High temperature low cycle fatigue of steels and their welds

High temperature low cycle fatigue (LCF) is influenced by various time dependent processes such as creep, oxidation, phase transformations and dynamic strain ageing (DSA) depending on test conditions such as strain rate and temperature of testing. In this paper the detrimental effects of DSA and oxidation in high temperature LCF is discussed with reference to extensive studies on 316L(N) stainless steel and Mod. 9Cr-1Mo steel. DSA is reported to enhance the stress response and reduce ductility. It localizes fatigue deformation, enhances fatigue cracking and reduces fatigue life. High temperature oxidation is found to accelerate transgranular and intergranular fatigue cracking in Mod. 9Cr-1Mo steel. The variations in grain size and formation of brittle phases due to transformation of delta ferrite during testing in welds are used to illustrate the effect of microstructural changes on crack initiation and propagation and fatigue life. Artificial neural network approach to life prediction was found to be successful in LCF. Thermomechanical fatigue studies are suggested as more closer to the actual service conditions. Deformation mechanisms in materials used in high temperature applications vary with temperature and in these situations life prediction in TMF from isothermal fatigue tests is seen as a challenging task in the future.