The Influence of Prior Plasticity on the Creep Resistance of Two Elevated Temperature Power Plant Steels

Abstract

Analysis of the risk of creep–fatigue (C-F) cracking is an integral part of the condition assessment of elevated temperature structural components operating in modern power plants. The advanced martensitic steels used in such applications tend to exhibit a high degree of C-F deformation interaction relative to low-alloy bainitic steels. Of particular interest to this work is characterising the influence of plasticity on the creep resistance of two power plant steels from both a qualitative and quantitative perspective. Uniaxial strain-controlled cyclic/hold C-F tests, with a hold period at peak tensile strain, have been performed in this context on 1%Cr and 10%Cr steels at their respective peak operating temperatures of 550 and 600 °C. In addition to C-F tests, monotonic stress relaxation tests have also been conducted on both the steels at different elastic/inelastic strains (strain amplitudes). With this information the influence of magnitude of the loading transients, both monotonic and cyclic, on the subsequent creep behaviour is systematically studied. Experimental results indicate that the creep deformation resistance of these steels is clearly modified as a consequence of prior, but not exclusively cyclic, plasticity. In particular, creep deformation kinetics are found to be influenced by the magnitude of the first inelastic loading transient for both the monotonic and cyclic loading histories under consideration. This effect is therefore not limited to the stress relaxation response during the midlife cycle hold, but also to the creep strain accumulated during the first cycle hold when preceded by an inelastic loading transient. This work also examines the determination of forward creep properties from the obtained stress relaxation data (and vice versa) without and with prior monotonic or cyclic inelastic loading.