The influence of age hardening on microstructure, phase composition, and microhardness of high-nitrogen austenitic steel
Keywords:high-nitrogen steel, Fe-23Cr-17Mn-0,1C-0,6N, age hardening, σ-phase, Cr2N, austenite, precipitation hardening, microhardness
The authors studied the effect of duration of age hardening at the temperature of 700 °C on the microstructure, phase composition and microhardness of high-nitrogen Fe-23Cr-17Mn-0.1C-0.6N (wt. %) steel. The study showed that age hardening at the temperature of 700 °C for half an hour causes the complex of phase transformations: the decomposition of δ-ferrite (with the formation of σ-phase and austenite) and the formation of cells of discontinuous decomposition on the austenitic grains boundaries (the formation of particles based on the chromium nitride Cr2N and the depletion of austenite by interstitials). After age hardening for more than 10 hours, besides the discontinuous decomposition of austenitic grains, a homogeneous (continuous) precipitation of chromium nitride occurs in those austenitic grains, which have not undergone discontinuous decomposition in the initial stages of aging. With an increase in the aging duration up to 50 hours, the authors observed the growth of decomposition cells in austenitic grains and the formation of mixed structure. Such structure consisted of austenite grains, which underwent discontinuous decomposition with the formation of lamellar precipitations of chromium nitride in austenite; austenitic grains with the dispersed particles formed by the mechanism of continuous decomposition; and the grains with σ-phase, chromium nitrides, and austenite formed as a result of the high-temperature ferrite decomposition during aging. The aging caused the increase in the microhardness, which value depends on the mechanism of precipitation hardening – continuous or discontinuous decomposition in austenite or the precipitation of intermetallic σ-phase and chromium nitrides plates in the grains of high-temperature ferrite.
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