THE EFFECTS OF INITIAL MICROSTRUCTURE AND HEAT TREATMENT ON THE CORE MECHANICAL PROPERTIES OF CARBURIZED AUTOMOTIVE STEELS

摘要

SAE 8620 and other steels are typically used in the carburized condition for powertrain applications in the automotive industry. Steel parts are generally normalized before carburizing to produce a homogeneous austenite with a uniform grain size. Elimination of the intermediate normalizing treatment could lead to significant cost reductions. The intent of this work was to investigate the effects of composition and initial microstructure (as-received or normalized) on the hardenability as well as the core microstructure and mechanical properties of two case carburized (SAE 8620 and PS-18) steels. Both steels were received in the form of hot-rolled bars, but the PS-18 had received a special controlled-cool processing. This study complements our previous work on the effects of initial microstructure and heat treatment on the residual stress, retained austenite, and distortion of the same carburized automotive steels. The feasibility of using either a new steel or heat treatment process cycle requires acceptable properties in both the case and the core; both directly affect part quality and performance. Jominy curves revealed that the as-received hardenability of PS-18 was higher than that of the 8620 steel. The normalizing heat treatment was characterized for both steels by Jominy testing and by the effect on prior austenite grain size. In both the as-received and normalized heat treatment conditions, the PS-18 steel was noted to have higher ultimate tensile strength and lower Charpy impact toughness than the 8620 steel. The carburizing thermal cycle heat treatment could lead to unacceptable impact toughness values for the core of PS-18 steel components.