The cleanliness of steels used for gears is of great importance when looking to improve their life or increase loads. In this paper, carburizing steels with the same basic chemical composition, but with a varying cleanliness level, are compared in regards to cleanliness and performance. Steel cleanliness can be described as size and frequency of non-metallic inclusions in steel, such as Al_2O_3-MgO particles. Rotating bending fatigue has been used to evaluate the fatigue performance of carburized samples. For this purpose, samples are taken transverse to the rolling direction of the bar, whereas normally, samples are taken in the longitudinal direction. The aim has been to test the method in itself, as well as the most critical loading direction, since this determines the worst-case scenario of the inclusion orientation when forging a gear out of a steel blank. To determine material cleanliness, a standard micro-inclusion rating method such as ASTM E45 has been used in combination with automated inclusion assessment by scanning electron microscope (SEM) and 10MHz immersed ultrasonic evaluation. The investigations show a good qualitative correlation between fatigue performance and inclusion assessment made by ultrasonic evaluation and SEM. The results also show that traditional micro-inclusion rating methods are not sensitive enough (i.e., the evaluated areas are too small) to give a good indication of material performance. Fatigue testing in the transverse direction of the bar proves to be a good technique that can more relevantly determine fatigue performance of steels for gears without involving costly and somewhat complicated gear manufacturing.
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