To determine the degree of cleanliness, six microsections are currently evaluated according to steel test specification (SEP) 1571, method K. The results show that the standards can still be applied, but more effort must be applied. However, due to the inhomogeneity of ultra-clean steels, the question arose if the values derived according to the standards are still representative of ultra-clean steel batches. ![]() However, as a result, the remaining non-metallic inclusions are inhomogeneously distributed in the steel volume in terms of both size and location. These ultra-clean gear steels were achieved by various measures in the steel production process. It is for this reason that, over the last decade, much effort has been invested by steel manufacturers to reduce the non-metallic inclusion content of gear steels so as to mitigate or even completely prevent such crack initiation. This failure type can lead to a reduced endurance fatigue limit of the gears. Here, the crack is initiated at a non-metallic inclusion in the center of a so-called fisheye. ![]() Shot-peened, case-hardened gears can fail in the tooth root due to crack initiation below the surface of the steel matrix.
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