394 Section 6 Oxyfuel Gas Processes Copyright Goodheart-Willcox Co., Inc. Oxyfuel Gas Cutting Principles Oxyfuel gas cutting (OFC) is a process used to cut metal by rapidly oxidizing it. The heat of a gas fl ame and pressurized pure oxygen are used in this process. Most materials, including steel, will burn. Oxyfuel gas cutting is also referred to as burning or fl ame cutting by some welders. If the temperature of paper is raised to its ignition temperature, the paper will burn. The ignition temperature is the temperature at which a material will burn if enough oxygen is present. Oxygen must be present for burning to occur. When paper is burned, the oxygen comes from the air. The ignition temperature for steel is 1500°F (816°C). One or more oxyfuel gas flames are used to heat steel to this temperature when oxyfuel gas cutting. Steel is a bright red or orange-red color at this temperature. When the steel attains its ignition temperature, a jet stream of high- purity (99%+) oxygen is directed at it. This stream of oxygen rapidly oxidizes, or burns, the steel. A continuous, smooth cut through the steel will result if the 1500°F (816°C) temperature and oxygen jet are maintained using a proper travel speed. Various gases are used as the fuel for oxyfuel gas cutting. These gases include acetylene, hydrogen, natural gas, propane, and methylacetylene-propadiene (MPS). Acetylene and MPS are the most commonly used fuel gases in industry. Oxyfuel Gas Cutting Equipment Oxyfuel gas cutting equipment is essentially the same as oxyfuel gas welding equipment. The main differences in the cutting and welding outfi ts are in the oxygen regulator and the torch. For cutting thick steel, a larger volume of oxygen at a higher pressure is needed. Therefore, a pressure regulator with a larger volume capacity is used. (Note that this special pressure regulator will have an oxygen pressure gauge with a higher range.) Cutting operations on thinner steels will use the same regulator as welding operations. Preparing to Cut The step-by-step procedure for assembling an oxyfuel gas cutting outfit is explained in Chapter 25. Select either a cutting torch or a cutting torch attachment and connect it to the hoses. Check the cutting station to ensure that it is assembled correctly and all gas connections are tight and leakproof. Be sure to select the proper size cutting tip when preparing to cut. Cutting tip sizes are identifi ed by numbers ranging from 00–8. The numbers are stamped on the tip when they are made. Most cutting tip manufacturers use the same tip numbering system. Figure 26-1 suggests oxygen and acetylene pressures for use with various cutting tips. Suggested tip sizes for use with various metal thicknesses are also shown. Goss, Inc. Figure 26-1. Suggested tip sizes for cutting various metal thicknesses with a positive pressure torch. The approximate oxygen and acetylene gas pressures are listed. Also shown is the correct cutting speed. Material thickness, inches 1/8 1/4 1/2 3/4 1 1 1/2 2 4 5 6 8 10 12 Recommended tip number 00 0 1 1 2 2 3 3 4 6 6 7 8 Oxygen pressure setting, psig 20–25 25–30 30–35 30–35 35–40 40–45 40–45 40–50 45–55 45–55 45–55 45–55 45–55 Acetylene pressure setting, psig 3–5 3–5 3–5 3–5 3–7 3–7 5–10 5–10 6–12 7–13 8–14 10–15 10–15 Cutting speed range, ln/min 27–30 26–29 20–24 17–21 14–18 13–17 12–15 8–11 7–9 6–8 5–6 4–5 3–4