Collective Evaluation of Temperature and Stress Dependence of Creep Rupture Life in Austenitic Stainless Steels

Temperature and stress dependence of creep rupture life was examined on 24 heats of type 304, 316, 321 and 347 stainless steels. Collective evaluation of the results indicated the existence of three regions, H, M and L, with different activation energy, Q, and stress exponent, n, for rupture life : region H at a shorter rupture life, region L at a longer rupture life, and region M in between the two regions. The values of Q and n decrease with increasing rupture life as expected from creep fracture theories, suggesting that a longer rupture life than the actual value would be predicted erroneously if it is estimated from the short term data taking the high values of Q. Transgranular and intergranular fracture take place respectively in the regions H and M, and the fracture process in the two regions is controlled by creep deformation. The intergranular fracture in the region L starts from cavities nucleated at sigma-phase/matrix interfaces, and this fracture process is controlled by grain boundary diffusion. The changes in Q and n coincide fairly well with the changes in creep mechanism. This fact points out that the changes in Q and n are inherent to the austenitic stainless steels and the same thing will happen in any materials similar to the four types of stainless steels.