Cranks are important parts in a car body. In the processing, moulded forged pieces were quenched and tempered, then the local
areas around the finished dowel holes were high
frequency quenched and medium and low temperature tempered. The dowels and the dowel holes should be fitted statically, and the whole bodys were phosphorized. Before this batch of parts were assembled, by means of examination we discovered that 28% of these parts were cracking or fractured on the thin walls around the dowel holes. Three cracked cranks were selected. And their material, hardness, microscopic structure and crack characteristic were analysed, and the fractured surfaces were analysed by means of the secondary electron photograph of scanning electron microscope (SEM) and their elements were qualitatively analysed by the X ray energy distribution spectrometer (EDS). Conclusions were drawn as follows: The hardness of the dowel holes became higher after high frequency heat treatment and deformation happened, and the dowels and the dowel holes were fitted too closely, thus resulting in the bigger inner stress around the dowel holes. The high frequency quenching process was not controlled properly so that the decarbonisation on the surfaces of the dowel holes emerged, thus the material′s ability to resist the formation and extension of the cracks was reduced. In the course of phosphorization, these parts were eracking due to the hydrogen
corrosion.