Effect of laser surface remelting on the fatigue crack propagation rate of 40Cr steel

In the present work, quenched and tempered 40Cr steel was treated by laser surface remelting, followed by microstructure evolution, hardness, residual stress field and external morphology investigation to understand the effect of laser surface remelting on surface characteristics, as well as the grain refinement mechanism. Then, fatigue tests were carried out to obtain the fatigue crack growth rate and fracture morphology, thereby further discussing the fatigue crack propagation and fracture mechanism. Results indicate that the grain refinement mechanism of 40Cr steel treated by laser surface remelting can be attributed to the rapid melting-solidification process in surface, during which tempered sorbite is transformed into fine martensite. In addition, the residual compressive stress can be produced by the thermal effect. The fine microstructure and residual compressive stress lead to a significant decrease in crack propagation rate due to the high density of grain boundaries which hinder crack propagation.