Decomposition mechanisms of continuously cooled bainitic rail in the critical heat-affected zone of a flash-butt welded joints

The joining process of bainitic rails is significant in terms of their industrialization in high-speed and heavy-loaded railways. This paper demonstrates the microstructure changes in the critical zone of the welded joint, which is responsible for the greatest deterioration in mechanical properties. Extensive progress in the decomposition of the retained austenite and bainitic ferrite occurs in the low-temperature heat-affected zone (LTHAZ) of the flash-butt welded joint of low-carbon bainitic rail. The decomposition products of the retained austenite were mainly a mixture of cementite and ferrite. The cementite was mainly precipitated at the boundary of the bainitic ferrite laths, which indicates lower thermal stability of the filmy austenite. Moreover, it was found that a part of the refined blocky retained austenite was decomposed into the ferrite and nanometric cementite, while another remained in the structure. The decomposition mechanisms are rather heterogeneous with varying degrees of decomposition. A relatively high proportion of dislocations and stress fields prove the occurrence of residual stresses formed during the welding process.