Influence of Varying W and Ta on Low Cycle Fatigue and Creep-Fatigue Interaction Behavior of Reduced Activation Ferritic/Martensitic Steels

The objective of the present paper is to provide first hand experimental data and analysis on the low cycle fatigue (LCF) and creep-fatigue interaction (CFI) performance of different chemical compositions of reduced activation ferritic-martensitic (RAFM) steels. As a part of the development programme, LCF and CFI experiments were conducted on different RAFM steels with varying W and Ta obtained in the normalized and tempered condition. Effect of varying Ta and W upon the resultant LCF and CFI life seemed to be interrelated and an optimum combination of both W and Ta worked out to be the best for CFI life. Stress relaxation that occurs under the application of hold has unique information of the underlying deformation and damage in the RAFM steels. It has been observed that stress relaxation is directly related to the amount of plastic strain accumulated, inversely related to CFI life and strongly dependent on chemical composition. Hence it can be used to explain the deformation and damage accumulation in ferritic-martensitic steels. The plastic strain accumulated during stress relaxation is accommodated as microstructural coarsening. EBSD based microstructural characterization reveals that the recovery process is closely related with the chemical composition and various test parameters. Breakage of protective chromia layer accounts for the more damaging effects of oxides under compressive hold.