Abstract: Some 42CrMo steel high strength bolts of diesel engine fractured with their heads dropped in the fastening process. The reasons of fracture were analyzed synthetically by means of macroscopic examination, chemical compositions analysis, fiber streamline examination, transition fillet inspection, metallographic test and SEM analysis, and combined with the bolt processing analysis. The results show that in the process of quenching and tempering, quenching cracks and old fracture surfaces are formed in the head-rod joint zone with stress concentration, which reduces the effective bearing area of the bolt, so that the bolt fractures due to relative overload under assembly stress. The key factor causing the quenching cracks is that the transition fillet radius in the head-rod joint zone is too small and with poor machining.

Key words: high strength bolt, 42CrMo steel, quenching crack, fracture morphology, fillet radius