| Railway plays an irreplaceable important role in our country’s transportation system, and the wheels, as one of the most important constituent part of the transportation system, has a significant influence for the normal operation of the railway system. With the rapid development of China’s heavy-duty and high-speed railway, causing the increase of speed and axle load on rail, which leads the service condition of track structure materials worse. Surface stripping and wear caused by rolling contact fatigue become worse and worse, and the wheel damage and the method to improve the ability to resist fatigue failure have become one of the most important questions puzzling the railway workers.High-energy shot peening treatment has been used on the CL60 steel, and the microstructure and hardness after HESP have been measured to compared with the surface deformation zone on the ineffective wheel. The microstructure and formation mechanism of the white layer on the surface of the ineffective wheel has been researched in this paper, and the feasibility of shot peening strengthening treatment on the wheel steel is discussed at the same time. Shot peening treatment were applied to the experimental material wheel steel CL60, with 1mm steel shot at a speed of 45 m/s as time varies in order to analyze the changes of the surface roughness, stress, microstructure and hardness after shot peening treatment. The results showed that: The damage of different degree exist on the surface of ineffective wheel. The inside region of the wheel tread has the slightest image, which is mainly made of plastic deformation organization. The hardness of the inside region can reach about 380 HVand the thickness of the deformation layer is about 100μm.The middle region of the wheel tread and the rim has a more serious image compared with the inside region, and the surface structure consists of two parts, discontinuous white layer and plastic deformation organization. The white layer is martensite, its hardness of the white layer can reach about 900 HV. There is a certain thickness plastic deformation organization between the white layer with the matrix structure. The cementite in the white layer begin to deform and fracture, and finally dissolve into the ferrite, and finally develop many grains about 50-200 nm in diameter, what’s more,with the effect of plastic deformation, slip band can be found on the surface of grain. In order to improve the strength of wheel steel in the process of actual operation, different parameters shot peening treatment are applied on the CL60 steel. The surface roughness of the treated samples are all lower than 4.9μm,which is accord with the wheel service specification..After shot peening treatment, residual compressive stress of about 400μm thickness changing along with depth generated at the surface of the sample, the maximum compressive stress is 762 MPa after shot peening for 2 minutes. With increasing time of shot peening treatment, hardness increased gradually,the hardness of the surface is about 375 HV after shot peening treatment for 30 mins,and reached the maximum 480 HV when peened for 360 mins,the thickness of the hardened layer increase at the beginning, and reach the maximum after peened for 1h,about 130μm,then remains stationary. After shot peening treatment, the cementite on the surface of wheel steel begin to deform and fracture, and finally dissolve into the ferrite, the ferrite on the surface refines significantly,the grain size of ferrite can be refined to 70 nm after peened for 180 mins.The hardness of the wheel steel can be improved effectually without damage on the surface after peened for 60mins~180mins, which is considered as the optimum processing of shot peening strengthening on the wheel steel. The formation mechanism of white layer are the combination of the temperature mechanism and the plastic deformation mechanism; And the white layer is composed of martensite, retained austenite. |
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