Copyright Goodheart-Willcox Co., Inc. Chapter 8 Manual Welding Techniques 131 Joint Preparation Joint edges prepared by thermal cutting processes have a heavy oxide fi lm on the surface, as shown in Figure 8-3. This oxide should be completely removed prior to welding in order to prevent porosity in the weld metal. Joint edges prepared by shearing should be sharp without tearing or ridges. Otherwise, dirt or oil can become trapped in the joint and result in faulty welds. Rough edges can be removed prior to welding with a light grinding. Weld Backing for Steel Groove welds that are welded from one side of the joint, where 100% penetration is required, can be backed with argon gas or a solid bar. Solid bars can be made of copper or stainless steel. These backing methods exclude air from the back- side or penetration side of the joint, which assists in wetting of the penetration. A wetted penetration has a smooth junction and even fl ow of the penetration onto the base metal. The exclusion of air also prevents the formation of scale and oxide. Preheating Steels Carbon steels less than 1″ (25.4 mm) thick and with less than .30% carbon generally do not require preheat. Welding on highly restrained joints is an exception. These joints should be preheated to 50°–100°F (10°–38°C) to minimize shrinkage cracks in the base metal and the weld deposit. A preheat temperature that is too low causes welds made on low-alloy steels, such as the chrome- moly steels, to have hard heat-affected zones. The hard heat-affected zones are caused by the rapid cooling rate of the base material and the formation of martensitic grain structures. A 200°–400°F (93°–204°C) preheat temperature slows down the cooling rate and prevents the formation of a martensitic structure. Martensite is a metallurgical term that defi nes a type of grain structure obtained by heating and quenching. The martensitic grain structure makes the metal hard. Parts that are welded are, in effect, heated and quenched. If the carbon content of the material is suffi cient, and the preheat temperature is too low, the material in the heat affected zone will harden during welding. This results in high tensile properties with very low ductility. The formation of hard martensite- type grains increases the possibility of cracking as the weld metal cools and shrinks. Tool steels have a very high carbon content and are prone to cracking in the heat-affected zone without suffi cient preheat. Figure 8-4 lists some recommended preheat temperature ranges for welding tool steels. Quenched and tempered steel requires preheat and interpass temperature control to retain the orig- inal mechanical properties of the metal. The manu- facturer’s recommendations for these temperatures should be strictly followed. Torch Angles and Bead Placement Proper manipulation of the welding torch is very important in making a good weld. The torch is usually held like a pencil, as shown in Figure 8-5. When welding is done in the fl at position, the hand should be placed lightly on a surface, so the hand can Figure 8-3. The oxide scale and small gouge indentations were formed on this piece of metal by the oxyacetylene cutting process. (Mark Prosser) Annealed Base Hardened Base Material Material Preheat Material Preheat Type and Postheat and Postheat Temperature Temperature W1, W2 250–450°F (121–232°C) 250–450°F (121–232°C) S1 300–500°F (149–260°C) 300–500°F (149–260°C) S5 300–500°F (149–260°C) 300–500°F (149–260°C) S7 300–500°F (149–260°C) 300–500°F (149–260°C) 01 300–400°F (149–204°C) 300–400°F (149–204°C) 06 300–400°F (149–204°C) 300–400°F (149–204°C) A2 300–500°F (149–260°C) 300–400°F (149–204°C) A4 300–500°F (149–260°C) 300–400°F (149–204°C) D2 700–900°F (371–482°C) 700–900°F (371–482°C) H11, H12, H13 900–1200°F (482–649°C) 700–1000°F (371–538°C) M1, M2, M10 950–1100°F (510–593°C) 950–1050°F (510–566°C) Figure 8-4. Thoroughly preheat the part to be welded to the required temperature. Do not allow the part to cool below the minimum temperature until postheat is complete.