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A hypoid pinion made from 4820 Ni-Mo alloy steel was the driving member of a power unit operating a rapid transit car. The pinion had been removed from service at the end of the initial test period because it showed undue wear. The mode of failure was severe abrasive wear. The cause of failure was insufficient surface hardness, resulting from improper heat treatment. A service recall for the remaining pinions was immediately initiated.

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Lester E. Alban, Abrasive Wear Failure of a Hypoid Pinion, ASM Failure Analysis Case Histories: Failure Modes and Mechanisms, ASM International, 2019,

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