The pinion gear suffered random, open
progressive pitting on the loaded flanks of the teeth. Other areas of the loaded gear teeth exhibited visual images of subsurface fatigue cracks that had not developed into open pits yet. The companion bull gear had not pitted. Microstructural investigation of a representative
subsurface pit showed that it propagated parallel to the case hardened surface and contained
white etching areas (WEA's). Other subsurface,
intergranular cracks were also present that had initiated at inclusions.
Results of the failure analysis showed that the gear tooth pitting was due to surface-contact fatigue cracking. The WEA's have been described as "butterfly wings" and white bands of altered martensite. They reportedly occur in gear teeth that have experienced heavy shear or impact loads.