Microstructure and Properties of Nitriding and Laser Quenching Composite Modified Layer on 38CrMoAl Steel Surface and Its Thickness Prediction

38CrMoAl steel was nitrided at 460 ℃ for 8 or 16 h and then laser quenched. The microstructure and properties of composite modified layer were studied. The nitrogen concentration distribution field and laser quenching temperature field of the composite modified layer were simulated by Matlab software and Abaqus finite element software, respectively. The thickness of the composite modified layer under different processes was predicted, and was verified by tests. The results show that the surface structure of the composite modified layer was composed of martensite containing susaturated nitrogen, and the subsurface consisted of quenched martensite. At the same time, there were iron oxides (Fe₃O₄, FeO, Fe₂O₃) and iron nitrides (ε-Fe_2-3N, γ´-Fe₄N) in the composite modified layer, and the content of nitrides under 16 h nitriding and laser quenching was more. There were platform zones in the section hardness distribution curves of composite modified layers under different processes. The hardness of the platform zone of the composite modified layer after 16 h nitriding and laser quenching was 720 HV, which was about 100 HV higher than that of the composite modified layer after 8 h nitriding and laser quenching. The thicknesses of the composite modified layer after 8, 16 h nitriding and laser quenching were 530, 590 μm, respectively. Compared with those after 8 h nitriding and laser quenching, the stable friction coefficient and wear volume of the composite modified layer after 16 h nitriding and laser quenching were lower, which were 0.293 and 1.012 mm³, respectively, and the wear resistance was better. The predicted thicknesses of the composite modified layer after 8,16 h nitriding and laser quenching were 531, 583 μm, respectively, and the relative error was less than 3%, which verified the effectiveness of the prediction method.