Low Cycle Fatigue and Creep-Fatigue Interaction Behaviour of Reduced Activation Ferritic Martensitic (RAFM) Steels with Varying W and Ta Contents

Abstract

Reduced activation ferritic/martensitic (RAFM) steels are candidate materials for the test blanket modules of ITER. Several degradation mechanisms such as thermal fatigue, low cycle fatigue, creep fatigue interaction, creep, irradiation hardening, swelling and phase instability associated irradiation embrittlement must be understood to estimate the component lifetime. The current work focuses on the effect of tungsten and tantalum on low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behavior of four RAFM steels with varying W and Ta contents. Total strain controlled LCF experiments were performed under various strain amplitudes in the range +0.25% to +1% and temperatures (300 K to 873 K) in air at a constant strain rate of 3×10-3s-1 using a servo hydraulic fatigue testing system. CFI experiments were carried out at total strain amplitude of +0.6% and by applying strain hold of different durations (10 min and 30 min) in peak tension and peak compression. Both LCF and CFI life of the RAFM steels improved with the increase in tungsten and tantalum contents. Based on the amount of softening during continuous cycling, tungsten content was optimized at 1.4 wt. % and the tantalum content at 0.06 wt%. Stress relaxation obtained during creep-fatigue interaction studies showed close relation with the chemical composition of the RAFM steels. Other damaging parameters influencing fatigue life were dynamic strain ageing (DSA) occurring in the intermediate temperature regime and oxidation at elevated temperatures. Keywords: RAFM steel, low cycle fatigue, dynamic strain ageing, creep-fatigue interaction, oxidation