On the Roughening of Grey and Nodular-graphite Cast Irons during Surface Melting

Abstract

The gas tungsten arc (GTA) welding process has been used to melt the surface on a series of grey and nodular-graphite iron castings. The topography of the fusion zone was foum to be markedly different for the two types of cast iron: for grey cast iron the resolidified surface was very rough, with surface perturbations leading to localised height differences of up to 0.4 mm, while for nodul-graphite iron a macroscopically smooth surface was obtained. A model has been developed to account for the observed phenomena based upon the influence of sulphur on the interfacial tensions responsible for the spreading of the liquid phase on the surface of the underlying unmelted region. It is hypothesised that surface-active sulphur is available in grey cast irons to exert a depressive influence on the interfacial tensions, but that in nodular-graphite irons the sulphur is rendered immobile through its combination with magnesium. The results have implications for other directed energy surface melting processes such as laser anc electron-beam melting of cast irons.