Abstract
The methodology proposed here adopts short-term stress relaxation testing (SRT) to predict creep strength/life of X10CrMoVNbN 91 (P-91) steel. SRT has been performed at three temperatures (853, 873, 898 K), and at each temperature two prestrains (0.7 and 1.2%) were applied with a hold time of 7 h. The initial data, stress versus time, were converted to stress versus inelastic strain rate by computation and compared with conventional steady state creep rate. Creep life was predicted from the SRT results using the Monkman-Grant relationship, and compared with creep data. The comparison showed the predicted life to agree extremely well with the creep-rupture life (≈30 000 h). The calculated activation energy for SRT (719 kJ mol-1) was found to be higher than the creep (415 kJ mol-1). Detailed SEM and TEM studies revealed that the degradation of microstructure in terms of coarsening of M23 C6, and martensitic lath grains, had taken place and was commensurable to long term creep exposure.
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