Handbook-Welding Cast Iron

Nodular Iron. Nodular cast iron, sometimes called ductile iron, has many of the properties of malleable iron.

Nodular cast iron is made by inoculating the molten metal, just before casting, with a small amount of magnesium or cerium. This causes the free carbon in the finished casting to appear as rounded nodules of graphite, rather than as flakes. Each nodule is surrounded by a zone of ferrite (carbon-free iron) with the balance of the metal usually in the form of pearlite. Nodular iron has less ductility than malleable iron (which can have almost as much ductility as mild steel) but far more than ordinary gray iron, which has virtually none. It usually has high strength; in fact, the yield strength of a nodular iron is almost always greater than that of mild carbon steel. All nodular irons have one property which clearly sets them apart from most gray irons; they have a high modulus of elasticity. In simpler terms, they have excellent stiffness, a property much desired in parts like propeller shafts or forming rolls. Where most gray irons are much more elastic (less stiff) than steel, nodular cast iron is nearly as stiff as cast steel. Like malleable iron, nodular iron cannot be fusion welded and retain all of its original properties. This is especially true of nodular iron castings which have been heat-treated after casting. A fusion weld made in nodular iron may not cause loss of tensile strength, but will almost always reduce the shock resistance of the part. Braze welding can be used on nodular iron if some sacrifice of tensile strength can be tolerated. Alloy Cast Irons. Alloying ingredients – chromium, nickel, molybdenum, and, occasionally copper or aluminum – are added to cast iron for three principal purposes: to increase wear resistance, to increase resistance to scaling in high-temperature service, and to increase corrosion resistance. In some alloy cast irons, the silicon level is also increased substantially. Some of the extra-hard, abrasion-resistant alloy irons are white irons; they appear almost white when fractured because they contain virtually no free carbon. Others may have the general appearance of gray cast iron. The range of compositions is so great that no general statement about the weldability of alloy cast iron can be made. So far as the oxy-acetylene process is concerned, fusion welding is not recommended; braze welding will not permit retention of all the properties for which the alloy iron was originally specified.