Time–Temperature–Precipitation Relations for Nitrides and Evaluation of Internal Oxidation Theory for Nitridation of Austenitic Stainless Steel

Abstract

Internal nitridation kinetics were determined for a UNS N08810/800H alloy using a general model of the form $$x^{n}=kt$$ . Nitridation behavior was studied at service-relevant temperatures 800 °C to 1000 °C in a 95 pct $$\hbox {N}_2/5\,{\text{pct}}$$ $$\hbox {H}_2$$ atmosphere for times 50 to 750 hours. Optical and scanning electron microscopy were used for microstructural characterization and measurement of nitride penetration. AlN, $$\hbox {Cr}_2\hbox {N}$$ , and CrN were formed, and the experimentally observed precipitation sequence was consistent with equilibrium calculations for this alloy using Thermo-Calc. A combination of diffusivity data determined using DICTRA and experimentally verified equilibrium calculations showed that Wagner’s analysis for internal oxidation kinetics was valid for AlN penetration. Parabolic kinetics closely approximated measured AlN penetration. This suggests that extension of AlN penetration models to other temperatures and Fe-Ni-Cr-Al alloy systems is reasonable. $$\hbox {Cr}_2\hbox {N}$$ penetration did not conform to Wagner’s analysis. Deviation from parabolic behavior was evident, and general model penetration predictions for $$\hbox {Cr}_2\hbox {N}$$ were experimentally validated. Using the experimentally determined models, time–temperature–precipitation diagrams for AlN and $$\hbox {Cr}_2\hbox {N}$$ penetration were constructed.