Effect of grain refinement of carburized layer on fatigue properties of M50NiL steel

This study investigated the formation of recrystallized and sub-grain structures, achieved through the PSN mechanism and pinning effects of second-phase particles, in the subsurface of the carburized layer in M50NiL steel during carburizing heat treatment. This process refined the carburized layer's grain size to the submicron scale. The submicron crystal region (SMCR), characterized by numerous grain boundaries and high-angle sub-grain boundaries, reduced martensite size and promoted the dispersed precipitation of carbides during tempering. Meanwhile, the high carbon content and significant lattice distortion increased the formation of twinned martensite. The above microscopic strengthening mechanism explains the hardness improvement of SMCR by 10-20HV₁. Rotating bending fatigue test results indicated that by fabricating the SMCR in carburized layer, the fine grain strengthening and the dispersion precipitation strengthening were realized, the strength and plasticity of the carburized layer are improved simultaneously, the generation and expansion of fatigue cracks were prevented. This method successfully prolonged the fatigue life of M50NiL steel and increased the fatigue limit by about 50 MPa. TEM analysis revealed that the main mechanism for fatigue crack initiation in M50NiL steel is related to the specific orientation relationships of martensite laths. When [100] _Martensite//[111] _Martensite, {110} _Martensite//{110} _Martensite at adjacent martensite laths, the phase boundary exhibits lower fatigue fracture strength and higher stress concentration. These make the local region more susceptible to becoming the origin of fatigue cracks. The parallel of the closely packed plane reduces the binding force of the two martensite laths and causes the crack initiation direction along the {110} _Martensite plane. As the crack propagates, it will increasingly favor the direction perpendicular to the stress.