Research On The Rules And Mechanisms Of Material Removal During Rail Grinding

As the most commonly used rail maintenance technique,the rail grinding technique is the process of grinding rail using the end surface of a grinding wheel.Rail grinding process is under high thermo stresses and contact stresses,and is affected by many parameters.In this case,the removal behaviours between the rail material and grinding wheel are very complicated.Therefore,the factors affecting the grinding process should be studied,and the material removal mechanism should be clarified for improving the rail grinding technique and guiding the selection of rail grinding parameters in the field.The rail grinding experiments were performed using a rail grinding simulation apparatus under different grinding parameters(grinding pressure,rotational speed of grinding wheel,forward speed of rail grinding train).The grinding amount was measured and the grinding debris was observed to explore the influences of grinding parameters on the grinding efficiency.In addition,the rail surface morphology,white etching layer,and residual stress were measured to investigate the influences of grinding parameters on the rail grinding quality.Through the establishment of models of grinding force and temperature field during rail grinding,the influences of grinding parameters on the grinding force and specific energy were analyzed.In addition,the temperature distribution on the rail surface and cross section was studied,and the influence of grinding wheel diameter on the grinding temperature was investigated.Furthermore,the relationships between the grinding temperature and surface burn and white etching layer were established.Finally,the microstructural transformation of a rail material and the formation of grinding debris were studied.Based on the cutting behaviours of an abrasive grain,the material removal mechanism during rail grinding was analyzed.The main conclusions are as follows:(1)A rail with fatigue cracks was used to perform the grinding experiments.When the grinding pass increased from one to four,the fatigue cracks were gradually invisible from the rail surface,but were not completely removed on the cross section of the rail.After grinding,the blue burn was observed on the rail surface,especially around the fatigue cracks.In addition,the ground rail was covered with a white etching layer.(2)The increase in the grinding pressure could improve the rail grinding efficiency but the surface roughness,surface burn,thickness of white etching layer,and residual stress also increased.Therefore,increasing the grinding pressure could improve the grinding efficiency but reduce the grinding quality.Taking both the rail grinding efficiency and grinding quality into consideration,it was suggested that the optimal grinding pressure were 2000 N,1800 N and 1600 N when grinding the rail crown,rail shoulder and rail gauge corner,respectively.(3)The increase in the rotational speed could improve the grinding efficiency but would reduce the rail grinding quality.With the rotational speed increasing,the grinding amount increased,but the surface burn and the thickness of white etching layer were also increased.The increase in the forward speed could reduce the grinding amount,but the surface burn,thickness of white etching layer also decreased,meaning that the grinding quality could be improved.It was suggested to increase the rotational speed and forward speed at the same time,which had potential to improve both rail grinding efficiency and grinding quality.(4)The longitudinal grinding force was zero at the zero swing angle of grinding wheel.With the increase in the swing angle,the longitudinal grinding force was increased,and the vertical grinding force was decreased.With the forward speed and grinding pressure increasing,the grinding forces were increased.The increase in the rotational speed and granularity of grinding wheel resulted in the decrease in grinding force.(5)The temperature in the center of a grinding band was higher than that at the edge.Under the same grinding parameters,the temperature at the rail gauge corner was the highest,followed by the rail shoulder,and the rail crown.The increase in the grinding pressure and rotational speed could lead to the rise of grinding temperature,and the increase in the forward speed resulted in the decrease in grinding temperature.The grinding temperature decreased gradually along the direction of rail depth,and basically dropped to the room temperature level at about 2 mm.In addition,the inner and outer radii of a grinding wheel had significant impacts on the grinding temperature.(6)When the grinding temperature was around 468.8 ℃,the yellow burn started to occur on the rail surface after grinding.In addition,with the grinding temperature increasing,the colour of surface burn was changed to brown and then to blue.When the grinding temperature was around 400 ℃,the white layer was found on the rail surface after grinding.In addition,the thickness of white layer increased with the grinding temperature increasing.Furthermore,the essence of the surface burn might be a layer of oxide film which covered on the rail surface.(7)The austenitic was found in the rail material after grinding,and the rail material underwent serious plastic deformation.The white etching layer was the martensite caused by the phase transformation of the rail material due to the high temperature,and a high contact pressure on the rail material applied by an abrasive grain decreased the austenitizing temperature.In essence,the rail grinding process was composed of numbers of cutting processes accomplished by the abrasive grains.Due to the extrusion process by abrasive grains,the rail material was removed in the form of strip debris.The abrasive grain might be worn in the cutting process,and the main wear patterns were abrasive grain fracture,adhesion wear and abrasion wear.