This study investigated the effects of Fine Particle Peening (FPP) and Laser Peening (LP) treatments on the fatigue strength of carburized and quenched chromium-molybdenum steel (SCM420 steel) . It was found that LP treatment, compared to FPP, formed a deeper surface modification layer while minimizing the deterioration of surface properties and imparting higher compressive residual stress. As a result, the fatigue strength of the carburized and LP-treated material (C＋LP) improved, with its fatigue limit reaching 1150 MPa. Additionally, the compressive residual stress increased after fatigue testing in the C＋LP material. These results suggest that improving hardness and applying compressive residual stress, without changing surface properties, is crucial for enhancing fatigue strength, as demonstrated by LP treatment.

Bending fatigue strength of the case hardening steel subjected to combined heat treatment with high-concentration vacuum carburizing and subsequent induction hardening was evaluated. The purpose of this study is to clarify the effects of these factors on bending fatigue strength by focusing on the distribution and morphology of undissolved cementite, the amount of retained austenite and its stability. As the austenitizing temperature increased, the cementite decomposed and the amount of retained austenite increased. Compressive residual stress caused by deformation-induced martensitic transformation was generated on the specimen surface during the test, leading to an increase in high-stress and low-cycle fatigue strength. In low-stress, high-cycle fatigue failure, deformation-induced martensitic transformation is unlikely to occur during testing, so the compressive residual stress prior to testing becomes critical. Furthermore, it is beneficial to reduce the stability of the retained austenite and facilitate deformation-induced martensitic transformation.