Grey cast irons for thermal-stress applications

Abstract

Pearlitic flake-graphite grey cast irons have long been widely used for the manufacture of diesel-engine combustion-chamber components. Nonetheless, very few pertinent high-temperature data are to be found in published work. The investigation described attempts to enlarge current knowledge on the high-temperature behaviour of grey cast irons in view of requirements highlighted by high thermal ratings. A general consideration of the problem showed that the often quoted Eichelberg material-quality factor is largely irrelevant and an alternative assessment formula is proposed. This is broadly confirmed by a systematic examination of the mechanical and physical properties of 166 plain and alloyed cast irons and a limited number of engine components. Different engine components impose different demands upon the material properties but for economic reasons a single cast-iron melt capable of meeting most of the requirements of all components is needed. It is shown that this situation is best met with an alloyed iron containing 1.3 per cent of copper-nickel and 0.4 per cent of molybdenum. The scatter in properties obtained from production casts was found to be typical for such irons. For components having safety factors of 2 or less, a prerequisite of economic engineering design, the service reliability is crucially dependent upon the scatter of material properties. Thus, there are advantages in reducing this scatter and it is shown that this can be achieved by attention to the cleanliness of the matrix and tight control of the chemistry of the melt. In general, the absolute values of mechanical and physical properties of flake-graphite cast irons appear to be governed mainly by the form and size of the graphite flakes and the chemistry of the matrix (i.e. CEV and alloying additions). Regression analysis of the results permitted the derivation of empirical formulae for the prediction of the iron properties. The general findings are used to suggest possible ways of improving the thermal resistance.