Some Peculiarities in Deformation and Fracture Behaviour of Austempered Ductile Cast Iron

Abstract

Tensile tests and fracture toughness tests of an unalloyed austempered ductile cast iron (ADI) with the matrix created by upper bainite and containing 30 to 35 % of retained austenite were performed in a temperature range –196 °C to +200 °C to study its mechanical behaviour. The temperature dependence of true fracture stress, elongation to fracture, and reduction of area is separated into three regions with different fracture behaviour. Unmonotonous temperature dependence of yield points defined by plastic strain of 0.05 to 2 % is evidence of the stress induced phase transformation of retained austenite into martensite before these values of plastic strain are reached. The transformation of retained austenite is also substantiated with an increase of hardness and in several cases the decrease of retained austenite content was directly determined with X-ray quantitative phase analysis. Also higher values of dynamic than of static fracture toughness are evidence of unusual behaviour of ADI due to retained austenite presence in ADI structure