Effect of long-term aging on the low cycle fatigue behavior and microstructure of a 10% Cr martensitic steel with low nitrogen and high boron contents at 650 °C

Abstract

The effect of thermal aging for 10,000 h at 650 °C on the low cycle fatigue (LCF) behavior and microstructural changes of the 10% Cr steel at 650 °C was studied. A series of uniaxial strain-controlled LCF tests were carried out at 650 °C on the aged steel under fully reversed tension-compression loading conditions. Comparison of the fatigue results of the aged and unaged steels revealed that long-term aging did not affect significantly the number of cycles to failure. After aging, the slightly lower stress amplitude and slightly higher rate of cyclic softening during LCF testing provided an approximately 10% reduction in the cyclic strength coefficient K′ of the Morrow’s equation, whereas the cyclic strain hardening exponent n′ did not change due to the remaining effect of dynamic strain aging. The parameters of the Basquin-Coffin-Manson relationship between fatigue life and strain amplitude slightly changed compared to the unaged steel. Larger cyclic softening of the aged steel was associated with faster annihilation of dislocations and lath boundaries, respectively.