Fatigue, Fracture and Strain Hardening of High Carbon Hardened Alloy Steel

摘要

Medium and high carbon alloys steels have been heat treated to microstructures of low-temperature tempered martensite and retained austenite. Four point bending fatigue testing of 0.8 pct C steels showed that low cycle fatigue resistance was directly related to retained austenite content. The strain-induced transformation of retained austenite substantially increased strain hardening rates of the composite tempered martensite-austenite microstructures at high strains and increased the number of cycles required to initiate fatigue cracks at prior austenite grain boundaries in specimens with the highest retained austenite content. Transmission electron microscopy identified the transition carbides formed on tempering as the orthorhombic eta carbide, and the increasing density of the transition carbides with increases in carbon content was the major carbon-dependent structural parameter which correlated with flow stresses and strain hardening rates in medium carbon tempered martensite. Elastic limits, as measured with strain gages mounted in compression specimens, decreased with increasing retained austenite content. In medium carbon steels with lath martensite morphologies the retained austenite transformed to martensite by stress induced mechanisms, and in high carbon steels with plate martensite morphologies, the retained austenite transformed by strain-induced mechanisms.