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

High-strength steels typically fail from inclusions. Therefore, to increase the fatigue limit of high-strength steels it is necessary to modify the inclusions and/or the surrounding matrix. The goal must be a higher threshold for crack initiation and/or crack propagation. One possibility to reach this goal seems to be a deep cryogenic treatment which is reported to completely transform the retained austenite as well as to facilitate the formation of fine carbides. Therefore, specimens were annealed before or after deep cryogenic treatment, which was carried out with different cooling and heating rates as well as different soaking times at -1960 C. Hardness and retained austenite measurements and fatigue experiments were used to evaluate the different sequences of treatments mentioned above. The fatigue limit increases only after some of the sequences. The results show that the soaking times are not relevant for the fatigue limit but it is very important to temper the specimens before the deep cryogenic treatment. Also, repeated deep cryogenic treatments had a positive influence on the fatigue limit.