Failure Analysis on Axial Cracking of a 42CrMo Steel High Strength Bolt

A 8.8 grade high strength bolt made from 42CrMo steel cracked along the axis during thread rolling process after heat treatment and the cracking causes of the bolt were analyzed by means of chemical composition analysis, macro examination and metallographic examination. The results show that the cracking of the bolt was mainly due to that there was linear distribution forging fold defect in the raw material surface before heat treatment. And the forging fold defect caused stress concentration and became a cracking source when quenching, which induced the axial cracking of the bolt under the action of powerful quenching stress and rolling stress.