low compared to those for machine tools. Such equipment is portable and self-contained, requiring no outside power, and well suited for field use When properties and dimensional accuracy of gas cut plate are acceptable, oxyfuel gas cutting can replace costly machining operations. It offers reduced labor, overhead, material, and tooling costs, and faster delivery With advanced machinery, oxyfuel gas cutting lends itself to high-volume parts production Large plates can be cut in place quickly by moving the gas torch rather than the plate Two or more pieces can be cut simultaneously using stack cutting methods and multiple-torch cutting machines

The disadvantages of the oxyfuel process include:
• • • •

Dimensional tolerances are poorer than they are for machining and shearing Because oxyfuel gas cutting relies on oxidation of iron, it is limited to cutting steels and cast iron Heat generated by oxyfuel gas cutting can degrade the metallurgical properties of the work material adjacent to the cut edges. Hardenable steels may require preheat and/or postheat to control microstructure and mechanical properties Preheat flames and the expelled red hot slag pose a fire hazard to plant and personnel

Factors Affecting Oxyfuel Gas Cutting
Oxygen consumption varies widely in practice, depending on whether maximum economy, speed, or accuracy is

sought. Literature supplied by torch and related equipment suppliers provides general guidelines for the amount of oxygen consumed for varying metal thicknesses.

As the cutting oxygen flows down through the cut, the quantity available for reaction decreases. If the flow of oxygen is relatively large and sharply coherent, the rate of cutting through the depth of the cut is not affected; that is, the cutting face will remain vertical if the oxygen is in excess and the cutting speed is not too great. However, if the oxygen flow is insufficient, or the cutting speed is too high, the lower portions of the cut will react more slowly. As a result, the cutting face will become curved, as shown in Fig. 1. The horizontal distance between the points of entry and exit is called drag. Drag often is expressed as a ratio or as a percentage of the metal thickness.

Fig. 1 Cross section of work metal during oxyfuel gas cutting showing drag on cutting face.

Drag can be stabilized; at the proper drag ratio, the heat from the molten metal flowing down the curve is efficiently

used. Drag is a rough measure of cutting quality and of economy in oxygen consumption. In metal thicknesses up to 50 or 75 mm (2 or 3 in.), a 10 to 15% drag is associated with good quality of cut and good economy. Higher quality demands less drag; more drag indicates poorer quality and low oxygen consumption. Too much drag may lead to incomplete cutting. In very thin sections, drag has little meaning; the main problem is control of high heat input compared to low heat sink. In very thick sections, the opposite is true; the problem is to avoid excessive drag. All the input variables controlled by the operator (size and type of cutting tip, preheat flames, oxygen flow, and cutting speed) can be used to control drag.
Oxygen purity, as well as the alloy content of the steel being cut, affects the chemical reaction in oxyfuel gas cutting.

Oxygen purity also affects combustion heat. The oxygen supplied from cylinders for oxyfuel gas cutting is usually at least 99.5% pure. A 0.5% departure from this purity (99% O2) decreases the cutting efficiency. At 90% purity, cutting is very difficult, and at lower purities it is often impossible. The impurities consist of inert gases and water vapor. The effective purity of oxygen can also be reduced by gaseous combustion products from the preheat flames and from the metal being cut.
Alloying of iron affects oxyfuel gas cutting, usually by reducing the rate of oxidation. The total alloy content in lowalloy steel usually does not exceed 5%, and the effect on cutting speed is slight. Alloying elements affect oxyfuel gas cutting of steel in two ways. They may make the steel more difficult to cut, or they harden the cut edge, or both. In highly alloyed steel, the oxidizing characteristics of alloying elements and the constituents formed in alloying may make sustained oxidation difficult or even impossible. The effects of alloying elements on cutting are evaluated in Table 1. In any steel, preheat accelerates the chemical reaction; higher alloy steels, therefore, may need preheating beyond that provided by the preheat flames of the gas torch to promote cutting.

Table 1 Effects of alloying elements on resistance of steel to oxyfuel cutting
Element Effect on oxyfuel cutting

Aluminum

Extensively used as a deoxidizer in steelmaking; has no appreciable effect on oxygen cutting unless present in amounts above 8 to 10%; above this percentage, plasma arc cutting or metal powder cutting should be used

Carbon

Steels containing up to 0.25% C can readily be flame cut; higher-carbon steels should be preheated to prevent hardening and cracking; graphitic carbon makes flame cutting of cast iron difficult; cast iron containing up to 4% C can be flame cut when a powder, flux, or filler rod is used as a supplemental oxidizing agent

Chromium

Steels containing up to 5% Cr can be flame cut without difficulty; steels with chromium content of 10% or more require metal powder, chemical flux, or plasma arc cutting

Cobalt

When present in the amounts normally used in steelmaking, cobalt has no noticeable effect on flame cutting

Copper

Up to 3% Cu has no effect on flame cutting

Manganese

Has no effect on flame cutting of carbon steels; steel containing 14% Mn and 1.5% C are difficult to cut and must be preheated

Molybdenum

Steels with up to 5% Mo can be cut easily; this is true of AISI 41XX steels; high molybdenum-tungsten steels require metal powder or plasma arc cutting

Nickel

Steels with up to 3% Ni and less than 0.25% C may be readily cut by OFC; up to 7% Ni requires flux additions to the oxygen stream; stainless steels, from 18-8 to 35-15 types, require chemical flux, metal powder, or plasma arc cutting

Phosphorus

The amount usually found in steel has no effect on flame cutting

Silicon

No effect in steels with up to 4% Si; in higher-silicon steels with high carbon and manganese contents, preheating and postannealing are usually needed to avoid hardening and cracking

Sulfur

Amounts usually found in steel have no effect; higher sulfur content slows cutting speed and emits sulfur dioxide fumes

Tungsten

Steels containing up to 14% W are readily flame cut, but cutting is more difficult with a higher percentage; high red-hardness tungsten steels are difficult to flame cut and require preheating

Vanadium

The amounts normally found in steel do not interfere with flame cutting

Preheating may consist of merely warming a cold workpiece with a torch or it may require furnace heating of the work beyond 540 °C (1000 °F). For some alloy steels, preheat temperatures are 200 to 315 °C (400 to 600 °F). Carbon steel billets and other sections occasionally are cut at 870 °C (1600 °F) and higher.

In oxyfuel gas cutting, preheating is accomplished by means of the oxyfuel gas flame, which surrounds the cutting oxygen stream. At cut initiation, the preheat flame, the result of oxygen and fuel gas combustion, brings a small amount of material to ignition temperature so that combustion can proceed. After cutting begins, the preheat flame merely adds heat to compensate for heat lost by convection and radiation or through gas exhausted during cutting. The flame also helps to remove or burn off scale and dirt on the plate surface; the hot, combusted gases protect the stream of cutting oxygen from the atmosphere. Preheating may also be applied over a broader area of the work. It may include soaking the entire workpiece in a furnace to bring it up to 100 to 200 °C (200 to 400 °F), or a simple overall warm-up with a torch to bring cold plate to room temperature. A preheat significantly improves cutting speed, allowing faster torch travel for greater productivity and

reduced consumption of fuel gas. Broader preheat smooths the temperature gradient between the base metal and the cut edge, possibly reducing thermal stress and minimizing hardening effects in some steels. Combustion of Gases Each cutting job entails a different type or volume of work to be completed. Consequently, the best gas for all cutting in a fabricating plant is found through experimentation. Evaluating a gas for a single job requires a test run that monitors fuel gas and oxygen flow rate, labor costs, overhead, and the amount of work performed. If plant production varies from week to week, gas performance should be measured over a long enough period to achieve an accurate cost analysis. Any of the fuel gases may perform well over a range of flow rates. When comparing gases, performance should be rated at the lowest flow rate that gives acceptable results for each gas. The most important preheat fuel gases are acetylene, natural gas, propane, propylene, and Mapp. Their properties are given in Table 2. These gases are hydrocarbons, which give off carbon dioxide and water vapor as the products of complete combustion. Flames of hydrocarbon gases are complex, displaying successive cones as a result of stepped chemical reactions. With acetylene, the products of complete combustion cannot exist at the temperature of the inner cone. Combustion is completed in the cooler, outer sheath of the flame. Chemical equations for combustion reactions of hydrocarbon gases often are simplified by treating the reactions as though the products were formed in only one step.
Table 2 Properties of common fuel gases
Acetylene Propane Propylene Methylacetylene-propadiene (Mapp) Natural gas

Chemical formula

C2H2

C3H8

C3H6

C3H4 propadiene)

(Methylacetylene,

CH4 (Methane)

Neutral flame temperature

°F

5,600

4,580

5,200

5,200

4,600

°C

3,100

2,520

2,870

2,870

2,540

Primary flame heat emission

Btu/ft3

507

255

433

517

11

MJ/m3

19

10

16

20

0.4

Secondary flame heat emission

Btu/ft3

963

2,243

1,938

1,889

989

MJ/m3

36

94

72

70

37

Total heat value (after vaporization)

Btu/ft3

1,470

2,498

2,371

2,406

1,000

MJ/m3

55

104

88

90

37

Total heat value (after vaporization)

Btu/lb

21,500

21,800

21,100

21,000

23,900

kJ/kg

50,000

51,000

49,000

49,000

56,000

Total oxygen required (neutral flame)

vol O2/vol fuel

2.5

5.0

4.5

4.0

2.0

Oxygen supplied through torch (neutral flame)

vol O2/vol fuel

1.1

3.5

2.6

2.5

1.5

ft3oxygen/lb fuel (60 °F)

16.0

30.3

23.0

22.1

35.4

m3oxygen/kg (15.6 °C)

1.0

1.9

1.4

1.4

2.2

Maximum allowable regulator pressure

psi

15

Cylinder

Cylinder

Cylinder

Line

kPa

103

Explosive limits in air, %

2.5-80

2.3-9.5

2.0-10

3.4-10.8

5.3-14

Volume-to-weight ratio

ft3/lb (60 °F)

14.6

8.66

8.9

8.85

23.6

m3/kg (15.6 °C)

0.91

0.54

0.55

0.55

1.4

Specific gravity of gas (60 °F, 15.6 °C)

Air = 1

0.906

1.52

1.48

1.48

0.62

Source: American Welding Society

Acetylene (C2H2) combustion produces a hot, short flame with a bright inner cone at each cutting-tip port; the hottest

point is at the tip of this inner cone. Combustion starts in the inner cone and is brought to completion in a cooler, blue, outer flame. The sharp distinction between the two flames helps to adjust the ratio of oxygen to acetylene. Depending on this ratio, the flame may be carburizing (reducing), neutral, or oxidizing. A neutral flame results when just enough oxygen is supplied for primary combustion, yielding carbon monoxide (CO) and hydrogen (H2). These products then combine with oxygen in ambient air to form the blue, outer flame, yielding carbon dioxide (CO2) and water (H2O). The neutral ratio of oxygen to acetylene is about 1 to 1, and the flame temperature at the tip of the inner cone is about 3040 °C (5500 °F). This flame is used for manual cutting. When the oxygen-to-acetylene ratio is reduced to about 0.9 to 1, a bright streamer begins to appear, and the flame becomes carburizing, or reducing. A carburizing flame is sometimes used for rough cutting of cast iron. When the oxygen-to-acetylene ratio is increased to more than 1 to 1, the inner cones are shorter, "necked in" at the sides, and more sharply defined; this flame is oxidizing. Flame temperature increases until, at a ratio of about 1.7 to 1, the temperature is maximum, or somewhat over 3095 °C (5600 °F) at the tip of the cones. An oxidizing flame can be used for preheating at the start of the cut, and for cutting very thick sections. According to the equation: 2C2H2 + 5O2 4CO2 + 2H2O

an oxygen-to-acetylene ratio of 2.5 to 1 is required for a complete reaction. For complete combustion, however, as much as 1.5 parts of oxygen is taken from ambient air. In oxyacetylene cutting, part of this oxygen may be supplied from the cutting oxygen, but total oxygen consumption is relatively low, an advantage of acetylene over all other fuel gases. Operation of oxyacetylene equipment in confined spaces, such as the inside of a closed tank or vessel, requires forced ventilation to supply the additional air needed for breathing and for flame combustion. Acetylene must be used at pressures below 105 kPa (15 psi), which is a stable operating range. Safety codes specify equipment and handling practices for acetylene. When supplied in special cylinders, acetylene is dissolved in acetone, which is contained in a porous mass that fills the cylinder. This technique eliminates the sensitivity of acetylene at pressures over 105 kPa (15 psi). Such cylinders can be filled to pressures exceeding 105 kPa (15 psi), but not greater than 1725 kPa (250 psi). Acetylene may also be supplied from generators. With either means of supply, safety regulations must be observed to avoid sudden decomposition and explosion. Despite some disadvantages, acetylene has been used for cutting for a longer time than any other gas. Its performance is well understood, equipment for it is perfected and widely marketed, and it is readily available. It has become the standard against which other gases are compared.
Natural gas is a mixture of gases, but consists principally of methane, and therefore is usually given the chemical

symbol for methane (CH4). One source defines the most widely used mixture as 85% methane (CH4), 4% ethane (C2H6), and 11% (N2, H2, O2, H2O). Some wells produce natural gas with large proportions of ethane and propane. The chemical equation for complete combustion: CH4 + 2O2 CO2 + 2H2O

indicates an oxygen-to-methane ratio of 2 to 1; this ratio is used for the preheat flame. Maximum flame temperature at the tip of the inner cones is about 2760 °C (5000 °F). Both higher and lower temperatures have been reported; also, the optimum oxygen-to-gas ratio is about 2 to 1. The flame is more diffuse than with acetylene; heat intensity is lower; and adjustment for carburizing, neutral, and oxidizing flame is less clearly defined. Initial cutting speeds are slower, and oxygen consumption is greater. Also, more time is required for preheating with natural gas than with acetylene. An excess of oxygen shortens preheat time, but increases consumption of oxygen. Furthermore, natural gas cannot be used for welding of steel, so extra installations are needed if this operation is to be performed. Despite these disadvantages, the use of natural gas for cutting has increased. It is the lowest-cost commercial fuel gas and, with careful torch adjustment, produces excellent cuts in light-to-heavy-gage material.

Neither acetylene nor natural gas accumulates in low pockets. When burned alone in air, the flame of natural gas does not produce soot.
Propane (C3H8) is a petroleum-base fuel usually supplied as a liquid in storage tanks from which it is drawn off as a gas.

The gas is dispensed from bulk storage tanks through pipelines. It has a narrow range of flammability and is relatively stable, but is heavier than air. Complete combustion requires an oxygen-to-propane ratio of 5 to 1. However, about 30% of the oxygen needed is taken from the ambient air. When the ratio of oxygen to propane is 4.5 to 1, the flame temperature is about 2760 °C (5000 °F) at the tip of the inner cones. At 4.25 to 1, the flame temperature is about 2650 °C (4800 °F). Flame properties are similar to those of natural gas, with respect to diffuseness, heat intensity, flame adjustment, and cutting speed. When burned alone in air, the flame is soot-free.
Propylene is a liquefied gas similar to propane. It has a higher flame temperature than propane. The flame temperature of propylene is about equal to Mapp gas, although its heat content is slightly less. On a volume basis, propylene is usually less expensive than acetylene; it does, however, consume more oxygen during combustion. The combustion equation for propylene is:

2C3H6 + 9O2

6CO2 + 6H2O

The combustion ratio for propylene is 4.5 to 1. Line oxygen for a neutral flame is about 3.5 to 1. Distributors sell propylene under various trade names, either pure or as improved mixtures with propane and other hydrocarbon additives.
Mapp gas (stabilized methylacetylene-propadiene) is a proprietary gas mixture; it is shipped and stored as a liquid, either in bulk storage tanks or in portable cylinders.

Both methylacetylene and propadiene have the chemical symbol C3H4 and by themselves are unstable, giving off their heat of formation during decomposition. As with acetylene, this heat is in addition to the heat of combustion. However, the methylacetylene-propadiene mixture in Mapp gas is stabilized by the addition of other hydrocarbons. The composition of Mapp gas is not disclosed, so the chemical equation for complete combustion in oxygen is not given. However, when the flame is neutral, the ratio of oxygen to fuel gas is about 2.3 to 1; the normal operating ratio for cutting varies from 2.5 to 1 to 4 to 1, depending on speed and thickness. Maximum flame temperature at the tips of the inner cones, reported as 2925 °C (5300 °F), occurs at oxygen-to-fuel ratios from 3.5 to 1 to 4 to 1. Flames can be adjusted for carburizing, neutral, or oxidizing conditions. Mapp gas is heavier than air, but it has a strong odor to reveal its presence in case it leaks or has collected in low pockets. At low temperatures, Mapp gas withdrawal rates from the cylinder are reduced. At about 0 °C (32 °F), methylacetylene has a vapor pressure of only 14 kPa (2 psi). Effect of Oxyfuel Cutting on Base Metal During the cutting of steel, the temperature of a narrow zone adjacent to the cut face is raised considerably above the transformation range. As the cut progresses, the steel cools through this range. The cooling rate depends on the heat conductivity and mass of the surrounding material, on loss of heat by radiation and convection, and on speed of cutting. When steel is at room temperature, the rate of cooling at the cut is sufficient to produce a quenching effect on the cut edges, particularly in heavier cuts in large masses of cold metal. Depending on the amount of carbon and alloying elements present and on the rate of cooling, pearlitic steel transforms into structures ranging from spheroidized carbides in ferrite to harder constituents. The heat-affected zone (HAZ) may be 0.8 to 6.4 mm ( to in.) deep for steels 9.5 to

150 mm ( to 6 in.) thick. Approximate depths of the HAZ in oxyfuel gas cut carbon steels are given in Table 3. Some increase in hardness usually occurs at the outer margin of the HAZ of nearly all steels.
Table 3 Approximate depths of HAZ in gas-cut carbon steels
Plate thickness HAZ depth

mm

in.

mm

in.

Low-carbon steels

<13 <

<0.8 <

13

0.8

150

6

1.4

High-carbon steels

<13 <

<0.8 <

13

0.8-1.6 -

150

6

1.4-6.0 -

Note: The depth of the fully hardened zone is considerably less than the depth of the HAZ. For most applications of gas cutting, the affected metal does not have to be removed.
Low-Carbon Steel. For steels containing 0.25% C or less, cut at room temperature, the hardening effect is usually

negligible, although at the upper carbon limit it may be significant if subsequent machining is required. Short of preheating or annealing the workpiece, hardening may be lessened by ensuring that the cutting flame is neutral to slightly oxidizing, the flame is burning cleanly, and the inner cones of the flame are at the correct height. By increasing the machining allowance slightly, the first cut usually can be made deep enough to penetrate below the hardened zone in most steels. Mechanical properties of low-carbon steels generally are not adversely affected by oxyfuel gas cutting.
Medium-Carbon Steels. Steels having carbon contents of 0.25 to 0.45% are affected only slightly by hardening

caused by oxyfuel gas cutting. Up to 0.30% C, steels with very low alloy content show some hardening of the cut edges, but generally not enough to cause cracking. Over 0.35% C, preheating to 260 to 315 °C (500 to 600 °F) is needed to avoid cracking. All medium-carbon steels should be preheated if the gas cut edges are to be machined.
High-Carbon and Alloy Steels. Gas cutting of higher-carbon (over 0.45% C) and hardenable alloy steels at room

temperature may produce, on the cut surface, a thin layer of hard, brittle material that is susceptible to cracking from the stress of cooling. The cooling stress that causes cracking is similar to the stress that causes distortion. Microcracks, or even incipient cracks, can be dangerous, because in service under tension they can develop into large fractures. The problems of hardening and the formation of residual stress can be alleviated by preheating and annealing.
Preheating serves three purposes. It:

• • •

Reduces the temperature gradient near the cut during cutting. This lowers differential expansion, which may cause distortion or upsetting of the metal. Metal upset during the heating cycle can produce excessive stress in cooling Increases the cutting speed and improves the surface of the cut, especially in heavier sections and in the difficultto-cut steels Reduces the cooling rate in the annealing range for the heat-affected portion of the cut during the cooling cycle.

By slower cooling, more ductile microstructures are obtained, and the formation of the hard martensitic structures is suppressed

If the higher-carbon and alloy steels are adequately preheated (and, in certain instances, annealed afterward), no cracks will occur. Ordinarily, a preheat temperature of 260 to 315 °C (500 to 600 °F) is sufficient for high-carbon steels; alloy steels may require preheating as high as 540 °C (1000 °F). Preheat temperature should be maintained during cutting. Thick preheated sections should be cut as soon as possible after the piece has been withdrawn from the furnace.
Local preheating involves heating that area of the workpiece that encloses what will become the HAZ of the cut. If the

area to be heated is small and the section is not too thick, the preheating flame of a cutting torch may be used, but usually a special heating torch is required. Local preheating is used when it is impossible or impractical to preheat the entire workpiece. It is important to heat the workpiece uniformly through the section to be cut, without causing too steep a temperature gradient. A multiflame heating torch is sometimes mounted ahead of the cutting torch in machine-guided cutting. Local preheating also can be accomplished using a preheat adaptor.
Annealing serves two main purposes in controlling the effects of gas cutting in carbon and low-alloy steels. It restores

the original structure of the steel, whether it be predominantly pearlitic or predominantly ferritic with spheroidized carbide, and it also provides stress relief. Many steels do not require annealing if they have been properly preheated. (See Heat Treating, Volume 4 of the ASM Handbook, for annealing practices for specific steels.)
Local annealing, also called flame annealing, is a localized postheat treatment that can be used to prevent hardening or

to soften an already hardened cut surface. Either the preheating flame of the cutting torch or a special heating torch may be used for local annealing, depending on the mass of the workpiece and the area to be covered. The heat-affected portion of the workpiece should be heated uniformly, and the temperature gradient at the boundary of the heated mass should be gradual enough to avoid distortion of the workpiece. Local annealing is not a substitute for preheating; it cannot correct damage done during cutting, such as upsetting of the metal or cracking at the cut edges. Local annealing is limited to steel plate up to 40 mm (1 in.) thick. From 40 to 75 mm

(1 to 3 in.) thick, heat should be applied to both sides of the plate. This method is not suitable for thicknesses over 75 mm (3 in.). If local annealing cannot be done simultaneously with cutting, the cut edges should be tempered after cutting with a suitable heating torch.
Stainless steels do not support oxyfuel combustion and therefore require metal powder cutting, chemical flux cutting,

or plasma arc cutting processes. Except for stabilized types, stainless steels degrade under the heat of metal powder or chemical flux processes. Carbide precipitation occurs in the HAZ about 3 mm ( in.) from the edge, where the metal has been heated to 425 to 870 °C (800 to 1600 °F) long enough for dissolved carbon to migrate to the grain boundaries and combine with the chromium to form chromium carbide. The chromium-poor (sensitized) regions near grain boundaries are subject to corrosion in service. This type of corrosion can be prevented by a stabilizing anneal, which puts the carbon back into solution. However, the required quench through the sensitizing temperature range may distort the material. Water quenching of the cut edge directly behind the cutting torch may avert sensitization. Because it takes about 2 min at sensitizing temperature for carbide precipitation to occur, water quenching must be done immediately. Distortion is more likely with this method than with the stabilizing anneal. Still another procedure is to remove the sensitizing zone entirely by chipping or machining.
Distortion, which is the result of heating by the gas flame, can cause considerable damage during cutting of thin plate

(<8 mm, or in., thick), cutting of long narrow widths, close-tolerance profile cutting, and cutting of plates that contain high residual stresses. The heat may release some of the locked-in stress, or may add new stress. In either case, deformation (warpage) may occur, thereby causing inaccurate finished cuts. Plates in the annealed condition have little or no residual stress.

Deformation. In cuts made from large plates, the cutting thermal cycle changes the shape of narrow sections and leaves

residual stress in the large section (see Fig. 2). The temperature gradient near the cut is steep, ranging from melting point at the cut to room temperature a short distance from it. The plate does not return to its original shape unless the entire plate is uniformly heated and cooled.

Fig. 2 Effects of oxyfuel gas cutting thermal cycle on shape of sections. (a) Plate with large restraint on one side of kerf, little restraint on the other side. Phantom lines indicate direction of residual stress that would cause deformation except for restraint. (b) Plate with little restraint on either side.

As the metal heats, it expands, and its yield strength decreases; the weakened heated material is compressed by the surrounding cooler, stronger metal. The hotter metal continues to expand elastically in all directions until its compressive yield strength is reached, at which point it yields plastically (upsets) in directions not under restraint. The portion of this upset metal at about 870 °C (1600 °F) is virtually stress-free; the remainder is under compressive stress that is equal to its yield strength. Metal that expands but does not upset is under compressive strength below yield. The net stress on the heated side of the neutral axis causes bowing of a narrow plate during cutting, as shown in Fig. 2. As the heated metal begins to cool, it contracts, and its strength increases. First, the contraction reduces the compressive stress in the still-expanded metal. When the compressive stress reaches zero and the plate regains its original shape, previously upset metal also has regained strength. This metal is now in tension as it cools, and its tensile yield strength increases. Tension increases until the metal reaches room temperature. Residual tensile stress in the cooling side of the neutral axis causes the bowing of narrow plates after cooling (Fig. 2). Controlled upsetting is the basis of flame straightening.
Control of Distortion. Preheating the workpiece can reduce distortion by reducing differential expansion, thereby

decreasing stress gradients. Careful planning of the cutting sequence also may help. For example, when trimming opposite sides of a plate, both sides should be cut in the same direction at the same time. When cutting rings, the inside diameter should be cut first; the remaining plate restrains the material for the outside-diameter cut. In general, the larger portion of material should be used to retain a shape for as long as possible; the cutting sequence should be balanced to maintain even-heat input and resultant residual stresses about the neutral axis of the plate or part. Equipment Commercial gases are usually stored in high-pressure cylinders. Natural gas--primarily methane--is supplied by pipeline from gas wells. The user taps into local gas lines. Acetylene, dissolved in acetone, is available in clay-filled cylinders. High-volume users often have acetylene generators on site. For heavy consumption or when many welding and cutting stations use fuel gas, banks of gas cylinders are maintained at a central location in the plant, and the gas is manifolded and piped to the point of use.

Manual gas cutting equipment consists of gas regulators, gas hoses, cutting torches, cutting tips, storage tanks, reverse flow check valves, and flashback arrestors. Auxiliary equipment may include a hand truck, tip cleaners, torch ignitors, and protective goggles. Machine cutting equipment varies from simple rail-mounted "bug" carriages to large bridgemounted torches that are driven by computer-directed drives.
Gas regulators reduce gas pressure and moderate gas flow rate between the source of gas and its entry into the cutting

torch to deliver gas to the cutting apparatus at the required operating pressure. Gas enters the regulating device at a wide range of pressures. Gas flows through the regulator and is delivered to the hose-torch-tip system at the operating pressure, which is preset by manual adjustment at the regulator and at the torch. When pressure at the regulator drops below the preset pressure, regulator valves open to restore pressure to the required level. During cutting, the regulator maintains pressure within a narrow range of the pressure setting. Regulators should be selected for use with specific types of gas and for specific pressure ranges. Portable oxyacetylene equipment requires an oxygen regulator on the oxygen cylinder and an acetylene regulator on the acetylene cylinder, which are not interchangeable.
High-low regulators conserve preheat oxygen when natural gas or propane is the preheat fuel used in oxyfuel gas

cutting. These gases require a longer time to start a cut than do acetylene or Mapp gas. High-low regulators reduce preheat flow to a predetermined level when the flow of cutting oxygen is initiated. When the regulator switches from high to low, preheat cutback may range from 75 to 25% as plate thicknesses increase from 9.5 to 200 mm ( to 8 in.). Highlow regulators are used for manual and automatic cutting with natural gas propane and liquefied petroleum gas (LPG).
Hose. Flexible hose, usually 3 to 13 mm (

to in.) in diameter, rated at 1380 kPa (200 psig) maximum, carries gas from the regulator to the cutting torch. Oxygen hoses are green; the fittings have right-hand threads. Fuel gas hoses are red; the fittings have left-hand threads and a groove cut around the fitting. For heavy cutting, two oxygen hoses may be necessary, one for preheat and one for cutting oxygen. Multiple-torch cutting machines often have three-hose torches.

Cutting torches, such as the one shown in Fig. 3, control the mixture and flow of preheat oxygen and fuel gas and the

flow of cutting oxygen. The cutting torch discharges these gases through a cutting tip at the proper velocity and flow rate. Pressure of the gases at the torch inlets, as well as size and design of the cutting tip, limits these functions, which are operator controlled.

Fig. 3 (a) Typical manual cutting torch in which preheat gases are mixed before entering torch head. (b) and (c) Sections through preheat gas duct showing two types of mixers commonly used with the torch shown. After the workpiece is sufficiently preheated, the operator depresses the lever to start the flow of cutting oxygen. Valves control the flow of oxygen and fuel gas to achieve the required flow and mixture at the cutting tip.

Oxygen inlet control valves and fuel gas inlet control valves permit operator adjustment of gas flow. Fuel gas flows through a duct and mixes with the preheat oxygen; the mixed gases then flow to the preheating flame orifices in the

cutting tip. The oxygen flow is divided: A portion of the flow mixes with the fuel gas, and the remainder flows through the cutting-oxygen orifice in the cutting tip. A lever-actuated valve on the manual torch starts the flow of cutting oxygen; machine cutting starts the oxygen from a panel control. Fuel gases supplied at low pressure (usually below 21 kPa, or 3 psi), such as natural gas tapped from a city line, require an injector-mixer (Fig. 3b) to increase fuel gas flow above normal operating pressures. Optimum torch performance relies on proper matching of the mixer to the available fuel gas pressure.
Cutting tips are precision-machined nozzles, produced in a range of sizes and types. Figure 4(a) shows a single-piece acetylene cutting tip. A two-piece tip used for natural gas (methane) or LPG is shown in Fig. 4(b). A tip nut holds the tip in the torch. For a given type of cutting tip, the diameters of the central hole, the cutting-oxygen orifice, and the preheat ports increase with the thickness of the metal to be cut. Cutting tip selection should match the fuel gas; hole diameters must be balanced to ensure an adequate preheat-to-cutting-oxygen ratio. Preheat gas flows through ports that surround the cutting-oxygen orifice. Smoothness of bore and accuracy of size and shape of the oxygen orifice are important to efficiency. Worn, dirty bores reduce cut quality by causing turbulence in the cutting-oxygen stream.

Fig. 4 Types of cutting tips. (a) Single-piece acetylene cutting tip. (b) Two-piece tip for natural gas or LPG. Fuel gas and preheat oxygen mix in tip. Recessed bore promotes laminar flow of gas and anchors the flame when natural gas or propane is used.

The size of the cutting-tip orifice determines the rate of flow and velocity of the preheat gases and cutting oxygen. Flow to the cutting tip can be varied by adjustment at the torch inlet valve or at the regulator, or both. Increasing cutting-oxygen flow solely by increasing the oxygen pressure results in turbulence and reduces cutting efficiency. Turbulence in the cutting oxygen causes wide kerfs, slows cutting, increases oxygen consumption, and lowers quality of cut. Consequently, larger cutting tips are required for making heavier cuts. Standard tips, as shown in Fig. 5(a), have a straight-bore oxygen port. Oxygen pressures range from 200 to 400 kPa (30 to 60 psi) and are used for manual cutting. High-speed tips, or divergent cutting tips (Fig. 5b), use a converging, diverging orifice to achieve high gas velocities. The oxygen orifice flares outward. High-speed tips operate at cutting-oxygen pressures of about 700 kPa (100 psi) and provide cutting jets of supersonic velocity. These tips are precision made and are more costly than straight-drilled tips, but they produce superior results: improved edge quality and cutting speeds 20%

higher than standard tips. Best suited to machine cutting, high-speed tips produce superior cuts in plate up to about 150 mm (6 in.) thick. Above this thickness, advantages of their use decrease, and they are not recommended for cutting metal more than 250 mm (10 in.) thick.

Fig. 5 Oxyfuel cutting tips. (a) Standard cutting tip with straight-bore oxygen orifice. (b) High-speed cutting tip with divergent-bore oxygen orifice.

Equipment Selection Factors. Natural gas and liquefied petroleum gases operate most efficently with high-low gas

regulators; injector-type cutting torches; and two-piece, divergent, recessed cutting tips. Acetylene cutting is most efficient with divergent single-piece tips. If acetylene is supplied by low-pressure generators, an injector-type torch is ideally suited to most cutting applications. Two-piece divergent cutting tips are best suited for use with Mapp gas; the tip recess should be less than one used for natural gas or propane. Injector-type torches and high-low regulators are not required with Mapp gas.
Guidance Equipment. In freehand cutting, the operator can usually follow a layout accurately at low speeds, but the

cut edges may be ragged. For accurate manual cutting at speeds over 250 mm/min (10 in./min), the torch tip should be guided with a metal straightedge or template. Circles and arcs are cut smoothly with the aid of a radius bar, a light rod clamped and adjusted to the torch at one end, while the other end is held at the circle center. Machine guidance equipment includes magnetic tracing of a metal template, manual spindle tracing, optical tracing of a line drawing, guidance by numerically controlled tape or by programmable controllers, and computer-programmed guidance equipment (Fig. 6).

Fig. 6 Gantry shape cutting system. (a) CNC-controlled cutting tool incorporating oxyfuel torches, plasma arc torches, 90° indexing triple-torch oxyfuel stations for straight-line beveling, and zinc powder or punch markers. (b) Close-up of CNC control console. Courtesy of ESAB North America, Inc.

Portable cutting machines are used primarily for straight-line and circular cutting. Components include a torch

mounted on a motor-driven carriage that travels on a track or other torch guidance device. The operator adjusts travel speed and monitors the operation. Machine cutting torches are of heavy construction of in-line design. The torch casing has a rack, which fits into a gear on the torch holder, for raising and lowering the torch over the work. Ducts and valves are encased in a single tube. The cutting tip is mounted axially with the tube. A valve knob or a lever-operated poppet valve replaces the spring-loaded cutting-oxygen lever of the manual torch. On some portable machines, gases are supplied to connections on the carriage, rather than directly to the torch to avoid hose drag on the torch. Short hoses are used from machine connections to the torch. Some carriages can accommodate two or more torches operating simultaneously, for such operations as squaring and beveling. The operator follows the carriage to make adjustments. When plates are wavy or distorted, the operator may need to adjust torch height to avoid losing the cut. When carefully operated, track-guided torches can produce cuts at speeds and quality approaching those obtainable with stationary cutting machines.
Stationary cutting machines, as shown in Fig. 6 and 7, are used for straight-line and circular cuts, but their primary

use is for cutting complex parts, that is, for cutting shapes. Plate to be cut is moved to the machine.

Fig. 7 Stationary oxyfuel gas cutting machine.

On shape-cutting machines, cutting torches move left and right on a bridge mounted over the cutting table. The bridge moves back and forth on supports that ride on floor-mounted tracks. The combined movement of the torches on the bridge and the bridge on the track allows the torch to cut any shape in the x-y plane. Bridges are either of cantilever or gantry design. Suppliers classify cutting-machine capacity by the maximum width of plate that can be cut.
Machine Directions. Methods for directing the motion of shape cutting machines have become increasingly

sophisticated and include manual, magnetic, and electronic means of control. The simplest machines have one or two torches and use manual or magnetic tracing. For manual tracing, the operator either steers an idler wheel or spindle around a template or guides a wheel or focused light beam around an outline on paper. Cutting speed is controlled by setting the speed of the tracing head (pantograph director) or by setting the speed of the torch carriage (coordinate drive). Cutting speed in manual tracing is about 350 mm/min (14 in./min), depending on operator skill. Magnetic tracing is done with a knurled magnetized spindle that rotates against the edge of a steel template. The spindle is linked to a pantograph. Direct-reading tachometers, showing cutting speed in inches per minute, assist in adjusting cutting speed. These control methods are relatively slow. Faster, electronic tracers use a photo-electric cell that scans the reflection of a beam of light directed on the outline of a template. Templates are line drawings on paper, white-on-black paper cutouts, or photonegatives of a part outline. To hold tolerances closer than in. continuously, templates of plastic film, glass cloth, or some other durable, dimensionally stable material should be used. In scanning the edge of a white-on-black template, the circuit through the photoelectric cell balances when the cell senses an equal amount of black and white. A change in this balance sends an impulse to a motor that moves the tracing head back to balance. In line tracing, the photoelectric cell scans the line from side to side. As long as the light reflects equally from both sides of the line, the steering signals balance. When the photocell scans more light on one side of the line than on the other, the scanner rotates to balance.

Some machines adjust to permit parts to be cut about in. larger or smaller than the template. This feature, called kerf compensation, is useful for cutting to close tolerances, especially when the template has insufficient kerf allowance. Coordinate-drive machines translate motion 1 to 1 or in other ratios. Such ratio cutting permits the use of templates in any proportion, from full-scale to one-tenth of part size.
Tape Control. Cutting machine movement may be controlled by electronic signals from punched tape (numerical control). These machines do not require templates, and the tape may be easily stored and used many times.

Some cutting machines receive directions from a microprocessor, programmed directly or from punched tape. The most sophisticated machines take directions from a computer (computerized numerical control, or CNC) and use computer graphics (Fig. 6). Nesting of Shapes Savings in material, labor, and gas consumption can be gained by nesting parts in the stock layout for single-torch or multiple-torch operation. Savings can be realized whenever one cut can be made instead of two. Sometimes a shape can be modified for better nesting. The advent of computer graphics allows cutting-machine programmers to create layouts of part patterns on cathode ray tube screens (Fig. 8), manipulating cutting patterns for greatest plate use. Several firms offer programs that closely optimize parts nesting.

Fig. 8 Parts programming system for nesting of shapes. Layouts of part patterns can be performed on-screen using such a system, resulting in optimum material use. Courtesy of ESAB North America, Inc.

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