Metallurgical Examination of Failed Spiral Bevel Gear P196, Part No. 145D6302
ARMY RESEARCH LAB ABERDEEN PROVING GROUND MD WEAPONS TECHNOLOGY DIRECTORATE
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A failed spiral bevel gear was examined by the U.S. Army Research Laboratory ARL, Weapons and Materials Research Directorate, Weapons Concepts Division WM-MD after the primary contractor Boeing Helicopters Co. performed the initial investigation. The gear was fabricated from X-2M steel. Light optical microscopy of the failed gear section revealed characteristics consistent with a fatigue failure. The fracture origin was characterized by a darkened, half-moon-shaped region. Energy-dispersive spectroscopy of the darkened region revealed the presence of sodium, evidence that the crack was open to the black oxide finish process. The pre-existing crack was oriented perpendicular to the direction of grinding, indicating the possibility of a grinding crack. Metallography confirmed that the pre-existing crack was a grinding bum, since evidence of rehardening and retempering was observed. Contributory to crack propagation was the presence of carbide networks within the carburized case. Metallographic examination, combined with microhardness testing, revealed a deeper than acceptable case in the damping ring groove region origin location. It was later learned that this region was mistakenly subject to a double carburization during processing. Although the morphology of the origin was featureless, fractographic examination of a secondary fracture most likely an additional grinding crack did reveal an intergranular rock candy morphology. This was evidence that the crack propagated along the carbide network within the carburized case. It was concluded that the cracks had formed during the grinding process and had propagated along the carbide network until final fast fracture.
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