Chapter 14 Oxyfuel Gas Cutting 425 The thickness of the pipe wall is the controlling factor. The proper welding codes and procedures must be followed whenever pipe is cut for use in structural or pressure vessel applications. 14.6.5 Piercing and Cutting Holes Holes can be pierced in steel plates rapidly and with accurate results. The term pierce means to produce a relatively small hole (in comparison to the size of the metal surface) through a steel plate. The process consists of holding the cutting torch with the nozzle perpendicular to the surface of the metal, and preheating the spot to be cut until it is a bright cherry red. After the metal is brought up to the proper temperature (1400°F, or 760°C), the oxygen jet is turned on very slowly. At the same time, the nozzle is raised enough to prevent the slag from being blown back into the nozzle orifi ces. A greater amount of heat is required to preheat the surface for piercing than when starting on an edge. Therefore, it is recommended that the welder use at least the next larger tip in relation to the thickness of the metal than is recommended in Figures 14-8 and 14-12. Caution: Until the steel plate is melted through the bottom surface, molten metal is blown upward by the pressure of the cutting oxygen. To cut larger holes in steel plate, the typical steel cutting method described in this chapter is recom- mended. It is good practice to outline the hole fi rst, using special chalk. The welder can then follow the outline to cut an accurate hole. If the hole is very large, it is best to do the cutting with an automatic machine or with a radius bar attachment clamped to the torch head. 14.6.6 Cutting and Removing Rivet Heads Cutting torches are frequently used in salvage operations. They can cut through structural members and metal skin. They are especially useful for disman- tling large fabricated structures that have been assem- bled with rivets. Torches are used to remove the rivet heads, allowing the structure to be disassembled with minimal damage. Two typical rivet shapes are: ● Round-head. ● Countersunk-head. The procedure for removing these heads is funda- mentally the same as for any cutting operation, but one additional precaution should be observed. If possible, the welder should do the cutting without damaging the steel plate. To do this, it is very important that the size of the tip is carefully chosen. If too large a tip is used, the steel plate will be damaged as the rivet head is being removed. If too small a tip is used, the method becomes too slow. Prac- tically all welding equipment companies recommend special-shaped cutting tips for rivet cutting. The procedure for cutting the round-head rivet is to preheat the head of the rivet to a bright cherry red. The steel plate is usually adequately protected from the preheating fl ame by a coating of scale (oxide). The special cutting tip is placed on the base metal and the rivet head cut in the usual manner. See Figure 14-34. The appearance of an accurately performed job shows Figure 14-33. A cutting torch being used to cut a bevel on a steel pipe. 45° Figure 14-34. Recommended methods of removing rivet heads using a cutting torch. A—Special rivet-cutting tip. B—Special tip used to cut a round-head rivet. C—Regular cutting tip used to cut out a countersunk rivet. A B C