Improvement and Analysis of Fatigue Strength for Mild Steel 20MnCrS5 during Carburizing and Quenching

摘要

Distortion and fatigue are both important criteria for evaluating carburizing and quenching process. An optimized process was proposed to reduce distortion and improve fatigue strength simultaneously. Mild steel 20MnCrS5 were heat treated using standard condition and optimized condition respectively. The microstructure, hardness, residual stress, domain size, fatigue performance and crack growth rate with different conditions were studied. Due to carburization, the near surface of the materials have different microstructures with different carbon concentration. The carburized layer, subsurface layer and central layer were selected to prepare the fatigue specimens and to be evaluated. The strengthening effect was verified by comparing the fatigue limit and the crack growth rate. The strengthening mechanism was analyzed by comparing microstructure, retained austenite, residual stress and domain size. The results show that with the optimized condition the fatigue performance at different layers are improved while achieving higher surface hardness. The joint action of domain refinement, more compressive residual stress and less retained austenite results in the strengthening.