Failure of a combine harvester front axle

Abstract

This paper describes the failure analysis expertise of the combine harvester axle shaft, made of heat treated Cr-Mo steel. The axle shaft was received as fractured after three years of operation. It was in a tempered-And case hardened condition. The failure zones were examined visually and with light-And scanning electron microscope. Also, chemical composition and Vickers hardness tests were carried out. The results indicated that fatigue is the dominant failure mechanism of the axle shaft. It was observed that the fatigue crack initiated at the tooth at four initiation regions with corresponding beach marks. The main cause that reduced the stress of crack initiation was decarburising in flanks of gear teeth. This was shown by the existence of ferrite in lathe martensite matrix in decarburized regions. Such finding is confirmed by a lower value of Vickers hardness in the flank region in relation to the subsurface region, not affected by decarburisation.