Handbook-Welding Cast Iron

Gray cast iron can usually be welded without loss of essential properties. For fusion welding, preheating of the

casting is absolutely essential. Since a higher level of preheat is required for oxy-acetylene welding then for arc welding, arc welding is likely to be chosen where fusion welding is essential (as it is whenever good color match is desired). For many repair jobs, however, oxy-acetylene braze welding is the ideal method. Much less preheating is required; in many cases, preheating can be done with the torch. If the work is properly done, the braze-welded joint will have a strength equal to that of the base metal, and excellent machinability. Welding of gray iron castings which have chilled white iron surfaces is seldom attempted, since the desirable properties of white iron will always be affected by welding temperatures. Welding of white iron generally is limited to malleable iron foundries, where castings may be reclaimed by welding before conversion to malleable iron takes place. Malleable Iron. The chemical composition of malleable cast iron is much the same as that of a typical gray iron, but its properties are much different. It is tough; it can resist shock; it has ductility approaching that of mild steel. How is such a remarkable change achieved? By cooling the original casting so rapidly that white cast iron, with no free carbon, is formed; then heating the casting to about 800⁰C and holding it at that temperature for several days. Under those conditions, virtually all the carbon is released from the iron carbide to form fine rounded particles of graphite (sometimes called temper carbon) scattered among grains of ferrite. Malleable iron has good wear resistance, and is widely used for parts where the toughness of steel is required, and the economy of casting (instead of forming or machining) will result in lower cost. However, malleable iron is substantially more expensive to make than gray iron, and is usually selected only where its toughness and ductility are essential. Malleable iron cannot be successfully fusion welded and retain its unique properties; to put it another way, you can weld malleable iron as easily as you can weld gray iron, but in the act of welding you will convert some of the malleable iron casting into a gray iron casting. Seldom will that yield a satisfactory result. However, malleable iron castings can usually be braze welded successfully. You may wonder how to tell a malleable iron casting from a gray iron casting. There’s one almost infallible method: use a high-speed grinder to make a spark test. The difference between the spark streams produced by gray iron and malleable iron is quite pronounced. Spark testing is covered in the Appendix.