Chapter 14 Oxyfuel Gas Cutting 427 must be preheated to a white heat before the cutting oxygen is turned on. The cutting action is much more violent with stainless steels than with straight carbon steels. Cutting takes place with considerable sparking and blowing of the slag. In situations where the progress of the cutting is frequently interrupted by the presence of unmelt- able slag, the welder may consider holding a mild steel welding rod in the kerf of the metal. The mild steel, mixed with the alloy steel, dilutes or reduces the percentage of the alloys in the area of the cut. The cutting properties of the alloy metal in the area of the cut thus become more like those of mild steel. The cut proceeds more smoothly. Adding welding rod to the cut is also useful when cutting poor grade steels, cast irons, and old, oxidized steel castings. Powder cutting, plasma arc cutting, and inert gas arc metal cutting have proven much more practical for cutting steel alloys and nonferrous metals. For full details on these processes, refer to Chapters 10 and 23. 14.8 Cutting Cast Iron As mentioned previously, it is more diffi cult to cut cast iron than steel. This is because iron oxides of cast iron melt at a higher temperature than the cast iron itself. However, cutting has been performed success- fully on cast iron in many salvage shops and found- ries. When cutting cast iron, it is important to preheat the whole casting before starting the cut. The metal should not be heated to a temperature that is too high, since this oxidizes the surface and makes cutting diffi - cult. A preheat temperature of about 500°F (260°C) is usually satisfactory. When cutting cast iron, use a carburizing fl ame to prevent oxides from forming on the surface before the cutting starts. Because of oxidation-related prob- lems, the kerf in cast iron is always wider than it would be in steel. After completing a cut on cast iron, the casting should be cooled very slowly if gray cast iron is desired. Rapid cooling creates a white cast iron grain structure. It is diffi cult to cut cast iron with a typical oxyfuel gas cutting torch. Other more satisfactory methods of cutting have been developed. These include oxygen arc, plasma arc, chemical fl ux, metal powder, and exothermic cutting. See Chapters 10 and 23 for descrip- tions of these cutting processes. 14.9 Automatic Cutting Automatic cutting machines are constantly being improved. Cutting processes such as metal powder cutting (POC), plasma arc cutting (PAC), fl ux cutting (FOC), air carbon arc (CAC–A), exothermic cutting, and water jet cutting are often done using automatic machines. Automatic cutting equipment requires a controller or a computer to control the operation. The cutting process is completely controlled and does not require adjustment while cutting. The controller or computer is programmed with the path and speed of the torch or torches. Solenoid valves start and stop the fl ow of gases. Electric motors, actuated by the controller or computer, are used to very precisely control the move- ment of the torch or torches. Because of these features, automatic cutting equipment produces almost iden- tical parts every time. Feedback controls monitor the automatic cutting process and make necessary corrections. Feedback control items may include vision devices. These vision devices are used to keep the preheating fl ames at a specifi ed height above the base metal at all times. Semiautomatic cutting, or mechanized cutting, is similar to automatic cutting except that some changes to the process may be required while cutting is taking place. These processes often use electronic and magnetic tracers to follow a pattern. As the pattern is followed, electric motors move the torch or torches to cut the metal in the same shape as the pattern. Elec- tronic tracers permit a pattern to be followed so that extremely accurate shapes can be produced. Chapter 25 describes semiautomatic and automatic processes. Figure 14-36. A technique used for the oxyacetylene cutting of chromium steel. Note how the torch is raised and lowered to assist in removing the slag from the cut. Cutting nozzle is lowered a short distance, raised an even shorter distance, and so on as cut progresses from top to bottom Line of cut Casting Riser Cutting nozzle horizontal and parallel to top of riser
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