CUTTING CAST IRON

It is more difficult to cut cast iron than steel because the iron oxides in cast iron melt at a higher temperature than the cast iron itself. Before you cut cast iron, it is best to preheat the whole casting to prevent stress fractures. Do not heat the casting to a temperature that is too high, as this will oxidize the surface and make cutting

Figure 4-22.-Torch movements for cutting cast iron.

more difficult. A preheat temperature of about 500F is normally satisfactory.

When cutting cast iron, adjust the preheating flame of the torch to a carburizing flame. This prevents the formation of oxides on the surface and provides better preheat. The cast-iron kerf is always wider than a steel kerf due to the presence of oxides and the torch movement. The torch movement is similar to scribing semicircles along the cutting line (fig. 4-22). As the metal becomes molten, trigger the cutting oxygen and use its force to jet the molten metal out of the kerf. Repeat this action until the cut is complete.

Because of the difficulty in cutting cast iron with the usual oxygas cutting torch, other methods of cutting were developed. These include the oxygen lance, carbon-arc powder, inert-gas cutting, and plasma-arc methods.

GOUGING MILD STEEL

Cutting curved grooves on the edge or surface of a plate and removing faulty welds for rewelding are additional uses for the cutting torch. The gist of groove cutting or gouging is based on the use of a large orifice, low-velocity jet of oxygen instead of a high-velocity jet. The low-velocity jet oxidizes the surface metal only and gives better control for more accurate gouging. By varying the travel speed, oxygen pressure, and the angle between the tip and plate, you can make a variety of gouge contours.

A gouging tip usually has five or six preheat orifices that provide a more even preheat distribution. Automatic machines can cut grooves to exact depths, remove bad

Figure 4-24.-Using angle iron to cut bevels on steel plate.

spots, and rapidly prepare metal edges for welding. Figure 4-23 shows a typical gouging operation.

If the gouging cut is not started properly, it is possible to cut accidently through the entire thickness of the plate. If you cut too shallow, you can cause the operation to stop. The travel speed of the torch along the gouge line is important. Moving too fast creates a narrow, shallow gouge and moving too slow creates the opposite; a deep, wide gouge.

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