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 eitherby chemical reaction (oxidation) or melting (heat from arc). All cutting processes result in the severing or removal ofmetals. Additional information on cutting processes used in metal-forming operations can be found in the articles "LaserCutting" and "Abrasive Waterjet Cutting" in this Volume.

Oxygen cutting is accomplished through a chemical reaction in which preheated metal is cut, or removed, by rapidoxidation in a stream of pure oxygen. Typical oxygen cutting processes are oxyfuel gas, oxygen lance, chemical flux, andmetal powder cutting. Oxyfuel gas cutting and its modifications, chemical flux cutting and metal powder cutting, whichare 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, arccutting can be used to cut almost any metal. Modifications of the process include the use of compressed gases to causerapid oxidation (or to prevent oxidation) of the workpiece, thus incorporating aspects of the gas cutting process. Arccutting methods include air carbon arc, gas metal arc, gas tungsten arc, shielded metal arc, plasma arc, and oxygen arccutting. The methods of industrial importance that are covered in this article include plasma arc cutting, air carbon arccutting, 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 preheatedmetal 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 burningor flame cutting, can cut carbon and low-alloy plate of virtually any thickness. Castings more than 750 mm (30 in.) thickcommonly are cut by the oxyfuel process. With oxidation-resistant materials, such as stainless steels, either a chemicalflux or metal powder is added to the oxygen stream to promote the exothermic reaction. Equipment for such cutting issomewhat 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 flowregulators and gages, gas supply hoses, and a cutting torch with a set of exchangeable cutting tips. Such manuallyoperated equipment is portable and inexpensive. Cutting machines, employing one or several cutting torches guided bysolid template pantographs, optical line tracers, numerical controls, or computers, improve production rates and providesuperior cut quality. Machine cutting is important for profile cutting, that is, the cutting of regular and irregular shapesfrom 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 generatinglarge amounts of heat. This heat supports the continued oxidation of the metal as the cut progresses. Combusted gas andthe pressurized oxygen jet flush the molten oxide away, exposing fresh surfaces for cutting. The metal in the path of theoxygen 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 pressureregulators, adjusted by the operator. The cutting torch contains ducts, a mixing chamber, and valves to supply an oxyfuelgas mixture of the proper ratio for preheat and a pure oxygen stream for cutting to the torch tip. By adjusting the controlvalves 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 machinecutting, oxygen is normally controlled by the operator at a remote station or by numerical control. Cutting tips have asingle cutting oxygen orifice centered within a ring of smaller oxyfuel gas exit ports. The operator changes the cuttingcapacity of the torch by changing the cutting tip size and by resetting pressure regulators and control valves. Becausedifferent fuel gases have different combustion and flow characteristics, the construction of cutting tips, and sometimes ofmixing 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 ofthe 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 cutincreases, more heat is needed to keep the metal at its ignition temperature. Increasing the preheat gas flow and reducingthe 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 reactionat 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 surroundingatmosphere. 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 timerequired 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 factorsbesides those mentioned. Oxyfuel gas cutting involves control of more than twenty variables. Suppliers of cuttingequipment 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 typeand condition (scale, oil, dirt, flatness) of material, thickness of cut, type of fuel gas, and quality and angle of cut. (Whennot otherwise defined, a cut is usually taken to mean a through or "drop" cut, made in horizontal plates with the cutting tipin the vertical position.)

Higher cutting speeds with good cut quality are obtained during the oxyfuel process using a special tip and torchconfiguration that provides a curtain of oxygen around the cutting oxygen. The protective curtain maintains a higher levelof 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 tominimize the kerf (the width of metal removed by cutting) and to increase the smoothness of the cut edge. Carefulbalancing of all cutting variables helps attain a narrow kerf and smooth edge. The thicker the work material, the greaterthe 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 andsome nonferrous metals can be oxyfuel gas cut, although process modification may be required and cut quality may notbe as high as is obtained in cutting the more widely used grades of steel. High-alloy steels, stainless steels, cast iron, andnickel alloys do not readily oxidize and therefore do not provide enough heat for a continuous reaction. As the carbon andalloy contents of the steel to be cut increase, preheating or postheating, or both, often are necessary to overcome the effectof 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 oxidefilms. 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, manufactureof earth-moving equipment, machinery construction, and the fabrication of pressure vessels and storage tanks. Manymachine structures originally made from forgings and castings can be made at less cost by redesigning them for oxyfuelgas cutting and welding, with the advantages of quick delivery of plate material from steel suppliers, low cost of oxyfuelgas 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 salvageoperations. In steel mills and foundries, projections such as caps, gates, and risers can be severed from billets andcastings. Mechanical fasteners can be quickly cut for disassembly using oxyfuel gas cutting. Holes can be made in steelcomponents by piercing and cutting. Machine oxyfuel gas cutting is used to cut steel plate to size, to cut various shapesfrom 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-carbonsteel or other metal that hardens by rapid cooling, the hardening effect must be considered, especially if the workpiece isto 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 ( in.)

thick often is stacked for cutting of several parts in a single torch pass. Procedures for light cutting (<9.5 mm, or in.

thick), medium cutting (9.5 to 250 mm, or to 10 in. thick), heavy cutting (>250 mm, or 10 in. thick), and stack cuttingare 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 cuttingis compared to other cutting operations such as arc cutting, milling, shearing, or sawing. The advantages of the oxyfuelprocess are:

• Metal can be cut faster. Setup is generally simpler and faster than is the case for machining and about equal tothat 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 indie-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 isportable 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 costlymachining 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 mechanicalproperties

• 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 issought. 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 isrelatively large and sharply coherent, the rate of cutting through the depth of the cut is not affected; that is, the cuttingface 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 moreslowly. As a result, the cutting face will become curved, as shown in Fig. 1. The horizontal distance between the points ofentry 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 efficientlyused. Drag is a rough measure of cutting quality and of economy in oxygen consumption. In metal thicknesses up to 50 or75 mm (2 or 3 in.), a 10 to 15% drag is associated with good quality of cut and good economy. Higher quality demandsless drag; more drag indicates poorer quality and low oxygen consumption. Too much drag may lead to incompletecutting.

In very thin sections, drag has little meaning; the main problem is control of high heat input compared to low heat sink. Invery thick sections, the opposite is true; the problem is to avoid excessive drag. All the input variables controlled by theoperator (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 least99.5% pure. A 0.5% departure from this purity (99% O2) decreases the cutting efficiency. At 90% purity, cutting is verydifficult, and at lower purities it is often impossible. The impurities consist of inert gases and water vapor. The effectivepurity of oxygen can also be reduced by gaseous combustion products from the preheat flames and from the metal beingcut.

Alloying of iron affects oxyfuel gas cutting, usually by reducing the rate of oxidation. The total alloy content in low-alloy steel usually does not exceed 5%, and the effect on cutting speed is slight. Alloying elements affect oxyfuel gascutting of steel in two ways. They may make the steel more difficult to cut, or they harden the cut edge, or both. In highlyalloyed steel, the oxidizing characteristics of alloying elements and the constituents formed in alloying may makesustained oxidation difficult or even impossible. The effects of alloying elements on cutting are evaluated in Table 1. Inany steel, preheat accelerates the chemical reaction; higher alloy steels, therefore, may need preheating beyond thatprovided 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 inamounts 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 preventhardening 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 morerequire 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 mustbe preheated

Molybdenum Steels with up to 5% Mo can be cut easily; this is true of AISI 41XX steels; high molybdenum-tungsten steelsrequire 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 tothe oxygen stream; stainless steels, from 18-8 to 35-15 types, require chemical flux, metal powder, or plasma arccutting

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, preheatingand 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 dioxidefumes

Tungsten Steels containing up to 14% W are readily flame cut, but cutting is more difficult with a higher percentage; highred-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 workbeyond 540 °C (1000 °F). For some alloy steels, preheat temperatures are 200 to 315 °C (400 to 600 °F). Carbon steelbillets 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 cuttingoxygen stream. At cut initiation, the preheat flame, the result of oxygen and fuel gas combustion, brings a small amountof material to ignition temperature so that combustion can proceed. After cutting begins, the preheat flame merely addsheat to compensate for heat lost by convection and radiation or through gas exhausted during cutting. The flame alsohelps to remove or burn off scale and dirt on the plate surface; the hot, combusted gases protect the stream of cuttingoxygen from the atmosphere.

Preheating may also be applied over a broader area of the work. It may include soaking the entire workpiece in a furnaceto 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 roomtemperature. 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 cutedge, 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 afabricating plant is found through experimentation. Evaluating a gas for a single job requires a test run that monitors fuelgas and oxygen flow rate, labor costs, overhead, and the amount of work performed. If plant production varies from weekto week, gas performance should be measured over a long enough period to achieve an accurate cost analysis. Any of thefuel gases may perform well over a range of flow rates. When comparing gases, performance should be rated at the lowestflow 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 offcarbon 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 completecombustion cannot exist at the temperature of the inner cone. Combustion is completed in the cooler, outer sheath of theflame. Chemical equations for combustion reactions of hydrocarbon gases often are simplified by treating the reactions asthough 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 (Methylacetylene,propadiene)

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 (neutralflame)

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

kPa 103

Cylinder Cylinder Cylinder Line

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 hottestpoint 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 justenough oxygen is supplied for primary combustion, yielding carbon monoxide (CO) and hydrogen (H2). These productsthen 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 about3040 °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 flamebecomes 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, thetemperature is maximum, or somewhat over 3095 °C (5600 °F) at the tip of the cones. An oxidizing flame can be used forpreheating 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 muchas 1.5 parts of oxygen is taken from ambient air. In oxyacetylene cutting, part of this oxygen may be supplied from thecutting 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 forcedventilation 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 specifyequipment 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 atpressures over 105 kPa (15 psi). Such cylinders can be filled to pressures exceeding 105 kPa (15 psi), but not greater than1725 kPa (250 psi). Acetylene may also be supplied from generators. With either means of supply, safety regulationsmust 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 iswell understood, equipment for it is perfected and widely marketed, and it is readily available. It has become the standardagainst which other gases are compared.

Natural gas is a mixture of gases, but consists principally of methane, and therefore is usually given the chemicalsymbol 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 thetip of the inner cones is about 2760 °C (5000 °F). Both higher and lower temperatures have been reported; also, theoptimum oxygen-to-gas ratio is about 2 to 1. The flame is more diffuse than with acetylene; heat intensity is lower; andadjustment for carburizing, neutral, and oxidizing flame is less clearly defined. Initial cutting speeds are slower, andoxygen consumption is greater. Also, more time is required for preheating with natural gas than with acetylene. An excessof oxygen shortens preheat time, but increases consumption of oxygen. Furthermore, natural gas cannot be used forwelding 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 notproduce 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 relativelystable, 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 flametemperature 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, flameadjustment, 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 temperatureof propylene is about equal to Mapp gas, although its heat content is slightly less. On a volume basis, propylene is usuallyless expensive than acetylene; it does, however, consume more oxygen during combustion. The combustion equation forpropylene 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 sellpropylene 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 theirheat 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 compositionof Mapp gas is not disclosed, so the chemical equation for complete combustion in oxygen is not given. However, whenthe flame is neutral, the ratio of oxygen to fuel gas is about 2.3 to 1; the normal operating ratio for cutting varies from 2.5to 1 to 4 to 1, depending on speed and thickness. Maximum flame temperature at the tips of the inner cones, reported as2925 °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), methylacetylenehas 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 thetransformation range. As the cut progresses, the steel cools through this range. The cooling rate depends on the heatconductivity 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 cutedges, particularly in heavier cuts in large masses of cold metal. Depending on the amount of carbon and alloyingelements 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. Someincrease 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, theaffected 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 usuallynegligible, although at the upper carbon limit it may be significant if subsequent machining is required. Short ofpreheating or annealing the workpiece, hardening may be lessened by ensuring that the cutting flame is neutral to slightlyoxidizing, the flame is burning cleanly, and the inner cones of the flame are at the correct height. By increasing themachining allowance slightly, the first cut usually can be made deep enough to penetrate below the hardened zone in moststeels. 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 hardeningcaused 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 avoidcracking. 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 roomtemperature may produce, on the cut surface, a thin layer of hard, brittle material that is susceptible to cracking from thestress 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 largefractures. 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 maycause distortion or upsetting of the metal. Metal upset during the heating cycle can produce excessive stress incooling

• Increases the cutting speed and improves the surface of the cut, especially in heavier sections and in the difficult-to-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 structuresis suppressed

If the higher-carbon and alloy steels are adequately preheated (and, in certain instances, annealed afterward), no crackswill occur. Ordinarily, a preheat temperature of 260 to 315 °C (500 to 600 °F) is sufficient for high-carbon steels; alloysteels 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 thearea to be heated is small and the section is not too thick, the preheating flame of a cutting torch may be used, but usuallya 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 theworkpiece uniformly through the section to be cut, without causing too steep a temperature gradient. A multiflameheating torch is sometimes mounted ahead of the cutting torch in machine-guided cutting. Local preheating also can beaccomplished using a preheat adaptor.

Annealing serves two main purposes in controlling the effects of gas cutting in carbon and low-alloy steels. It restoresthe original structure of the steel, whether it be predominantly pearlitic or predominantly ferritic with spheroidizedcarbide, and it also provides stress relief. Many steels do not require annealing if they have been properly preheated. (SeeHeat 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 orto soften an already hardened cut surface. Either the preheating flame of the cutting torch or a special heating torch maybe used for local annealing, depending on the mass of the workpiece and the area to be covered. The heat-affected portionof the workpiece should be heated uniformly, and the temperature gradient at the boundary of the heated mass should begradual 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 75mm (3 in.). If local annealing cannot be done simultaneously with cutting, the cut edges should be tempered after cuttingwith 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 hasbeen heated to 425 to 870 °C (800 to 1600 °F) long enough for dissolved carbon to migrate to the grain boundaries andcombine with the chromium to form chromium carbide. The chromium-poor (sensitized) regions near grain boundariesare subject to corrosion in service. This type of corrosion can be prevented by a stabilizing anneal, which puts the carbonback 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 atsensitizing temperature for carbide precipitation to occur, water quenching must be done immediately. Distortion is morelikely with this method than with the stabilizing anneal. Still another procedure is to remove the sensitizing zone entirelyby 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 containhigh 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 orno residual stress.

Deformation. In cuts made from large plates, the cutting thermal cycle changes the shape of narrow sections and leavesresidual stress in the large section (see Fig. 2). The temperature gradient near the cut is steep, ranging from melting pointat the cut to room temperature a short distance from it. The plate does not return to its original shape unless the entireplate is uniformly heated and cooled.

Fig. 2 Effects of oxyfuel gas cutting thermal cycle on shape of sections. (a) Plate with large restraint on oneside of kerf, little restraint on the other side. Phantom lines indicate direction of residual stress that wouldcause 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 thesurrounding cooler, stronger metal. The hotter metal continues to expand elastically in all directions until its compressiveyield strength is reached, at which point it yields plastically (upsets) in directions not under restraint. The portion of thisupset metal at about 870 °C (1600 °F) is virtually stress-free; the remainder is under compressive stress that is equal to itsyield strength. Metal that expands but does not upset is under compressive strength below yield. The net stress on theheated 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 compressivestress 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 strengthincreases. Tension increases until the metal reaches room temperature. Residual tensile stress in the cooling side of theneutral axis causes the bowing of narrow plates after cooling (Fig. 2). Controlled upsetting is the basis of flamestraightening.

Control of Distortion. Preheating the workpiece can reduce distortion by reducing differential expansion, therebydecreasing stress gradients. Careful planning of the cutting sequence also may help. For example, when trimmingopposite sides of a plate, both sides should be cut in the same direction at the same time. When cutting rings, the insidediameter should be cut first; the remaining plate restrains the material for the outside-diameter cut. In general, the largerportion of material should be used to retain a shape for as long as possible; the cutting sequence should be balanced tomaintain 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 pipelinefrom 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 cuttingstations use fuel gas, banks of gas cylinders are maintained at a central location in the plant, and the gas is manifolded andpiped to the point of use.

Manual gas cutting equipment consists of gas regulators, gas hoses, cutting torches, cutting tips, storage tanks, reverseflow 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 bridge-mounted 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 cuttingtorch to deliver gas to the cutting apparatus at the required operating pressure. Gas enters the regulating device at a widerange 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 thepreset pressure, regulator valves open to restore pressure to the required level. During cutting, the regulator maintainspressure 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 oxyacetyleneequipment 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 gascutting. These gases require a longer time to start a cut than do acetylene or Mapp gas. High-low regulators reducepreheat 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.). High-low 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 gasfrom the regulator to the cutting torch. Oxygen hoses are green; the fittings have right-hand threads. Fuel gas hoses arered; the fittings have left-hand threads and a groove cut around the fitting. For heavy cutting, two oxygen hoses may benecessary, 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 theflow 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 areoperator 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. Afterthe 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 flowsthrough 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 throughthe 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, requirean injector-mixer (Fig. 3b) to increase fuel gas flow above normal operating pressures. Optimum torch performance relieson 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-pieceacetylene cutting tip. A two-piece tip used for natural gas (methane) or LPG is shown in Fig. 4(b). A tip nut holds the tipin the torch. For a given type of cutting tip, the diameters of the central hole, the cutting-oxygen orifice, and the preheatports increase with the thickness of the metal to be cut. Cutting tip selection should match the fuel gas; hole diametersmust be balanced to ensure an adequate preheat-to-cutting-oxygen ratio. Preheat gas flows through ports that surround thecutting-oxygen orifice. Smoothness of bore and accuracy of size and shape of the oxygen orifice are important toefficiency. 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. Fuelgas and preheat oxygen mix in tip. Recessed bore promotes laminar flow of gas and anchors the flame whennatural 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. Flowto 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 cuttingefficiency. Turbulence in the cutting oxygen causes wide kerfs, slows cutting, increases oxygen consumption, and lowersquality 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 to60 psi) and are used for manual cutting. High-speed tips, or divergent cutting tips (Fig. 5b), use a converging, divergingorifice to achieve high gas velocities. The oxygen orifice flares outward. High-speed tips operate at cutting-oxygenpressures of about 700 kPa (100 psi) and provide cutting jets of supersonic velocity. These tips are precision made and aremore 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 150mm (6 in.) thick. Above this thickness, advantages of their use decrease, and they are not recommended for cutting metalmore than 250 mm (10 in.) thick.

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

Equipment Selection Factors. Natural gas and liquefied petroleum gases operate most efficently with high-low gasregulators; injector-type cutting torches; and two-piece, divergent, recessed cutting tips. Acetylene cutting is mostefficient with divergent single-piece tips. If acetylene is supplied by low-pressure generators, an injector-type torch isideally 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 fornatural 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 thecut edges may be ragged. For accurate manual cutting at speeds over 250 mm/min (10 in./min), the torch tip should beguided with a metal straightedge or template. Circles and arcs are cut smoothly with the aid of a radius bar, a light rodclamped 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 aline drawing, guidance by numerically controlled tape or by programmable controllers, and computer-programmedguidance equipment (Fig. 6).

Fig. 6 Gantry shape cutting system. (a) CNC-controlled cutting tool incorporating oxyfuel torches, plasma arctorches, 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 torchmounted on a motor-driven carriage that travels on a track or other torch guidance device. The operator adjusts travelspeed 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 onthe torch holder, for raising and lowering the torch over the work. Ducts and valves are encased in a single tube. Thecutting tip is mounted axially with the tube. A valve knob or a lever-operated poppet valve replaces the spring-loadedcutting-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 avoidhose drag on the torch. Short hoses are used from machine connections to the torch. Some carriages can accommodatetwo 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 toadjust torch height to avoid losing the cut. When carefully operated, track-guided torches can produce cuts at speeds andquality 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 primaryuse 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 bridgemoves back and forth on supports that ride on floor-mounted tracks. The combined movement of the torches on the bridgeand the bridge on the track allows the torch to cut any shape in the x-y plane. Bridges are either of cantilever or gantrydesign. 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 increasinglysophisticated and include manual, magnetic, and electronic means of control. The simplest machines have one or twotorches 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 focusedlight beam around an outline on paper. Cutting speed is controlled by setting the speed of the tracing head (pantographdirector) or by setting the speed of the torch carriage (coordinate drive). Cutting speed in manual tracing is about 350mm/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 islinked to a pantograph. Direct-reading tachometers, showing cutting speed in inches per minute, assist in adjusting cuttingspeed. 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 atemplate. 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 sensesan equal amount of black and white. A change in this balance sends an impulse to a motor that moves the tracing headback to balance. In line tracing, the photoelectric cell scans the line from side to side. As long as the light reflects equallyfrom both sides of the line, the steering signals balance. When the photocell scans more light on one side of the line thanon 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 kerfcompensation, 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 anyproportion, from full-scale to one-tenth of part size.

Tape Control. Cutting machine movement may be controlled by electronic signals from punched tape (numericalcontrol). 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 mostsophisticated machines take directions from a computer (computerized numerical control, or CNC) and use computergraphics (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 ormultiple-torch operation. Savings can be realized whenever one cut can be made instead of two. Sometimes a shape canbe modified for better nesting. The advent of computer graphics allows cutting-machine programmers to create layouts ofpart patterns on cathode ray tube screens (Fig. 8), manipulating cutting patterns for greatest plate use. Several firms offerprograms that closely optimize parts nesting.

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