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CHAPTER 7

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CHAPTER 7

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  1. CHAPTER 7 Flame Cutting

  2. OBJECTIVES • After completing this chapter, the student should be able to • describe the eye protection that must be used for flame cutting. • discuss the oxyfuel gas cutting process including fuel gases, metals, regulators, torches, and cutting tips. • demonstrate how to safely set up, light, adjust, and maintain a cutting torch. • demonstrate how to make clean slag-free cuts in various material thicknesses both manually and using a cutting machine. • demonstrate how to make a square cut on pipe in the horizontal rolled position, the horizontal fixed position, and the vertical position.

  3. KEY TERMS • combination welding and cutting torch • cutting tips • drag • drag lines • flashback arrestor • hard slag • kindling point • kindling temperature • MPS gases • oxyacetylene hand torch • oxyfuel gas cutting (OFC) • regulator gauge • regulators • safety release valve • tip cleaners • venturi

  4. INTRODUCTION • Oxyfuel gas cutting (OFC) is a group of oxygen cutting processes that use heat from an oxyfuel gas flame to raise the temperature of the metal to its kindling temperature before a high-pressure stream of oxygen is directed onto the metal, causing it to be cut. • The kindling temperature of a material is the temperature at which rapid oxidation (combustion) can begin. • This cutting processes is group and identified by the type of fuel gas used with oxygen to produce the preheat flame. • Oxyfuel gas cutting is most commonly performed with oxyacetylene cutting (OFC-A). • More people use the oxyfuel cutting torch than any other welding process.

  5. METALS CUT BY THE OXYFUEL PROCESS • The process will work easily on any reactive metal that will rapidly oxidize at an elevated temperature. • It is most often used to cut low carbon steels with carbon contents up to 0.3%. • A few reactive nonferrous metals, such as titanium, zinc, and magnesium, can also be cut. • OFC cannot be used to cut nonreactive metals such as stainless steel, cast iron, brass, copper, aluminum and all most all other nonferrous metals.

  6. THE CHEMISTRY OF A CUT • The oxyfuel gas cutting torch works by first preheating the metal to its kindling temperature before a high pressure stream of pure oxygen is turned on to rapidly oxidizes or burns the meal away. • Kindling point and kindling temperature are too terms that have the same meaning they refer to the lowest temperature at which a material will combust or start burning. • The kindling temperature of most steels in pure oxygen is between1600° to 1800°F (870°C to 900°C), which is a dull red color.

  7. EYE PROTECTION FOR FLAME CUTTING • The National Bureau of Standards has identified proper filter plates and uses. • The recommended filter plates are identified by shade number and are related to the type of cutting operation being performed. • Goggles or other suitable eye protection must be used for flame cutting.

  8. CUTTING TORCHES • The oxyacetylene hand torch is the most common type of oxyfuel gas cutting torch used in industry. • The hand torch, as it is often called, may be either a part of a combination welding and cutting torch set or a cutting torch only, Figure 7-1.

  9. CUTTING TORCHES (cont.) • Oxygen is mixed with the fuel gas to form a high-temperature preheating flame. • The two gases must be completely mixed before they leave the tip and create the flame. • Two methods are used to mix the gases. • One method uses a mixing chamber, and the other method uses an injector chamber.

  10. CENTER CUTTING ORIFICE SIZE

  11. SPECIALTY CUTTING TIPS

  12. CUTTING TIPS • Most cutting tips are made of copper alloy, but some tips are chrome. • Tips for straight cutting are either standard or high-speed, Figure 7-10. • The high-speed cutting tip is designed to allow a higher cutting oxygen pressure, which allows the torch to travel faster. • The diameter, or size of the center cutting orifice, determines the thickness of the metal that can be cut.

  13. CUTTING TIPS (cont.) • Finding the correctly sized tip for a job can be confusing, especially if you are using the cutting unit for the first time. To make it easier to select a tip, you can use a standard set of tip cleaners to find the size of the center cutting orifice. • MPS gases are used in tips having eight preheat holes or in a two-piece tip that is not recessed, Figure 7-12.

  14. PRESSURE REGULATORS • All pressure regulators reduce the high cylinder or system pressure to the proper lower working pressure. • It is important that the regulator keep the lower pressure constant over a range of flow rates. • CAUTION • Although all regulators work the same way, they cannot be safely used interchangeably on different types of gas or for different pressure ranges without the possibility of a fire or an explosion.

  15. REGULATOR OPERATION • A regulator works by holding the forces on both sides of a diaphragm in balance, Figure 7-17. • The size of a regulator determines its ability to hold the working pressure constant over a wider range of flow rates. • Two-stage regulators, Figure 7-20, are able to keep the pressure constant at very low or high flow rates as the cylinder empties.

  16. REGULATOR GAUGES • There may be one or two pressure gauges on a regulator. • One pressure gauge shows the working pressure, and the other indicates the cylinder pressure, Figure 7-21.

  17. REGULATOR SAFETY PRESSURE RELEASE DEVICE • Regulators may be equipped with either a safety release valve or a safety disc to prevent excessively high pressures from damaging the regulator. • A safety release valve is made up of a small ball held tightly against a seat by a spring. • A safety disc is a thin piece of metal held between two seals, Figure 7-22.

  18. CYLINDER AND REGULATOR FITTINGS • A variety of inlet or cylinder fittings are available to ensure that the regulator cannot be connected to the wrong gas or pressure, Figure 7-23A through D.

  19. CYLINDER AND REGULATOR FITTINGS (cont.) • The following are the two most common types: • (1) adapt a left-hand male acetylene cylinder fitting to a right-hand female regulator fitting, or vice versa and • (2) adapt an argon or mixed gas male fitting to a female flat washer-type CO2 fitting, Figure 7-24.

  20. CYLINDER AND REGULATOR FITTINGS (cont.) • Regulator Safety Precautions • The regulator pressure adjusting screw should be backed off each time the oxyfuel system is being shut down. • This is done to release the spring and diaphragm pressures, which, over time, may cause damage. • Keeping a spring compressed and the diaphragm stretched can cause the spring to weaken and the diaphragm to be permanently distorted.

  21. CYLINDER AND REGULATOR FITTINGS (cont.) • CAUTION • Regulators that creep excessively or beyond the safe working pressure must not be used. • All work on regulators must be done by properly trained repair technicians. • Regulators should be located far enough from the actual work that flames or sparks cannot reach them.

  22. REGULATOR CARE AND USE • There are no internal or external moving parts on a regulator or a gauge that require oiling, Figure 7-27. • CAUTION • Oiling a regulator is unsafe and may cause a fire or an explosion.

  23. BACKFIRES • A backfire occurs when a flame goes out with a loud snap or pop. A backfire may be caused by one or more of the following: • Touching the tip against the workpiece • Overheating the tip • Operating the torch when the flame settings are too low • Loose tip • Damaged seats • Dirt in the tip

  24. FLASHBACKS • When a flashback occurs, the flame is burning back inside the tip, torch, hose, or regulator. • A flashback produces a high-pitched whistle. • A flashback that reaches the cylinder may cause a fire or an explosion. • When a flashback occurs, there is usually a serious problem with the equipment, and a qualified technician should be called.

  25. REVERSE FLOW AND FLASHBACK VALVES • The purpose of the reverse flow valve is to prevent gases from accidentally flowing through the torch and into the wrong hose. • A reverse flow of gas will occur if the torch is not turned off or bled properly. • CAUTION • If both valves are opened at the same time, one gas may be pushed back up the hose of the other gas.

  26. REVERSE FLOW AND FLASHBACK VALVES (cont.) • A flashback arrestor will do the job of a reverse flow valve, and it will also stop the flame of a flashback, Figure 7-30.

  27. CARE OF THE REVERSE FLOW VALVE AND FLASHBACK ARRESTOR • Both devices must be checked on a regular basis to see that they are working correctly. • The internal valves may become plugged with dirt, or they may become sticky and not operate correctly. • To test the reverse flow valve, you can try to blow air backward through the valve. • To test the flashback arrestor, follow the manufacturer’s recommended procedure. • If the safety device does not function correctly, it must be replaced.

  28. HOSES AND FITTINGS • Most welding hoses used today are molded together as one piece and are referred to as Siamese hose. • Hoses that are not of the Siamese type, or hose ends that have separated, may be taped together. • When taping the hoses, they must not be taped solidly. • They should be wrapped for about 2 in. (51 mm) out of every 12 in. (305 mm) of hose length, allowing the colors of the hose to be seen.

  29. HOSE CARE AND USE • When hoses are not in use, the gas must be turned off and the pressure bled off. • Turning off the equipment and releasing the pressure prevents any undetected leaks from causing a fire or an explosion. • Hoses are resistant to burns, but they are not burn-proof. • Hoses should be checked periodically for leaks. • The hose fittings can be changed if the old ones become damaged.

  30. LEAK DETECTION • A leak-detecting solution can be purchased premixed and ready to use or as a concentrate that must be mixed with water. • A leak-detecting solution must be free flowing so that it can seep into small joints, cracks, and other areas that may have a leak. • The solution must produce a good quantity of bubbles without leaving a film. The solution can be dipped, sprayed, or brushed on the joints. • CAUTION • Some detergents are not suitable for O2 because of an oil base. Use only O2-approved leak-detection solutions on oxygen fittings.

  31. OXYFUEL CUTTING, SETUP, AND OPERATION • The setting up of a cutting torch system is exactly like setting up oxyfuel welding equipment except for the adjustment of gas pressures.

  32. TORCH TIP CARE AND USE • Torch tips may have metal-to-metal seals, or they may have an O-ring or a gasket between the tip and the torch seat. Metal-to-metal seal tips must be tightened with a wrench. • Tips with an O-ring or a gasket may be tightened by hand. • Dirty tips can be cleaned using a set of tip cleaners. • Damaged tips or tips with cleaners broken in them can be reconditioned, but they require a good deal of work and some specialized tools, Figure 7-37.

  33. PRACTICE 7-1 • Setting Up a Cutting Torch • Demonstrate to other students and your instructor the proper method of setting up cylinders, regulators, hoses, and the cutting torch.

  34. PRACTICE 7-2 • Cleaning a Cutting Tip • Using a cutting torch set that is assembled and adjusted as described in Practice 7-1, and a set of tip cleaners, you will clean the cutting tip.

  35. PRACTICE 7-3 • Lighting the Torch • Wearing welding goggles, gloves, and any other required personal protective clothing, and with a cutting torch set that is safely assembled, you will light the torch.

  36. HAND CUTTING • When making a cut with a hand torch, it is important for the welder to be steady in order to make the cut as smooth as possible. • The torch should be braced with the left hand if the welder is right-handed or with the right hand if the welder is left-handed. • A slight forward torch angle helps the flame preheat the metal, keeps some of the reflected flame heat off the tip, aids in blowing dirt and oxides away from the cut, and keeps the tip clean for a longer period of time because slag is less likely to be blown back on it, Figure 7-45.

  37. STARTING A CUT • To start a cut on the edge of a plate, hold the torch at a right angle to the surface or pointed slightly away from the edge, Figure 7-47.

  38. STARTING A CUT (cont.) • The torch must also be pointed so that the cut is started at the very edge. • The edge of the plate heats up more quickly and allows the cut to be started sooner. • Fewer sparks will be blown around the shop. • Once the cut is started, the torch should be rotated back to a right angle to the surface or to a slight leading angle.