The fatigue limit of high strength steels is one of the most important mechanical properties for applications. With a suitable thermo-mechanical treatment (TMT) in the temperature range of maximal dynamic strain aging (DSA) the high cycle fatigue (HCF) limit may be increased.
During a TMT in the DSA temperature range dislocations interact with alloying element atoms (e. g. carbon in case of steels) and are impeded in their movement. Finally, a stabilised microstructure with increased dislocation density is formed which may exhibit a higher HCF strength.
In this study the effect of TMT on the fatigue limit of quenched and tempered steels 100Cr6 (AISI 52100) and 42CrMo4 (AISI 4140) has been investigated. In a first step, the temperature where the maximal DSA occurs has been determined searching a maximum of hardening under cyclic loading. At this temperature, the material was subjected to 50 cycles of sinusoidal loading with increasing stress levels to strengthen the microstructure (TMT). Subsequently, stress-controlled push-pull fatigue tests were conducted in the HCF-regime on solid round specimens with and without TMT as well as with polished and non-polished surface.
It was found that for non-polished specimens the effect of TMT on the fatigue strength is insignificant. For polished specimens the TMT enhances the fatigue life significantly. The fatigue strength of polished specimens without TMT is comparable to that of non-polished specimens. Thus, the positive effect of TMT on fatigue strength depends on the surface quality. To investigate the acting damage mechanisms specimen and fracture surfaces were analysed using SEM.