Effect of sulphide inclusion shape on plane-strain fracture toughness (K_IC) of heat-treated structural low-alloy steels

Abstract

Three low-alloy structural steels with different levels of nickel, chromium and molybdenum and a carbon content of 0.4 wt% have been studied to determine the effect of the sulphide inclusion shape on the plane-strain fracture toughness (K_IC) of ultra-high strength steels. The shape of the sulphide inclusions was changed by varying the hot-rolling reduction at a temperature of 1473 K. The shape of the inclusion was modified from a stringer to an ellipse at a similar volume fraction level by decreasing the hot-rolling reduction from 98 to 80%, independent of the steel. This had a differing response for the mechanical properties of each steel. For 0.4C-Ni-Cr-Mo steel, modifying the shape of the sulphide inclusions greatly improved K_IC at an increased Charpy impact energy and similar strength level, independent of orientation. However, for 0.4C-Cr-Mo steel the mechanical properties were less affected than those for 0.4 C-Ni-Cr-Mo steel. Changing the shape of the sulphide inclusions had little effect on the mechanical properties of the 0.4 C steel. The results obtained are briefly discussed in terms of metallographic observations, X-ray measurements and fractography.