Abstract
A carburized coarse-grained AISI 8620 steel was subjected to three postcarburization heat treatments: a) direct oil quench from the carburizing temperature (1700°F), b) direct oil quench, reheat to 1550°F and oil quench, and c) slow cool, reheat to 1550°F and oil quench, reheat to 1450°F and oil quench. The latter two treatments refined the austenitic grain size over that resulting from the direct quench and caused a reduction in the size of the marten-site plates and of the number and/or size of the microcracks within the plates. The refine-ment of the microstructure and the reduction of the number of microcracks resulted in greatly improved fatigue resistance,i.e., from fatigue limits of 140 to 250 ksi maximum cyclic stress for the direct quench and double reheat conditions, respectively. Subcritical crack growth was transgranular, but the mode of unstable crack propagation was mixed transgranular and intergranular in the direct quench and single reheat specimens. Obser -vations of microcrack coalescence and fracture surface features suggest that microcracks are instrumental in the transgranular mode of failure.
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