Oxyfuel Gas Cutting

2. 14 Chapter Oxyfuel Gas Cutting

3. 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.

4. 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.

5. 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

6. 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

7. 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.)

8. 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

9. 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

10. 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)

11. 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

12. 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

13. Lighting a Positive-Pressure Oxyacetylene Cutting Torch • 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.)

14. Lighting a Positive-Pressure Oxyacetylene Cutting Torch • 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

15. Safety • 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

16. 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

17. Lighting 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

18. Lighting an Injector-Type Oxyacetylene Cutting Torch • 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

19. Safety • 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

20. 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

21. 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

22. 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

23. 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)

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

25. 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

26. 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

27. Cutting Steel with the Oxyfuel Gas Cutting Torch • 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

28. Cutting Steel with the Oxyfuel Gas Cutting 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

29. Cutting Steel with the Oxyfuel Gas Cutting Torch • 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

30. Safety • 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

31. 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

32. 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

33. 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

34. 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

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

36. 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

37. 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

38. 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.)

39. 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

40. 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

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

42. 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

43. 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

44. 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

45. 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

46. 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

47. Cutting Alloy Steels • 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

48. Cutting Alloy Steels • 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

49. 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

50. 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