The Effect Of Laser Surface Strengthening On Wear And Damage Properties Of Rail Welding Joints

The increase of train speed and axle load has caused more serious damage of rail welding joints,which has affected the safety of railway transportation.Therefore,investigation on the influence of laser strengthening treatment on the wear and damage properties of the rail welding joints has significant guiding significance and application value for reducing the damage and extending the service life of rail welding joints.In this paper,an on-site service rail welding joint was analyzed.The differences in microstructure and damage between the joint and the base metal were studied.The surface of the rail welding joints were treated by laser quenching?LQ?and laser shock peening?LSP?,and the influence of laser strengthening treatment on microstructures and hardness of different zones of the welding joints were analyzed.Rolling wear tests were carried out with the laser treated welding joint samples on the MMS-2A rolling wear tester.The wear and damage properties of different zones of the welding joint were analyzed.The main conclusions are as follows:?1?The rail welding joints could be divided into the central line,the weld bead zone and the heat affected zone.A large amount of pro-eutectoid ferrite was produced in the central line,resulting in the hardness obviously lower than that of the base metal.The microstructure of the weld bead zone was composed of block pearlite and the hardness was close to the base metal.The microstructure of the heat affected zone was mainly composed of granular bainite,and the hardness was obviously lower than that of the base metal.?2?The plastic deformation of the side wear area was more severe than the top rail area.The plastic deformation of the base metal was less severe than that of the welding joint.The fatigue cracks in the weld bead zone and the heat affected zone were large and developed in different directions in the top rail area,and the crack propagation depth are consistent.The propagation direction of oblique cracks in the weld bead zone and the heat affected zone was consistent.The number of cracks was smaller than that in the base metal,but the crack length and depth were larger and the fatigue damage was more serious than those in the base metal.The fatigue cracks propagation mainly showed transgranular manner in the central line and the weld bead zone.The fatigue growth path was straight and the fatigue cracks tip was passivated in the heat affected zone.?3?Uniform and dense martensite was obtained on the rail surface treated by laser quenching.The thickness of the hardened layers was about 500?m,and the surface hardness was increased to 900 HV_0.5.The wear rate was reduced by 90%and the thickness of the plastic deformation layers was decreased.Small pieces of peeling were the damage form.The fatigue cracks mainly initiated on the surface,and propagated with larger angles.The crack length was increased.The expansion depth was larger than the plastic deformation layers.Laser quenching treatment improved the wear resistance of the rail material,but failed to effectively increase the fatigue damage resistance.?4?Laser shock peening can significantly refine the materials gains of rail surface.The surface hardness was increased by about 11%-16%and the influence depth was about 800?m.The wear rate was reduced by 4%-11%.Lamellar wear was the main fatigue damage form of the samples.Cracks mainly initiated on the surface and subsurface of the samples.Laser shock peening improved the wear resistance and plastic deformation resistance of the welding joint,reduced the generation of cracks,and improved the fatigue resistance of the rail welding joint.