Elephant grain growth in 12% chromium steels

Abstract

The phenomenon of abnormal grain growth in certain 12% Cr turbine steels has been found to result from secondary recrystallization, facilitated by the presence of stable Nb(C,N)particles together with at least 1% austenite retained in the alloy matrix. In the absence of either of these features abnormal grain growth does not occur. If suitable nucleation sites are available, discontinuous grain growth occurs when the average material grain size reaches a critical value, determined by the amount and size of the stable Nb(C,N)particles. Once the grain size has attained this limiting value normal grain growth stops, but a few crystals continue growing at the expense of their nearest neighbours until the whole structure has been consumed. This phenomenon is essentially independent of the degree of prior deformation and is, therefore, not a critical strain effect. The duration of the incubation period, before the onset of abnormal grain growth, has been shown to be consistent with the time taken for the average material grain size to reach the limiting value. It is shown that at least 1% austenite in the alloy matrix is necessary to provide the nucleation sites for the secondary recrystallization process that leads to abnormal grain growth.