Microstructural stability during creep of a niobium-stabilized austenitic steel

Abstract

Creep tests combined with transmission electron microscopy have been carried out on a niobium stabilized austenitic stainless steel which has been subject to a multi-mechanical thermal treatment (MMTT). It is found that the substructure introduced by the MMTT creates two distinct creep regimes, separated by a narrow transition temperature or stress range. In the low stress or low temperature regime creep is characterised by a low stress exponent (about 5) and a low activation energy (~300kJ/mole). The diffusion-controlled motion of jogged screw dislocations is suggested to be the most likely rate-limiting process. In the high stress or high temperature regime creep is characterised by a high stress exponent (about 10) and a high activation energy (~ 600 kJ/mole). Subgrains stabilized by the NbC precipates are observed to elongate during creep and the rate-limiting process is the breakaway of subgrain boundaries from small particles.