Low-cycle fatigue behaviors of two heats of SA508 Gr.1a low alloy steel in 310 °C air and deoxygenated water-Effects of dynamic strain aging and microstructures

The low cycle fatigue (LCF) resistances of two heats of SA508 Gr.1a low alloy steel, heat FP with a ferrite-pearlite microstructure and heat BA with a bainite microstructure, were compared in 310 °C air and deoxygenated water. The LCF life of heat FP was shorter than that of heat BA in 310 °C deoxygenated water while the LCF life of both heats was comparable in 310 °C air. Such behaviors were explained considering the dynamic strain aging (DSA) behaviors in both environments and their microstructures. Based on cyclic stress and tensile behaviors, it was found that heat BA showed higher degree of DSA than heat FP in 310 °C air, but both showed similar degree of DSA in 310 °C deoxygenated water because the degree of DSA was reduced by absorbed hydrogen. In case of microstructure, heat FP had a lower fatigue cracking resistance due to the stress concentration effect in ferrite-pearlite interface. As a result, in 310 °C air, where the effect of DSA was large enough to override the microstructure effect, LCF life of heat BA was similar to that of heat FP. On the other hand, LCF life of heat BA was longer than that of heat FP in 310 °C deoxygenated water, where the effect of DSA was not so significant.