Influence of material and heat treatment aspects on tooth root strength of case carburized gears

At the end of the heat treatment process carburized case hardened gears are very often cleaned by peening, that means by blasting the surface with small metallic balls. This method is not very different to "shot peening" and therefore a similar increase of the load carrying capacity may be assumed. To determine the influence of various peening procedures on tooth root bending strength, two series of gears, all gears of a series having the same geometry and being heat treated in the same furnace batch, were tested in the FZG pulsator test stand. The focus was on different peen cleaning methods, but controlled shot peening was investigated too. Also the effects of peening on the material state is presented. To prove these results on a wider base of knowledge, other series of gears were tested, which were also made from the same material and with the same geometry, but heat treated in different companies after the standard heat treatment process. Each of these series was subdivided in an uncleaned series and a series that was cleaned by peening, also according to the standard method of these companies. The results show that peen cleaning in all cases significantly increases the bending strength of gears and that peen cleaning is a very important part in the process of carburizing and case hardening. As for each series the residual stress state is determined, comparison can be made between the load carrying capacity and the residual stress of test gears that are unpeened, peen cleaned with different intensity or shot peened. It is possible to derive values for the residual stress sensitivity acc. to Macherauch's [8] proposal. Based on the residual stress sensitivity the influence factor Y_RS is developed, with which the influence of the residual state on the endurance strength and the allowable stress number σ_Flim of the DIN 3990 [1] or ISO 6336 [2] method may be calculated.