Abstract
A truck diesel engine camshaft was found cracked following a straightening operation after the camshaft was carburized. The camshaft is made of 16MnCrS5 steel and was required to be surface-carburized. The cracking occurred just at the transition fillet root of a cam at a near middle position on the camshaft. This location bore the maximum tensile stress in the straightening process, a process involving three-point bending. The crack surfaces once exposed exhibit cleavage morphology, indicative of brittle fracture as the failure mechanism. Microstructural observation revealed an intergranular network of carbides and intergranular microcracks present in the carburized layer and a banded structure consisting of ferrite and pearlite in the core. These metallurgical defects decreased the deformation capacity of the carburized camshaft, in fact creating a “metallurgical stress concentration” that promoted the cracking of camshaft.
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