Challenges in determination of microscopic degree of cleanliness in ultra-clean gear steels

Shot-peened, case-hardened gears can fail in the tooth root due to crack initiation below the surface of the steel matrix. Here, the crack is initiated at a non-metallic inclusion in the center of a so-called fisheye. This failure type can lead to a reduced endurance fatigue limit of the gears. 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. These ultra-clean gear steels were achieved by various measures in the steel production process. However, as a result, the remaining non-metallic inclusions are inhomogeneously distributed in the steel volume in terms of both size and location. 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. The results show that the standards can still be applied, but more effort must be applied. To determine the degree of cleanliness, six microsections are currently evaluated according to steel test specification (SEP) 1571, method K. It is shown that an examination of 24 microsections starting from size class 0 seems beneficial to get more reliable and comparable results of the degree of cleanliness of these ultra-clean gear steels. In addition, it is shown that a high degree of cleanliness has been achieved for all steel batches investigated with the measures taken in the steel production process.