DISCONTINUOUS PRECIPITATION OF CR2N IN A HIGH-NITROGEN, CHROMIUM-MANGANESE AUSTENITIC STAINLESS-STEEL

The main purpose of the present work is to study the effect of a high nitrogen content (1 wt% N), on the microstructural evolution of a Cr-M n austenitic stainless steel aged over the [400-900 degrees C] tempera ture interval. Thermal treatments carried out between 700 and 900 degr ees C lead to the decem position of the nitrogen supersaturated austen itic matrix by discontinuous precipitation of Cr2N particles. The micr ostructural features of the reaction are described and analysed. In th e present case, the cellular precipitation of Cr2N is a peculiar and c omplex phenomenon which involves two diffusion mechanisms: the diffusi on of an interstitial element (nitrogen) and the diffusion of a substi tutional one (chromium). The nucleation of the discontinuous precipita tion arises from a reduction of the surface energy of the precipitates . Furthermore, the precipitation growth is a non-steady state process, because the reaction is governed at first by the intergranular diffus ion of chromium, and then tends to be controlled by its bulk diffusion . Consequently, the features of this discontinuous precipitation do no t fit in with the assumptions of usual theories, which have been estab lished for binary substitutional systems that transform in steady stat e conditions. This discontinuous precipitation brings about a slight h ardening. Then, the hardness of the aged samples can be described by a n additive relationship between the hardness of the precipitation cell s and that of the untransformed matrix. Beside the discontinuous preci pitation of Cr2N, sigma phase forms with significant volume fractions.