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The common methods used in cutting metal are oxygas flame cutting, air carbon-arc cutting, and plasma-arc cutting. The method used depends on the type of metal to be cut and the availability of equipment. As a welder, oxygas or air carbon-arc equipment is the most common type of equipment available for your use.
The oxygas cutting torch has many uses in steelwork Often, the welder finds the cutting torch an excellent tool for cutting ferrous metals. This versatile tool is used for operations, such as beveling plate, cutting and beveling pipe, piercing holes in steel plate, and cutting wire rope.
When using the oxygas cutting process, you heat a spot on the metal to the kindling or ignition temperature (between 1400°F and 1600°F for steels). The term for this oxygas flame is the preheating flame. Next, you direct a jet of pure oxygen at the heated metal by pressing a lever on the cutting torch. The oxygen causes a chemical reaction known as oxidation to take place rapidly. When oxidation occurs rapidly, it is called combustion or burning. When it occurs slowly, it is known as rusting.
When you use the oxygas torch method to cut metal, the oxidation of the metal is extremely rapid and part of the metal actually burns. The heat, liberated by the burning of the iron or steel, melts the iron oxide formed by the chemical reaction and accelerates the preheating of the object you are cutting. The molten material runs off as slag, exposing more iron or steel to the oxygen jet.
In oxygas cutting, only that portion of the metal that is in the direct path of the oxygen jet is oxidized. The narrow slit, formed in the metal as the cutting progresses, is called the kerf. Most of the material removed from the kerf is in the form of oxides (products of the oxidation reaction). The remainder of the material is molten metal that is blown or washed out of the kerf by the force of the oxygen jet.
The walls of the kerf formed by oxygas cutting of ferrous metals should be fairly smooth and parallel to each other. After developing your skills in handling the torch, you can keep the cut within close tolerances; guide the cut along straight, curved, or irregular lines; and cut bevels or other shapes that require holding the torch at an angle. Partial oxidation of the metal is a vital part of the oxygas cutting process. Because of this, metals that do not oxidize readily are not suitable for oxygas cutting. Carbon steels are easily cut by the oxygas process, but special techniques (described later in this chapter) are required for the cutting of many other metals.