Cracking reason of 40MnBH steel shaft head

Axial and circumferential cracks were found on the surface of the inner hole of a quenched and tempered 40MnBH steel shaft head during machining. The cause of the crack was analyzed by macroscopic observation, magnetic particle inspection, metallographic examination and scanning electron microscope analysis. The results show that the crack was a quenching crack, and the cause of the crack was the local stress concentration caused by the annular folding of the inner hole surface caused by the abnormal punching. The improvement of punching die and forging process could avoid the recurrence of such problems.