High cycle fatigue properties of gas-carburized 4140 steel were assessed to compare with those of 8620 steel which is widely used as a carburizing steel. Fatigue limit was evaluated associated with microstructure, case depth, and distribution of retained austenite and compressive residual stress near the surface. Test results indicated that the reheat quenching method of 4140 and 8620 steels produced a reduction in grain size, retained austenite level, and compressive residual stress at the surface and an increase in fatigue limit. The fatigue limit of direct-quenched 4140 steel shows substantially lower value than that of direct-quenched 8620 steel due to larger grain size of direct-quenched 4140 steel. However, the fatigue limit of reheat-quenched 4140 steel is greatly improved and is comparable to the reheat-quenched 8620 steel. This is attributed to the larger reduction ratio in grain size and deeper case depth of reheat-quenched 4140 steel as compared to direct-quenched and reheat-quenched 8620 steels.
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