Formation Mechanism of Abnormal Martensite in the Welded Joint of the Bainitic Rail

This investigation aims to explore the formation mechanism of abnormal martensite in the bainitic rail welding process by combining numerical simulation and physical experiment. For this purpose, the continuous cooling transformation (CCT) diagram of bainitic steel was measured to determine the transformation temperature and cooling rate range of bainite and martensite during phase transformation. Besides, the Eulerian multiphase solidification model was also used to accurately design the experimental plan of the thermosimulated test. The higher cooling rate and microsegregation of the welded joint are important factors affecting the formation of abnormal martensite. The effects of cooling rate and microsegregation on the microstructure during the welding process of the bainitic rail were examined against the martensite fraction and bainite fraction. When the cooling rate is greater than or equal to 1 K/s, abnormal martensite will appear in bainite after the thermosimulated test, regardless of whether there is microsegregation in the sample. The cooling rate, not the microsegregation, is the main factor promoting the formation of abnormal martensite. The reasonable postwelding treatment process has excellent potential for improving the mechanical properties of the welded joint of the bainitic rail.