Influence of test direction on hot ductility of austenite

Abstract

The influence of test direction on the hot ductility of a hot worked high S steel (0·15%S) has been examined over the temperature range 700–1100°C and for strain rates 3·3 × 10^?4 and 1·3 %times; 10^?2 s^?1. As with room temperature tensile testing, ductility was lowest in the short transverse direction and greatest in the longitudinal direction. Ductility troughs were observed for all directions at the lower strain rate. Increasing the strain rate improved hot ductility and removed the trough for samples tested in the longitudinal direction. Similar directional behaviour was observed for plate steel with standard S levels (≤0·01%), but the differences in hot ductility with direction were much reduced. Fracture was transgranular dimpled rupture when ductility was good, but intergranular for poor ductility. Intergranular failure occurred when the steels were austenitic. The elongated MnS inclusions were found to be situated in the γ-boundaries and it is believed that intergranular failure occurred by the inclusions restraining grain boundary movement and allowing voiding and decohesion at the inclusion/boundary interface, thus encouraging grain boundary sliding. Increasing the strain rate improved ductility by reducing the amount of grain boundary sliding and increasing the grain boundary migration rate making it difficult for cracks to link up.

MST/776