396 Section 6 Oxyfuel Gas Processes Copyright Goodheart-Willcox Co., Inc. Slag is iron oxide. It may form on the underside of the metal being cut. Figure 26-4A shows a plate that was turned over to reveal the slag. Slag that is easily removed is acceptable. Hard slag (slag that cannot be easily removed) is unacceptable. See Figure 26-4B. Enough oxygen pressure must be used to cut through the metal without leaving hard slag. The slot or opening produced in the metal when cutting is the kerf. See Figure 26-5. The kerf should be as narrow as possible. A wide kerf requires more oxygen to make a good cut. The width of the kerf is determined by the size and shape of the cutting tip. Too much oxygen pressure may result in a bell-mouthed kerf, Figure 26-6. When oxyfuel gas cutting, the base metal must be preheated (heated to its ignition temperature) before cutting oxygen is applied. The small orifi ces in the cutting tip supply the preheating fl ames. These fl ames preheat the metal prior to starting a cut. They also preheat the metal ahead of the oxygen stream. The cutting tip should be placed into the torch so it aligns with the cutting line, as shown in Figure 26-7. A minimum of two preheating orifi ces should line up with the cutting line. A kerf is formed under the cutting oxygen jet. The quality of the kerf depends on the following: Goodheart-Willcox Publisher Figure 26-5. An oxyacetylene cut in progress. A— Note the slag at the bottom of the kerf. B—Cutting oxygen slows down as it travels through the metal. Drag can result from both the oxygen slowing down as it penetrates thick plate and the travel speed of the torch. Goodheart-Willcox Publisher Jaochainoi/Shutterstock.com Figure 26-4. A—The front edge of the plate has a very clean cut with very little slag, while the back edge has considerable slag. B—The excessive amount of slag on this piece of cut steel pipe is unacceptable. Much grinding and additional edge preparation would be required to prepare this edge for welding. A B Direction of torch travel Steel plate Drag Kerf Slag A B