Bend Straightening of a Carbonitrided Gear Shaft – Consequences on Residual Stresses and Retained Austenite near the Surface

Carbonitriding of shafts in drive technology is strongly connected with distortion. Bend straightening is an important process step in order to eliminate distortion without removing the hardened surface layer. As shown in several investigations, even for simple part geometries, bending induced stresses and plastic deformations have a strong impact not only on the residual stress state but also on the microstructure after straightening. As gear shafts become increasingly filigree and complex in their geometry, the components state after straightening is only understood in rare cases. Due to indispensable, function based notches as well as changes in the cross section, these effects will occur especially in areas of small cross sections or notches in conjunction with high bending stresses. It is the objective of this work, to characterize these critical areas with respect to the distribution of residual stresses and retained austenite along the circumference as well as in the in-depth direction. Therefore, the measured distributions of residual stresses, integral width values and retained austenite near the surface will be presented and discussed. For this purpose a carbonitrided gear shaft taken out of a large production volume was investigated before and after a commonly used straightening operation.