Behavior of sulfide inclusions during thermomechanical processing of AISI 4340 steel

Abstract

The morphological and compositional modifications of sulfides in AISI 4340 low alloy steel, in which the sulfur level was raised to about 0.1 pct, were studied during hotrolling at 1223 K followed by homogenization at 1583 K for various lengths of time. The relative plasticity of sulfide inclusions with respect to the steel matrix increased with cooling rate during solidification, hence with iron content. The number of sulfides first increased with homogenization time, reached a maximum and subsequently decreased. Inclusion size exhibited the opposite variation. The sulfide matrix interface area per unit volume of sulfide decreased continuously with homogenization time. These variations were in agreement with observed morphological modifications of sulfides during homogenization. During early stages of homogenization the flattened and elongated sulfide plates in the as-rolled material coarsened and became cylindrical. The cylindrical sulfides, broken into segments which spheroidized and coarsened with time, assumed finally a faceted morphology. During homogenization iron was rejected from the sulfide phase into the surrounding matrix, whereas manganese was accepted, causing the formation of manganese depleted zone around the inclusions.