Handbook-Welding Cast Iron

5 The Importance of Preheating For Fusion Welding. If you are called on to weld cast iron, the material to be welded will almost always be gray iron. Gray iron is brittle; it has virtually no ductility. If the forces of expansion or contraction, as generated during the welding operation or in cooling after welding, are concentrated in one area of the casting, cracking of the casting, or of the cooling weld, will almost certainly occur. Even at elevated temperatures, gray cast iron has little ”give”; it will break, rather than stretch, when the force of expansion or contraction exceeds its yield strength. Therefore, whenever a casting must be fusion welded, it is usually necessary to preheat the entire casting, slowly and evenly, before welding is started, and then allow the casting to cool slowly after welding has been completed. This will permit all sections of the casting to expand and contract at a reasonably uniform rate. The temperature to which a casting must be preheated depends somewhat upon the welding process to be used. Oxy-acetylene fusion welding puts more heat into the casting than does arc welding, and therefore requires a higher level of preheat, usually to about 600⁰C (1100⁰F). The preheat temperature level is also somewhat dependent upon the size and form of the casting. Rather simple castings, without major variations in section thickness, usually require less preheat than complex castings. If a suitable furnace is not available for preheating a casting, one can be improvised out of fire brick, as suggested in Chapter 13. If the casting is preheated in a furnace, and then withdrawn for welding, it is essential that as much of the casting as possible be insulated during the welding operation, to hold the preheat as well as protect the welder. Asbestos paper will be found almost indispensable during the fusion welding of cast iron. For Braze Welding. When a casting is to be braze welded, some preheating is usually required, but the level of preheat temperature can be much lower, and many jobs can be done without preheating the entire casting. In braze welding, there is no danger of weld cracking. Bronze weld metal has extremely high ductility, and is capable of absorbing any contraction stresses to which it may be subjected. Because the temperature of the casting itself, even in the metal immediately adjacent to the weld metal, need never exceed 900⁰C, changes in the physical properties of the casting metal will seldom occur. That is why malleable iron castings can often be braze welded. Continued on next page...