Abstract: During the medium frequency induction hardening, multi-point melting cracks and single point melting cracks appeared in the 42 CrMo steel commercial automobile half-shafts. The failure reason was determined by microstructure analysis, banded structure analysis, hardness test and fracture morphology observation. The fracture analysis excludes the possibility of cracking caused by abnormal inclusions and slag in the steel. The microstructure analysis result shows that overheating leads to melting and cracking. In the process of induction quenching, the heater sticking makes the defect area overheat and melt, and the grain grows abnormally. The lath martensite is coarse, and obvious intergranular fracture is observed. The banded structure also increases the tendency of quenching cracking, and finally leads to the failure of half-shafts cracking.