Failure Investigation of a Diesel Engine Piston Pin

Transverse and longitudinal cracking took place on a diesel engine piston-pin used in a truck in service. The external circle and internal hole surfaces of piston-pin are specified to be carburized. The cracks originated from the internal hole surface and propagated, by fatigue, towards the external circle. The microstructure and microhardness profiles indicate that the serious decarburization occurred on the internal hole surface and that the hardening case was not present. The occurrence of the decarburization in the surface region of the internal hole decreases the fatigue strength to levels far below those for a carburized surface so failure initiated easily at the internal hole surface. To assure against similar failures in the future, make sure that the region around the internal hole obtains the specified depth of hardened layer.