Effect of Laser Shock Processing on Microstructure and Mechanical Properties of M50 Bearing Steel

By means of microstructure observation and mechanical property tests, the influence of the laser pulse energy of the laser shock processing (LSP) on the surface residual stress, microstructure, microhardness and fatigue limits of the M50 steel was studied. Results show that when the laser pulse energy increases from 4, 6 to 8 J, the surface damage of the sample becomes more and more pronounced, which gradually changes from pits to long-strip ravines. LSP increases the hardness of the steel, the maximum hardness occurs on the impact surface, and the microhardness gradually decreases as the distance from the impact surface increases. When the laser pulse energy rises from 4 to 8 J, the maximum hardness of the surface increases from 741 to 845 HV1, which are 2.90 and 11.33% higher than the steel matrix respectively. The depth of the hardened layer of the samples produced by LSP is also deepened when the laser pulse energy increases. The hardened layer depth is approximately 0.3 mm when the laser pulse energy is 4 and 6 J, which reaches 0.5 mm when the laser pulse energy is 8 J. LSP generates residual compressive stresses of − 423, − 565 and − 809 MPa on the surface of the samples when the laser pulse energy are 4, 6 and 8 J respectively. The residual compressive stresses can inhibit the expansion of cracks, so that the steel is less prone to fracture, and thereby the fatigue limit of the steel is increased by 45.95% from 740 MPa of the sample without LSP to 1080 MPa after LSP with the laser pulse energy of 8 J.