Low cycle fatigue behavior of a 10Cr–2W–Mo–3Co–NbV steel

Abstract

Low cycle fatigue (LCF) behavior of a 10% Cr–2% W–0.7% Mo–3% Co–NbV steel with 0.008wt.% B and 0.003wt.% N additions was studied under fully reversed tension–compression loading as a function of temperature from 20 to 650°С and constant strain amplitude from ±0.2% to ±1.0%. The effect of LCF on the steel’s microstructure was analyzed. It was demonstrated that the 10% Cr steel was susceptible to cyclic softening, even at room temperature, and could not withstand reversed plasticity. The fatigue lifetime curves at all temperatures could be described by the Basquin–Manson–Coffin relationship. The cyclic softening effect was attributed to the coarsening of the laths, primarily at 20 and 500°C, and the transformation of the tempered martensite lath structure (TMLS) to a subgrain structure at 600 and 650°C. In addition, a decrease in the dislocation density occurred under LCF. Increasing the reversed plasticity with the total strain amplitude accelerated the evolution of the TMLS and thus promoted cyclic softening. The propagation of fatigue cracks during Stage II was also facilitated by increasing the contribution of plastic deformation processes to the total strain amplitude, which led to a drop in the fatigue life with an increase in the constant strain amplitude.