Creep deformation measurement of ex‐service 12% Cr steel over nonuniform stress fields using digital image correlation

Deterioration assessment of materials is essential to the continued effective operation of critical components in thermal power plants. Establishing the degree of creep exhaustion of power engineering alloys operating at high temperatures and stresses guides maintenance strategies to ensure reliable plant operation. Within progressive inspection philosophies, traditional laboratory‐based creep testing is often difficult to conduct on ex‐service steel due to the limited material availability from which to machine standard specimen geometries. This work investigates a technique for the measurement of creep strain curves at several stresses and at 600°C from a single test using a nontraditional specimen geometry together with full‐field strain measurement through digital image correlation (DIC). Of interest is ex‐service X20CrMoV12‐1 (X20) which is widely used in older, subcritical thermal power plants. The paper aims to show that multiple creep curves can be resolved over a spatially varying stress field using DIC whilst preserving material economy. Differences in creep behaviour between ex‐service X20 with varying levels of service exposure are evident from quantitative comparisons of the creep strain and rate curves through threshold stress computation which agrees with hardness measurements and microstructural observation of subgrains and precipitates using electron microscopy. These single‐specimen tests yield high densities of creep data which can be used in the calibration of creep damage models for characterisation of ex‐service X20.