A 1050 steel crankshaft with 6.4 cm (2.5 in.) diam journals that measured 87 cm (34.25 in.) in length and weighed 31 kg (69 lb) fractured in service. The shaft had been quenched and tempered to a hardness of 19 to 26 HRC, then selectively hardened on the journals to a surface hardness of 40 to 46 HRC. Visual inspection and 100x micrographs showed the fracture surface as having a complex type of fatigue failure initiated from subsurface inclusions in the transition zone between the induction-hardened surface and the softer core. The fractured shaft was examined for chemical composition...