426 Modern Welding a clean removal of the head without any score marks from the cutting jet on the steel plate. Removal of a countersunk-head rivet is more diffi cult than the round-head rivet. The rivet is usually tightly embedded in the plate. However, countersunk rivet heads can still be removed, with very little (if any) damage to the steel plate. This is done by care- fully selecting the tip size and by carefully cutting around the countersunk angle. 14.6.7 Gouging with the Cutting Torch Gouging is a process that removes metal from the surface of a part to a desired depth. When done properly, gouging cuts a U-shaped groove into the surface of the base metal. Gouging may be done using the oxyfuel gas process or one of several arc cutting processes. Gouging may be used to open up a defect (crack) that may occur in a part and provide a well-shaped U-groove in preparation for a welded repair. It can be used to cut out areas of a completed weld that are judged to be defec- tive, preparing the weld for rewelding. Oxyfuel gas gouging differs from oxyfuel gas cutting. For gouging, a lower oxygen cutting pressure and a special larger diameter cutting tip orifi ce are used. The resulting lower-pressure oxygen cutting jet allows the welder to move more slowly along the gouge line. The lower pressure also helps to prevent cutting through the base metal. See Figure 14-35. The less forceful oxygen stream oxidizes the surface metal only and penetrates more slowly. This enables the welder to gouge or groove the base metal with greater accuracy. In a gouging tip, there are fi ve or six preheat orifi ces to provide an even distribution of the preheat fl ames. Some gouging tips have a small bump on the underside of the tip. This small bump helps the welder to achieve a more even depth of gouge by keeping the tip at a uniform distance above the metal surface. In an automatic cutting machine, a gouging tip is capable of creating a gouge with a very accurate depth and width. If the gouging cut is not started properly, it is possible to cut too deeply or actually cut through the entire thickness of the base metal. The speed at which the torch is moved along the gouging line is impor- tant. Too-rapid movement of the torch creates a groove that is too shallow and narrow. Moving the torch too slowly creates a gouge that is too deep and wide. 14.7 Cutting Alloy Steels The introduction of many alloy steels to industry has made it necessary to develop new techniques for cutting these metals successfully and economically. Of the alloy steels, stainless steel is the most widely used. Stainless steel consists of chromium, nickel, and other elements added to iron. See Chapters 20, 27, and 28 for more information about alloy steels. Many of these alloy metals have melting temper- atures below that of mild steel. The oxides formed when cutting have a melting temperature higher than that of the original metal. These high-melting- point oxides must be reduced and/or blown out of the cut as it proceeds, or the cutting action stops. For the same relative thickness of metal, stainless steels need approximately 20% more preheating fl ame and 20% more cutting oxygen than is required to cut mild steel. It is also a good practice to use a slightly carburizing fl ame when preheating stainless steel. The metal to be cut should be placed so that the cutting tip and fl ame are in a horizontal posi- tion whenever possible. Setting up the base material in the vertical position is most preferable so that the molten slag can be blown away most effi ciently. Start the cut at the top of the metal and proceed downward in a vertical line. A slight, but quick, up and down motion of the torch facilitates the removal of the slag. Figure 14-36 shows how the torch should be moved up and down to facilitate the slag removal. Using a regular cutting torch, it is diffi cult to obtain as clean and narrow a kerf when cutting alloy metals as when cutting straight carbon steels. Like carbon steel, alloy steels must be preheated before the cutting operation is started. Stainless steels Figure 14-35. A typical oxygen gouging operation. A low- velocity cutting jet is used to maintain better control of the gouge width and depth.