subsurface was 30 mm which depicted the area of maximum shear stress concentration. The alternating stress cycles during rolling/sliding action induced high shear stress at theses subsurface crack in the hardened case. The active cracks propagated into the core and ended at zero stress level on the other side of the teeth. Drive gear teeth having visible coarse fatigue striations (FS) and cleavage features in the overloaded (OL) final fracture zone. The SEM examination revealed crack originating sites at the adjacent tooth with non-uniform radial fatigue progression marks in the crack propagation region which dictates clearly the variations in the loading cycles at the delineation front of crack surface. These coarse marks suggest the mechanism of failure as contact fatigue. It is also evaluated from the fractography that crack did not propagated continuously from teeth containing crack origin to the adjacent teeth but originated and grew independently under fluctuating load and fractured ultimately by cleavage due to overloading. The extreme wear and sever deformation of successive gear teeth (2–7) upto pitch plane occurred due to low core hardness compared to core. The hardened case was sheared off from the deformed core but cracked and pitted by the rolling action of driven gear (Fig. 4).