Low cycle fatigue behavior and microstructural evolution of modified 9Cr–1Mo ferritic steel

In the present paper, influence of hold time on the high temperature low cycle fatigue (LCF) behavior of modified 9Cr–1Mo ferritic steel in the normalized and tempered condition is addressed. Total axial strain controlled LCF tests at 873 K with hold times at peak strain up to 10 min in tension and compression were carried out employing total strain amplitude of ±0.6%. The alloy in general, showed a gradual and continuous softening regime. Fatigue life was found to decrease with increase in the duration of hold time in both tension and compression. Compression hold was found to be more damaging than the tension hold. The fatigue failure in compression hold tests were marked by extensive crack branching and formation of secondary cracks. Oxidation-assisted crack initiation and propagation contributed to life reduction at high temperatures in hold time tests. Additionally, the substructural changes of the starting microstructure were also found to be responsible for the reduction of the fatigue life. The combined effect of cyclic deformation and elevated temperature on the evolving microstructure have been evaluated. The application of hold during cycling at elevated temperature accelerates the conversion of initial heavily dislocated lath structure to equiaxed cells with low dislocation density and coarse carbides.