Experimental Investigation And Numerial Simulation Of Residual Stress Of High Strength Steel After Surface Strengthening

With the wider application fields of high strength steel, due to long time under alternating load, there are more failure events caused by the fatigue cracks in the surface of parts prepared by the high strength steel parts. Therefore, surface strengthening processes such as the shot peening, hole cold-extrusion process and carburizing were carried out on the high strength steel parts under cyclic loading, and the distribution of residual stress field was analyzed by the combined method of experimental research and numerical simulation, in order to improve its service life. It is significant both for theory and engineering.The effects of different surface strengthening processes(shot peening and hole cold-extrusion process) on residual stress field of the high strength steel, and the effects of different shot peening parameters(layer depth and different surface carbon content) on the residual stress field of the 20 Cr Mn Ti steel were investigated in this work. Meanwhile, the residual stress fields were simulated by finite element method.The residual stress after shot peening, hole cold-extrusion process and carburizing were determined by X-ray stress instrument, and the distributions of the residual stress were determined step by step using the electrolytic polishing method. Meanwhile, the crystal structures of the specimens before and after shot peening, hole cold-extrusion process and carburizing were analyzed by X ray diffractometer, and then the surface micro-hardness distributions were measured along the deep layer and the morphology of the specimens were observed. Finite element models of the residual stress fields after different processes of surface strengthening were established by using ANSYS software.The results show that, with the increase of shot peening time, the maximum depth of the residual stress and strengthen depth are increased, while, the surface residual stress is decreased. Compared with the small diameter projectile, the shot peening strengthening depth is larger using projectile with larger diameter. With the increase of shot peening strengthening, the maximum residual stress and strengthening depth are increased. The surface residual stress of the specimens can improved by using projectile of glass or ceramic to second shot peening. The number of dislocation and micro-hardness in the surface of the high strength steel are increased.The grains in the internal surface after hole cold extruding appear deformation and fibrosis, and a strengthening layer in the material surface is formed, then the material surface hardness is improved. The residual compressive stress formed within the range in the distance is about a hole aperture from the side of hole, and the maximum residual compressive stress is appeared in the distance is about 1 mm from the side of hole.A residual compressive stress layer is formed in the surface of the specimen after carburizing process, however, the maximum compressive stress is not in the surface but inside the carburized layer. The residual compressive stress value can be reduced by quenching and tempering after carburizing significantly. Lower surface carburizing time and carbon content will reduce the residual compressive stress value, and make the maximum of the residual compressive stress moves to the surface.Simulation results of the residual stress field after shot peening, hole cold-extrusion process and carburizing process are consistent with the experimental results, show the effectiveness of the model, which can be used for numerical simulation with different process parameters of shot peening, hole cold-extrusion process and carburizing process.With the increase of shot peening projectile speed, the maximum residual compressive stress, the depth of the maximum residual compressive stress and the strengthening depth are increased. The projectile elastic modulus has a little influence on residual stress field after the shot peening, and with the increase of the projectile diameter, the maximum residual compressive stress, the depth of the maximum residual compressive stress and the strengthening depth are increased.With the increase of compression strength, the residual compressive stress at the hole edge and the external residual tensile stress are increased. Meanwhile, the distance between the point of the peak residual compressive stress and the hole edge increase with the increase of the compression strength as well. With the increase of hole diameter, the peak residual compressive stress value is decreased, while the distribution width is increased. When the distance from pore to edge e/d is small, the residual compressive stress at the hole edge is small, but the external residual tensile stress is large. With the increase of e/d, the residual compressive stress is increased, the residual tensile stress is decreased, and the distribution of residual compressive stress is increased.The transient stress during cooling process of carburizing is simulated using the residual stress field model. When the cooling time is 50 s, the specimens present tension in surface and compression in the core. When the cooling time is 260 s, the specimens present compression in surface and tension in the core. When the cooling time is 1300 s, the tress distribution is same as the distribution at 260 s, the specimen present tension in surface and compression in the core, but the value of the compression stress is larger obviously, and the compressive stress reach the maximum value about 0.7 mm from the surface.