In-situ and ex-situ microstructure studies and dislocation-based modelling for primary creep regeneration response of 316H stainless steel

X Li,S.R Holdsworth,S Kalácska,L Balogh,J.-S Park,Y Arroyo Rojas Dasilva,X Maeder,A Cocks,E Mazza,E Hosseini

doi:10.1016/j.actamat.2021.117130

Abstract

The emergence of renewable energy sources with their variable and unpredictable nature, in addition to the variation of energy need for weekdays vs. weekends, demands an ever flexible operation of thermal power plants. Such a feature has therefore altered the typical steady creep loading of high-temperature components of power plants to stress-varying or cyclic creep conditions. The introduced load transients have been found to affect the strain hardening memory of the creeping alloys and might lead to multiple primary creep regeneration (PCR). Therefore, the creep strain accumulation can considerably increase under such conditions. Consideration of the PCR phenomenon is beyond the capability of conventional creep constitutive models which are based on strain- or time-hardening assumptions. The present study conducted in-situ and ex-situ experiments for 316H stainless steel. Various microstructural examination techniques, such as synchrotron high energy X-ray and neutron diffraction, and backscattered and transmission electron microscopy, have been employed for characterising evolution of the dislocation structure and the internal lattice strain/stress state of the alloy during stress-varying and cyclic creep conditions. The formation/annihilation of dislocation pileups and the bowing/unbowing of dislocation-lines were identified as the responsible mechanisms for PCR. A dislocation-based model was then formulated which could well represent the measured microstructural evolution and mechanical response of the steel during the conducted experiments at 650°C.

Highlights

In a creeping alloy, a load reversal may clear the previous hardening memory, and lead to a period of high creep strain rates upon reloading, i.e. primary creep regeneration (PCR) is seen
This study presents a detailed description of the PCR phenomenon for the steel and provides a basis for proposing guidelines which can realistically consider PCR in EDF’s R5 high temperature mechanical assessment procedure
Interpretation of the microstructural data suggests that the formation/relaxation of dislocation pileups and the bowing/unbowing of dislocation-lines are the responsible mechanisms for the PCR phenomenon

Summary

Introduction

A load reversal may clear the previous hardening memory, and lead to a period of high creep strain rates upon reloading, i.e. primary creep regeneration (PCR) is seen. Has been developed for representing the evolution of microstructural parameters and creep strain response of the alloy during stress-varying creep loading. A previous study by Petkov et al [5] proposed a dislocation-based model representing the creep response of 316H steel under stress-varying creep conditions. The model could reasonably predict the results of their mechanical experiments, it could not interpret the experimentally observed dislocation density evolution during the conducted in-situ and ex-situ stress-varying creep experiments in this study, which indicated a need for further elaboration of the model assumptions and formulations.

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Conclusion