40CrNiMo structural steel has excellent mechanical properties and is widely used in key components such as turbine shafts, gear shafts and large ring gears. In view of the problems of fatigue and fracture of these mechanical parts in harsh working conditions, the surface of 40CrNiMo structural steel is subject to laser shock peening (LSP) treatment by high-power laser beam. The work aims to improve the microhardness and wear resistance of 40CrNiMo structural steel by inducing residual compressive stress and refining grains through laser shock peening without coating. By means of metallographic microscope, XRD test, microhardness tester, residual stress tester, friction tester and laser scanning confocal microscope, the microstructure, microhardness, residual stress and friction and wear properties of the untreated sample, the LSP sample (with coating) and the LSPwC sample were compared and analyzed. The 40CrNiMo structural steel samples were subject to LSP treatment with/without coating under the action of aluminum foil coating/no-coating and deionized water confinement layer, respectively, which induced residual compressive stress and grain refinement. The diffraction peaks of the samples were all shifted, and the average grain size of the samples treated with LSP (with coating) and LSPwC decreased by 6.0% and 9.6%, the surface microhardness of the samples increased to 313.5HV and 336.9HV, an increase of 13.5% and 21.9%, and the maximum residual compressive stress on the surface could reach -405.3 MPa and -326.6 MPa. After the LSP treatment, the wear surface of the samples was less worn and the number of pits and furrows was reduced, but the width of the furrows varied. The friction coefficient of the LSP (with coating) sample was relatively stable, decreasing by about 14.1%. The friction coefficient of the LSPwC sample was staged relatively. In the early stage of friction and wear, the friction coefficient was reduced by 22.9% compared with that of the untreated sample and 3.2% lower than that of the LSP sample. The main reason was that in the LSPwC process, the laser was in direct contact with the surface of the sample, which induced thermal effects and plastic deformation, thus increasing the number of austenite nucleation. At the same time, the dispersed carbides prevented the growth of austenite grains, and the distribution was uniform, which improved the wear resistance of the sample surface, so that the friction coefficient of the outer layer was the lowest and the wear resistance was the best. In the middle and late stages of friction and wear, compared with that of untreated sample, the friction coefficient decreased by 7.9%, but increased by 4.8% compared with that of LSP sample. Compared to the LSP (with coating) sample, the LSPwC sample show an increased number of pits and oxides on the surface and the presence of larger furrows. As the depth increased, the thermal effect of LSPwC decreased rapidly, and because there was no coating in the LSPwC process, the absorption rate of laser energy was reduced, resulting in a smaller shock wave energy. Thus, the plastic deformation effect was weakened, and the wear resistance was also reduced. The wear amount of the untreated sample was 13 mg and those of the LSP (with coating) sample and the LSPwC sample were 6 mg and 8 mg, respectively, which decreased by 53.8% and 38.5%. On the whole, compared with LSP (with coating), LSPwC has a poorer effect on the wear resistance of 40CrNiMo structural steel, but LSPwC can be directly applied to complex working conditions and structures with complex structures, because LSPwC can be carried out under the condition of no coating, thereby effectively improving the processing efficiency and saving the cost of the coating. At the same time, the microhardness and wear resistance of 40CrNiMo structural steel are also improved to a certain extent. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.
