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 pm 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 mu m, 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. (C) 2012 Elsevier Ltd. All rights reserved.