Failure Analysis of Tractor Wheel Axle

A number of wheel axles of MF-240 Tractors, which had broken after unusually short times in the field (~ 2–4 months), were examined to determine the cause of their failure. The fracture surface showed typical fatigue fracture that had initiated from a circular crack (about 2–3 mm deep) in the induction-hardened layer at the ‘neck’ region of the axle. A scrutiny of the production data showed that the failed axles were all from those which were induction-hardened during the winter months. A further probe revealed that sometimes the induction-hardened axles were left overnight without tempering. It appeared highly likely that quenching stresses in the induction-hardened layer of those components which were left un-tempered during cold winter nights were responsible for the ‘delayed cracking.’ In an attempt to verify this hypothesis, a simulated test was conducted which confirmed that delayed cracking could occur if induction-hardened axles were kept un-tempered overnight at low ambient temperatures. Another interesting and rare feature of the present fracture was that the fatigued area was ‘curved,’ i.e., the fatigue crack, as it progressed, had gradually acquired an orientation that was normal to the maximum operative tensile stress.