Microstructural assessment of 310S stainless steel during creep at 800 °C

A thorough investigation of the microstructural evolution of 310S stainless steel subjected to creep at 800 °C shed new light on the nucleation and growth mechanism of M23C6 precipitates and the σ-phase. During creep of 310S, the M23C6 precipitates form on defects such as dislocations, Ti(C,N) precipitates and twin boundaries, within the austenitic matrix. These precipitates have a cube to cube orientation relationship with the matrix and are initially semi-coherent; maintaining coherency by forming mismatch dislocations. As the M23C6 precipitates are consumed by growing σ-phase particles, they can no longer pin sub-grain boundaries and a decrease in dislocation density of the material occurs, as measured by TEM. Under low stress, formation of sufficient quantities of the σ-phase eventually causes sensitization of the stainless steel leading to oxidation and void formation at grain boundaries. Eventually the diffusion of nitrogen, causes formation of an extensive network of widmanstatten Cr2N throughout the bulk.