Microstructure and fracture mechanism of a flash butt welded 380CL steel

Abstract High strength low alloy (HSLA) steels have been widely used in the manufacture of wheel rim on account of the higher strength and larger elongation compared to the traditional low carbon steels. This experimental investigation was aimed to evaluate the microstructure and fracture mechanism of a flash butt welded 380CL steel by comparing the failed wheel rim and the survived wheel rim. The results showed that the continuous banded structure in the HAZ and the serious Widmanstatten ferrite in the weld were the major reasons for the failure of the wheel rim. The micro-hardness of two joints was similar in the overall profile. For the joint of the failed rim, the micro-hardness at the weld was the highest at 254 HV and the lowest micro-hardness value was in the HAZ. The high yield ratio of the failed wheel rim resulted in poor formability in the flaring process. The fracture mechanism of the failed rim was the mixture of ductile and brittle fracture modes. The crack initiation was the fusion zone of the weld having highest micro-hardness, then the cracking propagated along the weld metal to the middle of the wheel rim, a limited deviation was found in the terminal of the crack. The large ferrite grain in the HAZ was the main reason for the brittle–ductile transition.