Precipitation strengthening of Nb-stabilized TP347 austenitic steel by a dispersion of secondary Nb(C,N) formed upon a short-term hardening heat treatment

A dense intra-granular dispersion of small secondary Nb(C, N) precipitates in commercial TP347 Nb-stabilized austenitic steel was formed during a short-term hardening heat treatment in the temperature window ΔT=1050–950°C, starting with isothermal annealing at 1050°C and followed by continuous cooling from 1050°C to 950°C. The precipitates are semi-coherent in cube-on-cube orientation relationship with the austenitic matrix, of characteristic faceted octahedral shape with {111} facets and typically d=15±4.6nm in size. The experimentally determined volume fraction f=1.4±0.2·10−3 correlates well with thermodynamic calculations. The precipitation gives a significant strengthening of the steel from σy initial~529±42MPa to σy hardened~703±39MPa, corresponding to a strength increment Δσy~174MPa (~33%) in good agreement with the prediction for Orowan strengthening. The present work is relevant to the exploration of the application potential of short term hardening heat treatments for precipitation strengthening austenitic steels.