Towards improved ODS steels: A comparative high-temperature low-cycle fatigue study

Within the frame of this work, the mechanical behaviour of a bimodal ferritic 12Cr-ODS steel as well as of a ferritic-martensitic 9Cr-ODS steel under alternating load conditions was investigated. In general, strain-controlled low-cycle fatigue tests at 550 degrees C and 650 degrees C revealed similar cyclic stress response. At elevated temperatures, the two steels manifest transitional stages, ie, cyclic softening and/or hardening corresponding to the small fraction of the cyclic life, which is followed by a linear cyclic softening stage that occupies the major fraction of the cyclic life until failure. However, it is clearly seen that the presence of the nano-sized oxide particles is certainly beneficial, as the degree of cyclic softening is significantly reduced compared with non-ODS steels. Besides, it is found that both applied strain amplitude and testing temperature show a strong influence on the cyclic stress response. It is observed that the degree of linear cyclic softening in both steels increases with increasing strain amplitude and decreasing test temperature. The effect of temperature on inelastic strain and hence lifetime becomes more pronounced with decreasing applied strain amplitude. When analysing the lifetime behaviour of both ODS steels in terms of inelastic strain energy calculations, it is found that comparable inelastic strain energies lead to similar lifetimes at 550 degrees C. At 650 degrees C, however, the higher inelastic strain energies of 12Cr-ODS steel result in significant lower lifetimes compared with those of the 9Cr-ODS steel. The strong degradation of the cyclic properties of the 12Cr-ODS steel is obviously linked to the fact that the initial hardening response appears significantly more pronounced at 650 degrees C than at 550 degrees C. Finally, the obtained results depict that the 9Cr-ODS steel offers higher number of cycles to failure at 650 degrees C, compared with other novel ODS steels described in literature.