Modeling and experimental study of long term creep damage in austenitic stainless steels

Different batches of austenitic stainless steels (316LN) are subjected to numerous creep tests carried out at various stresses and temperatures between 525 degrees C to 750 degrees C up to nearly 50.103h. Interrupted creep tests show an acceleration of the creep deformation only during the last 15% of creep lifetime which corresponds to macroscopic necking. The modeling of necking using the Norton flow law allows lifetime predictions in fair agreement with experimental data up to a few thousand hours only. In fact, the experimental results show that, the extrapolation of the 'stress - lifetime' curves obtained at high stress leads to large overestimations of lifetimes at low stress. After FEG - SEM observations, these overestimates are mainly due to additional intergranular cavitation along grain boundaries as often observed in many metallic materials. The modeling of cavity growth by vacancy diffusion along grain boundaries coupled with continuous nucleation proposed by Riedel is carried out. For each specimen, ten FEG SEM images (about 250 observed grains) are analyzed to determine the rate of cavity nucleation assumed to be constant during each creep test in agreement with many literature results. This constant rate is the only measured parameter which is used as input of the Riedel model. Lifetimes for long term creep are rather fairly well predicted by either the necking model or the Riedel model with respect to experimental lifetimes up to 200000 hours for temperatures between 525 degrees C and 700 degrees C. A transition time as well as a transition stress is defined by the intersection of the lifetime curves based on the necking and Riedel modellings. This is due to a change in damage mechanism. The scatter in lifetimes predicted by the Riedel model induced by the uncertainly of some parameter values is around 50%. This model is also validated for martensitic steels (Lim et al, 2011.) and for other austenitic SSs 304H, 316H, 321H (creep rupture data provided by Dr. F. Abe, NIMS). A transition from power-law to viscous creep behavior is reported in the literature at 650 degrees C -750 degrees C. It allows us to predict even better lifetimes up to 200000 hours at very high temperature. (C) 2014 Published by Elsevier Ltd.