Effects of austenite grain size and grain boundary segregation of impurity atoms on high temperature ductility

Abstract

The high temperature deformation characteristics of low alloy, high strength steel martensites have been studied with particular reference to the role of prior austenite grain size and the grain boundary segregation of impurity elements. It was confirmed that, in the region where the mean linear intercept of austenite grains is larger than 100 μm, slow tensile deformation at elevated temperatures nucleates microvoids on the boundaries and microvoid coalescence leads to final fracture. Elongation in this region is determined specifically by prior austenite grain size and is linearly proportional to the inverse of prior austenite grain size, if the mode of matrix strengthening is not varied. The addition of Cu or P impurities reduces austenite grain growth at temperatures above 1400 K and increases elongation. For higher P contents, the fracture mode transition from intergranular ductile to intergranular decohesion can be observed occasionally during the course of fracture. However, the effect of this transition on elongation cannot be detected by the unnotched tensile test used in these experiments because final fracture occurs following microvoid nucleation on prior austenite grain boundaries.