INVISIBLE MACRODEFECTS IN CASTINGS

There is a wide spectrum of controls which are required to be in place to ensure the maintenance of quality in castings. Most of these are well known, and are not therefore considered in this paper. The parameters which are often overlooked, and thus not controlled, are (i) the rate of flow of liquid metal in the mould to avoid surface turbulence and the generation of macroscopic crack-like defects as a result of folded-in surface films; and (ii) the rate of quench following solution heat treatment. As a result of failure to control these critical parameters castings traditionally exhibit random failure from leakage, and mechanical failure, especially fatigue. Mechanical failure is enhanced by internal stress which is superimposed on service stress to promote premature failure. It is considered that these are the main reasons why in the past castings have been found to be unreliable, compared to other production techniques such as forging. Techniques to control both surface turbulence and internal stresses include respectively (i) the limiting of flow velocities in moulds to less than 0.5 m/s, and (ii) eliminating the water quench from casting heat treatments. These actions are expected to revolutionise the concept of castings as totally reliable products.