Case internal oxidation and intergranular fracture of carburized splined-shaft

The splined-shafts used in truck diesel engine fractured after service of merely 7–8 h. The transverse fracture occurred at the root fillet between the tooth portion and the cylinder portion. The fracture surfaces, corresponding to the carburized layer, of the failed splined-shaft and manufactured artificially show intergranular fracture features. Intergranular facets within a depth of about 20 μm from the surface were found to be associated with the oxide compounds of Cr, Mn, Si and dimples. The fracture mechanism of the splined-shaft was ductile intergranular cracking. Microstructure observation indicates intergranular internal oxidation occurred in the carburized layer to a depth of about 20 μm, in which oxide compounds of Cr, Mn, Si are along the grain boundaries. The association of intergranular oxidation pre-cracks with microstructural embrittlement promoted the premature failure of the splined-shaft. Over-short axial free length between the tooth portion and the cylinder portion enhances the degree of stress concentration, which contributes to the failure of the splined-shaft.

► The splined-shafts used in truck diesel engine fractured after service of merely 7–8 h.
► The fracture mechanism of the splined-shaft was ductile intergranular cracking.
► Occurrence of intergranular internal oxidation in the carburized layer is mainly responsible for the failure.
► Great degree of stress concentration between the tooth and cylinder portions also contributes to the failure.