Nodular Cast Iron—its Present Position and Future Prospects as an Engineering Material, with Special Reference to its Suitability for Crankshafts

Abstract

The historical development of iron having graphite present in the nodular form as cast, is traced from the patent application by Morrogh and the British Cast Iron Research Association (1946)† to its selection as a crankshaft material by the Ford Motor Co. in 1952. The economics of the production of nodular iron by various methods is discussed and it is concluded that material produced by the magnesium-nickel process now in use cannot compete in price with established materials such as carbon steel and malleable iron. Methods for direct addition of magnesium to molten iron are described, and it is concluded that application of these methods to low-sulphur irons from basic cupolas may completely alter the situation. The engineering properties of nodular cast iron are compared with those of other ferrous metals. It is concluded that nodular irons in the as-cast condition may be used instead of high grade flake-graphite irons for components not subject to shock loading, and instead of malleable irons in applications where 2–4 per cent elongation at fracture is adequate. The annealed nodular cast irons may be used, where their extra ductility and impact resistance are considered essential, to replace malleable irons and cast or forged carbon-steels. Their chief advantage over malleable irons lies in the shorter heat-treatment they require, which becomes economically more important as the size of a casting increases and as its selling price per lb. falls. As a replacement for carbon steel, nodular cast iron will be attractive for components which are difficult to cast or which require high surface-hardness with medium strength and impact resistance. However, the extent to which nodular cast iron replaces any of these materials will be determined largely by the price at which it can be produced; this will have to be decreased very considerably before it becomes competitive. Ultimate output of nodular cast iron in Britain in peace time is likely to be limited by the high cost of the low-phosphorus ore which is necessary for its production; in war time, its output might be severely restricted by the difficulty in importing this ore. The particular application of nodular cast iron to crankshafts is discussed in detail. It can replace alloy flake-graphite irons with a gain in ductility and impact resistance, and is already replacing graphitic cast steel, chiefly because of its better founding properties, and because its lower pouring-temperature facilitates the use of mass-produced shell moulds. It seems doubtful whether its fatigue properties will permit of its replacing the forged low-alloy steel which is used for most British automobile crankshafts, unless some reduction in engine specific-power output is accepted.