Abstract
A repeat premature gear tooth breakage occurred in a load gearbox of a 40MW steam turbine. Factographic examination indicated that the breakage was a complex fatigue fracture. The oil deposit on the fracture surface has been applied as an auxiliary fractographic method. The root cause of the failure was due to improper heat treatment. The insufficient surface hardness and shallow case resulted in case/core separation through main fatigue crack propagation. The very coarse detrimental tempered low carbon martensites in core resulted in cleavage fracture in final fast fracture. The fractographic morphology of the butterfly has been revealed. Hertzian stress and non-metallic inclusions are not the necessary condition of the butterfly formation. The microcracks within the butterflies did not actively play the role of the fatigue rupture.
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