Failure of a rear axle shaft of an automobile due to improper heat treatment

A section of fractured rear axle shaft made of induction-hardened steel and removed from the scene of overturned automobile was analyzed to determine the most probable cause of failure. Light optical metallography and scanning electron microscopy combined with energy dispersive spectroscopy were used to characterize the microstructure and the mechanical strength was evaluated by microhardness measurements. Chemical analysis verified that the shaft was made of AISI 4140 steel as per specifications. However, microstructural characterization and microhardness measurements revealed that the shaft was improperly heat treated resulting in a brittle case, where crack propagation was found to occur by an intergranular mode in contrast with cleavage within the core. This behavior was related to differences in microstructure, which was observed to be martensitic-type within the case with microhardness equivalent to R _c 58, and a mixture of pearlite and ferrite within the core with R _c 25. Although it was not possible to reconstruct the exact sequence of events leading to fracture, it is possible that it was initiated by large overload within the extremely hard brittle case, which could lead to overturning of the vehicle and final fracture could have occurred by the impact of overturning. However, crack initiation due to hydrogen generated by rust and water pickup as well as the possibility that overturning of the vehicle was the cause of the fracture could not be ruled out.