MECHANICAL-PROPERTIES OF ISOTHERMALLY AGED HIGH-NITROGEN STAINLESS-STEEL

The effects of nitride (Cr2N) precipitation on the tensile, impact, and hardness properties of a typical high-nitrogen, low-carbon austenitic stainless steel(SS), nominally Fe-19Cr-5Mn-5Ni-3Mo-0.024C-0.69N, were determined. Annealed and cold-rolled (20 pet reduction in thickness) specimens were isothermally aged at 700 degrees C and 900 degrees C for times ranging from 0.1 to 10 hours. Only grain boundary Cr2N precipitation occurred in annealed materials aged at 700 degrees C. Precipitation at 900 degrees C occurred sequentially at grain boundaries, by cellular precipitation, and, finally, by transgranular precipitation within the matrix. Nitride precipitation had little effect on yield and ultimate strengths but reduced tensile ductility and impact toughness. Embrittlement occurred due to grain boundary separation (700 degrees C and 900 degrees C) and fracture through cellular precipitate regions, initiated at nitrides (900 degrees C). Prior deformation increased precipitation kinetics and had a controlling influence on nitride morphology, enhancing grain boundary and transgranular Cr2N and retarding cellular precipitation. Nitride structures produced in cold-rolled materials were just as detrimental to material plasticity as those produced in annealed materials, but prior deformation increased the kinetics of embrittlement. Due to strain recovery, the yield and ultimate strengths of cold-rolled materials decreased with aging time and temperature.