Increasing the fatigue limit of a high-strength bearing steel by thermomechanical treatment

High-strength steels have a maximum of the fatigue limit at a certain tensile strength. The decrease of the fatigue limit at still higher tensile strength can be explained by the failure mechanism of high-strength steels which fail due to crack initiation at inclusions or other internal flaws. To increase the fatigue limit two different thermomechanical treatments (TMTs) are applied to the material after classical martensitic hardening and tempering. A TMT results in a decrease of the hardness of about 10%. Different cyclic loading during the TMT can either reduce or increase the fatigue limit. In order to describe and compare the different treatments an evaluation of the stress intensity factors arising at the critical inclusions was carried out. To take into account different numbers of cycles to failure in the calculation a modification of the calculation of the stress intensity factor according to Murakami was successfully developed.