Isothermal low-cycle fatigue and fatigue-creep behaviour of boron-added 9% Cr martensitic stainless steel at 600°C

Abstract

In this article, strain-controlled Low-Cycle Fatigue (LCF) and fatigue-creep tests were conducted on COST FB2, a boron-added 9% Cr martensitic stainless steel, at 600°C. LCF tests were performed with a mechanical strain rate of $1\times 10^{-3}$/s, while the fatigue-creep tests involved either tensile or compressive strain dwells lasting 600 s. Both the LCF and fatigue-creep tests revealed cyclic softening behaviour, with the magnitude of relaxed stress decreasing with cycles in the fatigue-creep tests. This softening was associated with the coarsening of the laths and subgrains and a reduction in dislocation density, both of which were more pronounced for LCF loading at higher strain amplitudes and during fatigue-creep loading. Investigations into the damage mechanisms identified environmentally assisted transgranular cracking as the predominant failure mode, with the severity of oxidation-induced cracking increasing with higher applied strain amplitudes or during fatigue-creep loading with compressive dwell, while cracking was suppressed during tests with tensile strain dwell. Finally, a damage model combining the strain-life approach with a time-dependent damage term was proposed to effectively predict the reduction in lifetime during fatigue-creep tests compared to continuous LCF cycling.