Fatigue fracture analysis of the 42CrMo4 crankshaft used for mining dump truck

•Fracture of the crankshaft belongs to high-cycle bending fatigue failure.•Root of the fatigue fracture was the inadequate thickness of quenching layer at the fillets.•The bending moment could be promoted by the increasing thickness of the hardening layer in a certain range.•Thickness of the medium frequency induction hardening layer was recommended about 3.5 mm based on overall considerations. A failure investigation was performed into the fractured 42CrMo4 crankshaft used for wide-body mining dump truck in this paper. Fracture morphology, chemical composition, microstructure and mechanical properties of the crankshaft material were studied to investigate the premature failure mechanism. All the crankshaft broke at the 4th or 6th necks and a high-cycle bending fatigue type in this case could be identified by the observation of beach marks and fatigue striations in the propagation area according to fractography results. The fatigue source is underneath the hardening layer of R fillet and the root cause could be identified as the designed inadequate thickness of quenching layer at the fillets obtained through medium frequency induction quenching (MFIQ), which could not provide adequate fatigue strength upon cyclic loadings for long term. Fatigue validation test indicated that the bending moment increased with the increasing thickness of the quenching layer, and the appropriate thickness was recommended about 3.5 mm based on overall consideration.