Abstract: Martensite, as an abnormal microstructure in U71Mn steel rail flash welded joint, will decrease the safety of the joint, so it is necessary to study its formation causes and countermeasures. Taking U71Mn steel rail flash welded joint as the research object, the single normalizing treatment and dual-normalizing treatment were carried out by Gleeble-1500D thermal simulation testing machine. The microstructure and element composition before and after normalizing treatment were analyzed by means of optical microscope and scanning electron microscope. The distribution law and formation causes of martensite microstructure in the U71Mn steel rail flash welded joint were discussed, and the corresponding countermeasures were put forward and verified on the spot. The results show that the distribution of martensite in the flash welded joint of the U71Mn steel rail is consistent with the segregation of C, Mn element. The martensite in flash welded joint mainly occurs in the manganese rich and carbon rich area, in which the content of Mn element is more than twice that of the base metal. Flash welding will aggravate Mn element segregation in the U71Mn steel rail. Normalizing treatment has no obvious effect on Mn element segregation in U71Mn steel rail flash welding joint, but it can accelerate the diffusion of C element and reduce the occurrence of martensite to a certain extent. By reducing the content of Mn in U71Mn steel rail and providing proper normalizing treatment can greatly eliminate or even avoid the occurrence of martensite.

Key words: U71Mn steel rail, flash welding, martensite, segregation, normalizing treatment