Issue 48

J. Christopher et alii, Frattura ed Integrità Strutturale, 48 (2019) 554-562; DOI: 10.3221/IGF-ESIS.48.53 554 Focused on “Showcasing Structural Integrity Research in India” Comparative evaluation of two physically based models for the description of stress-relaxation behaviour of 9% chromium containing steel J. Christopher*, C. Praveen, B.K. Choudhary Materials Development and Technology Division, HBNI, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, 603102, India, e-mail: jchris@igcar.gov.in A BSTRACT . An attempt has been made to evaluate the applicability of two constitutive models related to the dislocation-obstacle interactions for the description of stress-relaxation behaviour of E911 tempered martensitic steel. The first one is Feltham model (Model-I) and the second model proposed by Christopher and Choudhary (Model-II) is based on the sine hyperbolic kinetic rate formulation coupled with the evolution of internal stress. The physical constants associated with these models have been determined by the minimization of errors between experimental and predicted relaxation stress vs. hold time data for two different strain hold levels of 1.3 and 2.5% at 873 K for E911 steel. Model-II provides better prediction of stress-relaxation behaviour of the steel as compared to Model-I. In addition to prediction of relaxation stress vs. hold time data, Model-II describes the evolution of internal stress, inter-barrier spacing and activation volume with the hold time. The predicted increase in inter-barrier spacing and activation volume with hold time indicated that substructural coarsening remains dominant in E911 steel under stress-relaxation conditions. K EYWORDS . Feltham model; Sine hyperbolic kinetic rate; E911 steel; Stress- relaxation behaviour Citation: Christopher, J., Praveen, C., Choudhary, B.K, Comparative Evaluation of Two Physically Based Models for the Description of Stress-Relaxation Behaviour of 9% Chromium Containing Steel, Frattura ed Integrità Strutturale, 48 (2019) 554-562. Received: 22.11.2018 Accepted: 28.02.2019 Published: 01.04.2019 Copyright: © 2019 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION % chromium tempered martensitic steels are favoured structural materials for high temperature heat exchanger applications in power generating industries. Among 9% Cr steels, E911 steel offers good combination of high creep strength and ductility, and microstructural stability over long exposures at elevated temperatures [1]. Understanding and modelling of inelastic deformation behavior of structural materials at elevated temperatures attract continued scientific and technological interest in view of improving the appropriate conditions for material processing and for reliable prediction of the performance of the components during service. Stress-relaxation testing is one of the potential techniques for understanding the high temperature inelastic deformation behaviour of materials [2]. During stress- relaxation testing, the externally imposed constraint i.e. total applied strain (  t ), is kept constant. Since the total strain rate is related to the sum of elastic (  e ) and inelastic (  in ) strain components, the total applied strain rate is equal to zero and it is represented as 9