Effect of laser hardening on the fatigue strength and fracture of a B–Mn steel

Abstract

The influence of laser hardening on plane bending fatigue behavior ( R=−1) of a B–Mn SS2131 (∼AISI 15B21H) steel has been studied. Laser hardening improves the fatigue limit of Q&T smooth and notched specimens by 18 and 56%. The fatigue strength improvement depends on the fatigue life. Surface and subsurface crack initiations occur independently of the applied load level. The residual stresses and hardness distributions caused by laser hardening generate improvement in the fatigue properties, however, the improvement is less than that seen for plasma nitriding on the same core material. The local fatigue strength approach predicts the existence of surface layers with good and bad fatigue properties. The detrimental factors that occur with laser hardening are the formation of soft surface layers and low compressive residual stresses at the overlapping zones. These factors produce a long narrow crack along the edge of the overlapping zone causing premature failure. Comparison of fatigue data for different laser-hardened steels shows a trend towards an increased fatigue limit with increasing hardness and compressive residual stress.