Development of the RCC-MR Creep Deformation Model for the Prediction of Creep and Stress Relaxation in Type 321 Stainless Steel

Abstract

In this paper a creep deformation model has been developed for Type 321 stainless steel which has been based on a modified version of the creep model that is used in the French fast reactor design code RCC-MR. The model has been evaluated using: 1) constant load creep data covering the temperature range from 550°C to 650°C and 2) constant displacement, stress relaxation data obtained from creep-fatigue tests at 650°C. Samples in the heat-treatment conditions of solution-treated, aged, and simulated ‘heat affected zone’ have been assessed. The standard RCC-MR model was fitted to the constant load data and provided good predictions of forward creep. However, when this model was used to predict stress relaxation it was observed that the model significantly over predicted creep strain rates and therefore the level of stress drop during each cycle. During constant load tests the stress remains relatively constant (noting that true stress does increase a small amount prior to rupture). However, in relaxation tests the stress varies significantly over the dwell. Due to the poor predictions of stress relaxation it was hypothesised that the fitted model did not capture the stress dependence of creep appropriately. The RCC-MR model was therefore modified to include a primary and secondary threshold stress term that is a function of the accumulated creep strain. This work indicates that the RCC-MR model, modified to include threshold stresses, can be used to provide good predictions of both forward creep and stress relaxation in Type 321 stainless steel. Further work is required to validate this model on stress relaxation data at additional temperatures and lower start of dwell stresses.