Microstructural evaluation of austenite reversion during intercritical annealing of Fe-Ni-Mn martensitic steel

Akin to the binary Fe-Ni alloys, ternary Fe-Ni-Mn alloys show lath martensite structure which transforms to austenite during annealing at high temperatures. The transformation is accompanied by precipitation and reversion either by diffusion or by displacive mechanisms which leads to the structural and property refinement. This study was aimed to clarify microstructural changes during reversion of an Fe-10Ni-7Mn alloy which is suffered from severe grain boundary embrittlement after aging. The alloy was annealed at 600 C for various holding times, where a mixed structure of α and γ phases are expected to be stable thermodynamically. Microstructure changes were investigated by means of electron back scattering diffraction (EBSD), X-ray diffraction (XRD), dilatometer and differential scanning calorimeter (DSC). In the solution-annealed sample only diffraction lines belonging to the bcc iron are found. However, after intercritical annealing bcc-martensite in combination with fcc austenite were revealed as the latter retained after subzero-cooling subsequently. Volume fraction of retained austenite was indicated to increase remarkably with holding time. It was found that austenite particles are nucleated at block and packet boundaries in the lath martensite structure. After annealing for prolonged times, austenite grows further as a result of which the initial lath martensite structure is subdivided, leading to fine (α + γ) dual phase structures.