Oxyfuel Gas Cutting

14Oxyfuel Gas Cutting

Chapter

Permission granted to reproduce for educational use only.© Goodheart-Willcox Co., Inc.

Objectives• Describe the function of each component of an

oxyfuel gas cutting outfit.• Correctly and safely assemble an oxyfuel gas cutting

outfit.• Correctly and safely test the cutting outfit for leaks.• Describe the difference between a positive-pressure

and injector-type cutting torch.• Properly turn on and shut down an oxyacetylene

cutting outfit.• Correctly select the proper cutting tip and working

pressures for cutting a particular thickness of steel.


Objectives• Cut base metal using an oxyacetylene cutting torch.• Distinguish between a well cut surface and a poorly

cut surface, and be able to adjust where necessary to perform a good cut.

• Describe the gouging process and state when it is used.

• Pass a test on safe practices and procedures while oxyfuel gas cutting or gouging.

• List all the safety equipment that should be worn and used when oxyfuel gas cutting in various positions.


Oxyfuel Gas Cutting

• An oxyfuel gas flame is used to heat the metal and an oxygen jet is used to perform the cutting

• Oxyfuel gas cutting (OFC) is also called oxygen cutting

• Many layers of metal can be cut at the same time (stack cutting)

• OFC is useful when shape-cutting metal parts


The Heat of Combustion of Steel

• Virtually all materials burn if they are heated to their ignition temperature in the presence of oxygen

• During the process of burning, steel releases heat that is measured in British thermal units (Btu) or joules

• This is called the heat of combustion


Oxyfuel Gas Cutting Process

• A special torch and tip is used in the OFC process• Cutting tips have one or more preheating orifices• Cutting oxygen emits from the central orifice when

the welder presses the cutting oxygen lever

(Victor, a division of Thermadyne Industries, Inc.)


Oxyfuel Gas Cutting Process

• The preheating flames are used to heat a spot on the base metal to its ignition temperature

• As the torch is moved along a line, the metal is cut, forming a kerf

• The preheating flames are kept burning throughout the entire cutting process


Cutting Outfit

• A cutting outfit includes all equipment required to make a cut

• A cutting station includes the outfit, lighting, ventilation, a cutting table, and possibly a booth

• The cutting torch, which provides an oxygen cutting jet, is quite different from a welding torch


Cutting Torch

• The cutting torch is connected to oxygen and fuel gas cylinders

• The welder controls the cutting operation using the cutting torch lever

(ESAB Welding and Cutting Products)


Cutting Torch

• Before the torch is turned on, the desired cutting tip must be inserted into the torch

• There are two types of OFC torches– Positive-pressure torches– Injector-type torches


Lighting a Positive-Pressure Oxyacetylene Cutting Torch

• Check the condition of all equipment• Inspect the regulators• Open the oxygen valve very slowly• Turn the cylinder valve all the way open• Open the acetylene cylinder valve 1/4 to 1/2 turn



• Open the torch oxygen valve one full turn

• Open the oxygen cutting valve

• Adjust the oxygen regulator to give the desired pressure

• Open the acetylene torch valve one turn

(Uniweld Products, Inc.)



• Turn in the acetylene regulator adjusting screw until the low-pressure gauge indicates the desired working pressure

• Open the torch acetylene valve 1/16 to 1/8 turn before lighting the torch

• Adjust the acetylene until most of the smoke clears from the flame

• Open the torch oxygen valve and adjust it to obtain neutral preheat flames


• The welder should stand to one side of the gauges when opening the oxygen valve

• Leave the acetylene cylinder valve wrench in place so the valve can be turned off quickly

• Never use acetylene at above 15 psig (103kPa)• Check the low-pressure gauge readings to make

sure that the pressures are not rising• A rising pressure indicates that a valve is leaking• The outfit should be shut down immediately• Use a flint lighter to ignite the acetylene

Safety


Lighting an Injector-Type Oxyacetylene Cutting Torch

• Several steps are required to turn on the outfit, purge the system, and light the torch

• This illustration shows typical gas flow through an injector-type oxyacetylene cutting torch



• Check the condition of all equipment

• Inspect the regulators• Open the oxygen

cylinder valve until the regulator high-pressure gauge reaches its maximum reading

• Turn the cylinder valve all the way open

• Slowly open the acetylene cylinder valve 1/4 to 1/2 turn

• Open the torch oxygen valve 1/4 turn

• Open the torch oxygen cutting orifice lever wide open

• Adjust the oxygen regulator screw for correct working pressure



• Close the torch oxygen valves

• Open the torch acetylene valve fully

• Adjust the acetylene working pressure

• Close the torch acetylene valve

• Check the low-pressure gauges

• Open the torch oxygen valve 1/4 turn

• Open the torch acetylene valve fully

• Use a spark lighter to ignite the fuel gas

• Press down the oxygen cutting lever and adjust the torch acetylene valve until the preheating flames are neutral


• Leave the wrench in place on the acetylene cylinder valve

• Check the low-pressure gauges for any gradual increase in pressures

• If the pressures are increasing, the regulator seat may be leaking

• If the pressure on either of the low-pressure gauges is increasing, shut down immediately

• Use a spark lighter to ignite the fuel gas

Safety


Using a Cutting Torch

• To cut, bring the tip of the inner cone of the preheating flames to the edge of the metal to be cut

• The jet of oxygen coming from the oxygen jet causes the heated metal to burn away, forming the kerf


Using a Cutting Torch

• Drag is a measurement made in the direction of travel between the entry and exit points of the cutting jet

• The slag stream can lag excessively behind the torch tip travel

• Possible causes include– The flame adjustment may be incorrect– The cutting oxygen pressure adjustment may be too low– The tip travel is too fast and the metal is not being

preheated enough


Cutting Attachments

• The cutting attachment is connected to the welding torch body to change the torch from a welding torch to a cutting torch

• For portable kits, the attachment saves space• The operation of the torch with the cutting

attachment is the same as the operation of a regular cutting torch


Cutting Attachments

• This illustration shows a cutting attachment installed on a torch

• The welding torch body has an oxygen valve and a torch acetylene valve

• The cutting attachment has a torch oxygen valve and a cutting oxygen lever

(CONCOA)


• The torch valves on the welding torch body must be turned off before the cutting attachment is disconnected

Safety


Cutting Tips

• Cutting tips normally have at least two orifices• One orifice is for the cutting oxygen• One or more small orifices are for preheating the

metal to be cut• One-piece tips are used for oxyacetylene cutting

only• Two-piece tips are used for all other gas cutting


Cutting Steel with the Oxyfuel Gas Cutting Torch

• Metals that can be cut with the oxyfuel gas cutting torch are divided into two classes– Metals whose oxides have a lower melting temperature

than the metal itself– Metals whose oxides have a higher melting temperature

than the metal itself• Practically all steel falls under the first classification



• The second class includes cast iron, some alloy steels, stainless steel, and nonferrous metals

• It is very important that refractory oxides be reduced by chemical action or be prevented from forming

• If too much oxygen is fed to the steel being cut, this leaves a bell-mouthed kerf on the side of the metal away from the torch



• The metal at the bottom of the cut is not burned away if the torch is moved too rapidly

• The large drag leaves a kerf that is very rough and irregular in shape



• Metal that is very dirty and rusty should be cleaned before starting the cutting operation

• In some cases the thickness of the metal requires an oscillating motion in order to obtain the necessary width of a cut

• The welder should stand in a comfortable position that permits looking into the cut as it is being formed


• Welders should wear– Safety boots with high tops– Trousers without cuffs– Leggings to protect against flying molten slag

• Welding leggings are pieces of leather that cover the shin and top of the shoe

• If trousers have cuffs, they must be covered to keep them from catching slag

Safety


Cutting Thin Steel

• Cutting steel that is 1/8″ or less in thickness requires the use of the smallest cutting tip

• A tip with only a few preheat holes is often used• The tip is usually pointed in the direction the torch is

traveling• Be careful to keep the end of the preheating inner

cone just above the metal


Cutting Thick Steel

• Steel over 1/2″ thick should be cut by holding the torch so the tip is perpendicular to the surface of the base metal being cut

• A cut is normally started at the edge of the stock


Cutting Thick Steel

• The thicker the steel, the greater the time required to raise the temperature of the steel high enough to make clean cuts

• To start a cut faster, a welder begins at the corner of the metal by slanting the torch in the direction opposite the direction of travel

• One method used to start cuts is to nick the edge of the metal with a cold chisel


Cutting Thick Steel

• Another method used to start cuts is to place an iron filler rod under the preheating flames at the edge of a thick plate

• Shaping cutting can be done by– Free-hand cutting– Using special attachments or guides– Setting up an automated cutting torch


• Be sure to read and carefully follow all manufacturer’s tables and safety charts prior to attempting a cutting operation for very thick metal

Safety


Cutting Chamfers (Bevels)

• Thicker pieces of steel must be prepared with a chamfered edge so the weld penetrates the metal

• Bevel angles can be cut at the same time the metal is being cut to size and shape


Cutting Chamfers (Bevels)

• A chamfer can also be cut as a separate operation• The technique used to cut a bevel is similar to that

used to cut thick metal• The torch is held at an acute angle to the base

metal• The cut is generally made at an existing edge of the

metal


Cutting Pipe or Tubing

• When cutting small-diameter pipe, it is best to keep the tip almost tangent to the inner circumference of the pipe

• For larger pipes, it is possible to keep the torch tip perpendicular to the pipe surface while cutting

(Thermadyne Industries, Inc.)


Cutting Pipe or Tubing

• If the pipe is being chamfered, most welders start the cut at the extreme edge of the pipe and cut back to the marked chamfer ring

• It is not the diameter of the pipe that determines the size of the cutting torch tip

• The thickness of the pipe wall is the controlling factor


Piercing and Cutting Holes

• The term pierce means to produce a relatively small hole through a steel plate

• A greater amount of heat is required to preheat the surface for piercing than when starting on a edge

• For this reason, a larger tip may be needed• It is good practice to outline the hole first, using

special chalk


• Until the steel plate is melted through the bottom surface, molten metal is blown upward by the pressure of the cutting oxygen

Safety


Cutting and Removing Rivet Heads

• Cutting torches are frequently used in salvage operations

• Torches are used to remove the two types of rivet heads– Round-head– Countersunk-head

• If possible, the welder should do the cutting without damaging the steel plate


Cutting and Removing Rivet Heads

• For cutting the round-head rivet, preheat the head of the rivet to a bright cherry red

• The steel plate is usually adequately protected from the flame by a coating of scale (oxide)

• Countersunk rivet heads can be removed by carefully cutting around the countersunk angle


Gouging with the Cutting Torch

• Gouging is a process that removes metal from the surface of a part to a desired depth

• For gouging, a lower oxygen cutting pressure and a larger diameter cutting tip orifice are used


Gouging with the Cutting Torch

• In a gouging tip, there are five or six preheat orifices to provide even distribution of the preheat flames

• The speed of torch movement is important• Moving the torch too rapidly creates a groove that is

too narrow and shallow• Moving the torch too slowly creates a gouge that is

too deep and wide


Cutting Alloy Steels

• Of the alloy steels, stainless steel is the most widely used

• Many alloy metals have melting temperatures below that of mild steel

• The metal to be cut should be placed so that the cutting tip and flame are in a horizontal position whenever possible



• A slight, quick up-and-down motion of the torch facilitates the removal of slag

• Like carbon steel, alloy steels must be preheated before the cutting operation is started



• Stainless steel must be preheated to a white heat before the cutting oxygen is turned on

• When the cutting is often interrupted by unmeltable slag, a welder may hold a mild steel welding rod in the kerf of the metal

• Adding welding rod is also useful when cutting poor grade steels, cast irons, and old, oxidized steel castings


Cutting Cast Iron

• It is more difficult to cut cast iron than steel

• This is because iron oxides of cast iron melt at a higher temperature than the cast iron itself

• It is important to preheat the whole casting before starting the cut

• Use a carburizing flame to prevent oxides from forming on the surface before cutting starts

• The casting should be cooled very slowly if gray cast iron is desired

• Rapid cooling creates a white cast iron grain structure


Automatic Cutting

• Automatic cutting equipment requires a controller or a computer to control the operation

• Feedback controls monitor the automatic cutting process and make needed corrections

• In semiautomatic cutting, some changes to the process may be required while cutting is taking place


Automatic Cutting

• There are many automatic and semiautomatic mechanisms available to perform cutting operations

(ESAB Welding and Cutting Products)


Automatic Cutting

• Practically all automatic cutting machines and their torches are moved by servomotors

• Special cutting torches are mounted on a light, rigid rail called a gantry

• A vernier gear mechanism permits fine adjustments to raise and/or lower the torch

• A magnetic tracer with a steel pattern can be used as a cutting torch guiding device


• Dangers can be greatly reduced through the correct handling of oxygen and fuel gas equipment

• Many potential hazards exist when cutting• Every welder must follow approved procedures to

eliminate these dangers

Safety


Safety in Oxyfuel Gas Cutting

• Floors on which cutting is done should be concrete or other fireproof material

• Shop furniture should be made of metal or fire-resistant material

• Flammable materials should not be in the room where cutting is done



• Leather or slow-burning fabrics should be worn• Welders’ trousers should not have cuffs• Welders’ pockets should not contain flammable

items• Tanks and containers should be welded only by

experienced welders


• It is generally required to pass an inert gas or steam through the tank as it is being cut

• The steam or inert gas is intended to displace any combustible gases in the tank

• The tank may also be filled with water except in the area of work

• In all cases the tank must be vented to prevent the entrapment of potentially explosive gases

Safety



• Certain metals, such as magnesium, can burn with an explosive force if flame-cut

• The face and hands must be protected from metal splatter

• A fire extinguisher should be at hand and a fire watch posted during cutting operations

• Follow correct procedures for using cylinders, regulators, hoses, and torches



• Review metal fume hazards• Know what metal is being worked on before

performing any cutting operation• Check that ventilation systems are working• As the oxygen content in an enclosed space is

increased above a certain percentage, there is an increasing danger of a spark or flame causing a fire or explosion


• Be certain of what metal is being cut• Never use acetylene at a pressure greater than

15 psig (103kPa)• Acetylene becomes unstable above that pressure

and may explode

Safety

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Oxyfuel Gas Cutting