Fatigue Strength of Laser Hardened 42CrMo4 Steel Considering Effects of Compressive Residual Stresses on Short Crack Growth

Abstract

The paper contains results of a study aimed at an experimental evaluation of fatigue resistance and initiation mechanisms of heat treated and laser hardened 42CrMo4 steel. Experiments were performed using quite small specimens of 8 x 8mm cross section, under three point bend fatigue loading. Two different parameters of laser hardening were used, one of them resulted in considerable longitudinal residual stresses – surface speed of laser beam 4mm/s. Results of fatigue tests of basic reference material, just heat treated, were characteristic by a surprisingly high scatter, particularly in the region near fatigue limit. Fractographical analyses indicated that this scatter was connected with presence of surface or subsurface defects, mostly inclusions, even quite large, which in some cases caused fatigue crack initiation. Compressive residual stresses after the laser treatment, dependent on laser treatment parameters, improved fatigue strength and reduced the scatter, likely due to short crack retardation in the compressive residuals stress field. Further analyses were carried out using Murakami method of evaluation of fatigue life of materials containing defects. The Murakami model of fatigue limit evaluation of specimens containing defects was just slightly conservative, when applied to specimens just conventionally heat treated, but quite strongly conservative particularly in case of laser surface hardening with high longitudinal compressive residual stresses. The results indirectly confirm significant beneficial effects of compressive stresses induced by laser hardening on fatigue resistance caused by retardation or arrest of short fatigue cracks emanating from microstructure defects.