The failure analysis of the repeat geartooth breakage in a 40 MW steam turbine load gearbox and the butterfly in the carburized case

A repeat premature geartooth breakage occurred in a load gearbox of a 40 MW steam turbine. Fractographic examination indicated that the fatigue crack originated from the root fillet of the non-active flank and propagated to the active flank. The oil deposit on the fracture surface has been applied as an auxiliary fractographic method to distinguish the fatigue crack path. The root cause of the fatigue failure was the improper heat treatment, including the very coarse microstructures due to the direct quenching from the carburizing, plus the shallow case and the insufficient surface hardness by the post-quenching grounding. The solution was successful. Some butterfly characteristics in this case study contradicted to the previous theories. The butterflies initiated from neither non-metallic inclusion nor carbide. The butterfly cracks did not propagate into the martensitic matrix. A butterfly observed in the subsurface of the gear-root indicated that the butterfly formation was irrelevant to the Hertzian stress.