The Influence of Hardness and Retained Austenite on the Fatigue Limit after Shot Peening
This thesis was performed at Scania CV AB and aimed to optimize the effect of shot peening in order to increase the bending fatigue limit of planetary gears. In this project three parameters affecting the performance of the shot peening were examined in more detail, specifically the hardness of the shot peening media, the hardness of the material and the amount retained austenite.
Shot peening and measurements were performed on carburized gears consisting of ss 92506 steel. In order to find out how the fatigue limit is affected by the material hardness, a number of gears were tempered whereas other remained untempered. Similarly the retained austenite content was altered in some of the gears with a cryotreatment. The shot peening of these gears was performed by dual shot peening using media of two different hardness. The fatigue limit was evaluated by a servo hydraulic material testing machine.
The results show that the fatigue limit is enhanced when the shot peening was performed with hard media. This was expected since the harder media can cause larger plastic deformation in the material giving rise to larger compressive residual stresses which is known to increase the fatigue limit. The fatigue limit was also improved for samples with lower surface hardness. The amount of retained austenite does not seem to affect the fatigue limit for samples shot peened with media of sufficient hardness in order to cause adequate plastic deformation. The surface roughness remained unchanged for samples shot peened by different media, which indicates that this parameter should not hinder a change of media.
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