Effect Of Laser Shock Peening On Residual Stress And Fatigue Performance Of U75VG Rail Flash-butt Welding Joints

With the prosperous development of the railway industry,the advantages of continuous welded rail(CWR)are more and more recognized by the whole world,especially the countries along “the Belt and Road Initiatives”.As an important part of CWR,the significance of welded joints has become increasingly prominent,too.Due to the residual stress after welding,the smoothness variation of the welded joint and the discontinuity of the structure and performance,the welded joint becomes a weak part in CWR,which is easily damaged by the cyclic stress and prone to rail breakage.Laser shock peening(LSP)is a surface treatment method that can effectively improve the residual stress distribution and fatigue performance.Besides improving the quality of welded joints,the research on the effect of LSP on the residual stress and fatigue properties of welded joints can also provide an important reference for the subsequent strengthening process of rail welded joints and promote the application of laser shock strengthening in the field of rail transit field.In this paper,the flash-butt welding and post-weld-heat-treatment process of U75 VG rail which is in accordance with the Chinese railway standards is obtained through many tests.The technological parameters of LSP are designed.The distribution of residual stress,surface roughness and hardness of welded joint are measured and analyzed.Through comparison and selection,an LSP process for rail welded joints with the laser energy of 5J and the impact number of 1 was obtained.It is found that LSP can induce significant compressive residual stress on the surface of welded joint base,and it gradually attenuates with the increase of depth.Within a certain range,the compressive residual stress increases with the increase of laser energy,and the attenuation slows down at depths above 1 mm.Multiple impacts will affect the existing residual stress distribution on surface,but it can increase the depth of the compressive residual stress layer and reduce the attenuation rate of the surface compressive residual stress.LSP increases the surface roughness and hardness of rail welded joints.As the laser energy and the number of impacts increase,the surface roughness and hardness increase.A three-point bending fatigue test is designed to analyze the effect of LSP on the fatigue properties of rail welded joints and fatigue tests are carried out on the rail welded joint specimens before and after LSP under different load conditions.It is found that LSP has a improvement on the fatigue performance of welded joints,which can improve the fatigue limit by about 7%.On this basis,the mechanism of LSP of rail welded joints is analyzed from the perspectives of introducing residual compressive stress,improving surface morphology,increasing density of surface dislocation and refining grains.Based on the analysis of the morphology of the normal grinding scratches,an experimental design is carried out on the influence of the grinding process of the rail welded joint,and the influence of the residual scratch direction during rough grinding on the fatigue performance of the rail welded joint is compared.For railway lines with large axle loads such as freight train lines,the perpendicular rotation direction of the grinding wheel to the weld seam during rail base rough grinding after welding is beneficial to the improvement of the fatigue life of welded joints,the number of cycles can be increased by more than 5 times.For light axle load lines such as high-speed railways,the grinding direction will not have a significant impact on the bending fatigue performance of the welded joint.In addition,it is found that different residual scratch directions will not affect the effect of LSP.