Abstract: Using optical microscope and SEM metallographic techniques to inspect cracking fractures of U75V rail flash welding joint,it was found there was a lot of high-carbon martensite in center areas of rail head and base,the microstructure was normal in other areas.For the rail flash welding process,it is made sure when the surface temperature of joint drop down to 400 ℃,the temperature of center areas of rail head and base is still higher than transformation temperature,so the martensite is changed by forced water cooling after normalizing heat treatment,this change causes the rupture.This rupture could be refrained from altering the water cooling site rearward.