The Research On Microstructure And Failure Of Bainitic Joint

With the development of the heavy haul railway, the bainitic rails with excellentwear resistance are used for heavy haul railway. However, the quality of bainiticjoints need to be improved. Therefore, it is necessary to study on the problems ofbainitic joints.In this paper, bainitic flash welding joints and bainite-pearlite dissimilar thermitewelding joints are applied as sample. Not only the reason of bainitic joint failure arestudied, but also discussed the influence of flash welding joint welding Heattreatment. The main results are as follows:There are white micro-region and banded structure in the neighborhood of flashwelded metal. The cause of white micro-region appeared in welding joints is due toelement segregation. Its Metallographic Figures is due to element segregation andorganizational differences. The composition segregation of steel is the reason offormation of banded structure. The reason of flash welding joint failure ismicro-crack which exists in white micro-region. The propagation of micro-crack goesalong banded structure. The segregation of P in white micro-region is the cause ofthe micro-crack.In the fusion zone of thermite welding joint exists element diffusion. Diffusion ofcarbon is far stronger than alloying elements. The fusion zone near bainitic basemetal has a higher element content and the plate martensite comes to form. Theemergence reasons of intergranular crack in this area are plate martensite and coarsegrain.Reducing the cooling rate can slightly decrease hardness, increase plasticity andtoughness. Meanwhile strength basically unchanged. Improve the heatingtemperature can improve the impact toughness of flash welding joint. Both methodscan reduce the appearance of banded structure.