D. S. Sarma

Prediction of creep life of X10CrMoVNbN-91 (P-91) steel through short term stress relaxation test methodology

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.

Analysis of tertiary creep in an NiAl hardened austenitic steel

Abstract The present study aims to identify the various factors responsible for the early onset and extended tertiary creep observed in a precipitation hardened Fe–Ni–Cr–Al alloy by analysing the tertiary creep at 600°C. Although the forms of the individual creep curves qualitatively appear to be compatible with the concept of particle coarsening control, quantitative analysis of the results in this present study are totally inconsistent with the predictions of a particle-coarsening model for tertiary creep. Creep rate acceleration with creep strain accumulation during the tertiary creep regime of the Fe–Ni–Cr–Al alloy indicates that strain softening is the origin of the onset of tertiary creep in this alloy. Further, acceleration of tertiary creep rate depends on the applied stress and is also related to the rupture ductility such that the rate of increase of tertiary creep rate becomes progressively more rapid as the applied stress is reduced or when rupture ductility is lower. The higher incidence of grain boundary cavitation combined with limited rupture ductility, and an exponential variation of creep rate with strain over most part of tertiary creep regime suggests that cavitation damage (internal loss of section) is primarily responsible for tertiary creep in the Fe–Ni–Cr–Al alloy when in the peakaged condition with lower rupture ductility. Wider but limited grain boundary cavities, which are localized very close to the fracture face together with a higher rupture ductility, indicate that the contribution from cavitation damage to tertiary creep of the alloy in the over-aged condition is significant only late in the tertiary creep regime. While linear softening makes a significant contribution early in the tertiary regime. Linear softening is subsequently replaced by exponential softening (external loss of section) during later part of deformation and can exist over longer periods.

First order reaction kinetics for transient creep in NiAl hardened austenitic steel

Abstract Transient creep of an NiAl hardened austenitic steel was analysed in the temperature range of 823 to 923 K at stresses ranging from 150 to 450 MPa in the frame work of first order reaction kinetics. The present analysis is aimed: to correlate various transient creep parameters with steady state creep rate following first order reaction rate theory to obtain correlation constants; and to arrive at a unified equation to describe primary and steady state regimes of the creep curves in terms of correlation coefficients thus derived. Good correlation of transient creep parameters with steady state creep rate has been obtained over the test conditions studied indicating that the basic mechanism of deformation is the same for all the three stages of creep. Unified equation that fits the experimental creep strain time data for different test conditions over transient and steady state regimes has been obtained in terms of correlation coefficients.