Effect Of Hammering Precompression Stress Treatment On Fatigue Properties Of S355J2 Steel Welding Zone

The bogie is an extremely important part of the high-speed train,and it plays a decisive role in the safety performance of the train.However,the welding part of the bogie is very prone to fatigue failure,causing safety accidents and seriously threatening people's life and property safety.Therefore,it is very necessary to strengthen the welding part to improve its fatigue performance.Compared with other strengthening methods,the hammering method has low cost,simple operation and wide application range,and can effectively introduce residual compressive stress on the surface of the welded structure to improve the fatigue performance of the weld.In order to explore the distribution law of the plastic zone inside the material after hammering treatment to achieve the best strengthening effect,this study uses a self-made hammering device,taking the S355J2 steel used in the train bogie as the research object,and the S355J2 base metal and weld are carried out.In the hammering test,the plastic zone produced by the hammering was deeply studied by the pressing hammering method;the influence of the hammering treatment on the fatigue performance of the S355J2 welded joint was explored,and the fatigue performance changes before and after hammering were compared through the fatigue test.It provides a basis for the technology of hammering method in the strengthening direction of high-speed rail bogie welding structure.The experimental results show that:(1)The hammering plastic zone of S355J2 base metal and weld is circular on the abutting surface and spherical in space.Both the depth and area of the hammering plastic zone increase with the increase of the hammering stroke.Under the same hammer stroke,the depth and area of the plastic zone of the base metal are larger than those of the weld,and the greater the hardness,the smaller the plastic zone.In this hammering test,the maximum depths of the base metal and weld hammered plastic zone are 3.25 mm and 3.05 mm,respectively.The number of single-point hammering does not affect the depth of the plastic zone.The multi-point hammering plastic zone is the spatial superposition of the single-point hammering plastic zone,and the spacing of the hammering points will affect the spatial superposition result of the plastic zone.(2)Through the finite element analysis,it is found that the depth of the plastic zone of the hammering test is very close to the simulated depth and has the same trend of change,which proves the reliability of the pressing hammering method.(3)The fatigue limit of the as-welded specimens without hammering strengthening treatment is 179.97 MPa;the fatigue limit of the specimens hammered at a height of 60 mm hammering becomes 194.73 MPa,which is 8.2% higher than that of the as-welded state;The fatigue limit becomes 202.86 MPa,which is 12.7% higher than that in the as-welded state;the deeper the pre-pressure layer,the better the fatigue performance of the welded joint;the fatigue limit of three times of 100 mm hammering height is 223.15 MPa,which is higher than that of the as-welded state.24%,the fatigue data after three times of hammering is relatively stable,and under the same stress amplitude,the fatigue life of welded joints after three times of treatment is increased by more than 5 times.After three times of hammering,the roughness is smaller than that of single-pass hammering,and the precompression stress is also more uniform.Hammering treatment effectively improves the fatigue performance of S355J2 welded joints.(4)The as-welded samples all broke at the weld,and the hammer-strengthened samples basically broke at the base metal,and the hammer treatment had a significant strengthening effect on the weld.The fatigue fractures of the samples before and after hammering all have crack source regions,crack propagation regions and transient break regions,but the morphology is different.There are tear edges and cleavage steps in the expansion zone,but the fatigue streaks of the samples after hammering are not obvious compared to the welded state.Some secondary cracks and rock sugar-like structures can be seen in different areas of the fracture.The welded and hammered samples have plastic fracture at the end of crack propagation,and a large number of fracture dimples can be seen,and the surface is dark gray fibrous.