Improvement of fatigue performance of low-alloy steels with a sulfurized layer using fine particle peening as pre-treatment under rotating bending

• A hybrid surface-modification method that combines FPP and sulfurizing was proposed. • Part of the surface-modified layer formed by FPP was replaced by the sulfurized layer. • The residual stress relaxation behaviour during fatigue tests was revealed. • The fatigue limit for sulfurized steels was increased by FPP using steel-particles. The influence of a hybrid surface modification combining fine particle peening (FPP), which is defined as peening using particles less than 200 μm in diameter, and sulfurizing on the fatigue properties of low-alloy steels (AISI4120) were investigated. Three types of sulfurized samples were prepared by electrochemical polishing and FPP with iron(II) sulfide (FeS) particles or steel particles as a pre-treatment. Fatigue tests were conducted under rotating bending to evaluate the fatigue performance of sulfurized specimens pre-treated with FPP in ambient air. FPP using steel particles increased the fatigue limit of specimens treated with sulfurizing. Moreover, residual stress was measured using the cos α method before and after fatigue tests under constant stress amplitude to examine the mechanism by which the fatigue limit increases considering residual stress relaxation. In addition, the microstructure of surface-modified specimens was analysed using electron backscattered diffraction and X-ray diffraction. Fine crystal grains were formed by FPP; however, the fine-grained layer formed by FPP using FeS particles disappeared during the subsequent sulfurizing. By contrast, fine grains formed by FPP using steel particles remained during the following sulfurizing, which resulted in an improvement of the fatigue performance of sulfurized steels.