Precipitation Behavior and Mechanical Properties of High-Nitrogen Austenitic Stainless Steel According to Chemical Composition and Heat Treatment Temperature

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The precipitation behavior and mechanical properties of high-nitrogen austenitic stainless steels were investigated for various chemical compositions and solution treatment temperatures. For this study, Fe-22.1Cr-12.2Ni-5.1Mn-2.1Mo alloys with three different sets of Nb, V, and N contents were prepared, and two distinct solution treatment temperatures were applied. An increase in Nb or N content resulted in finer austenite grain size, attributed to the formation of additional precipitates, such as Z-phase and (Nb,V)(C,N), at the grain boundaries. Notably, the effect of a 0.07 wt% increase in Nb content was found to be comparable to more than a 100oC reduction in solution treatment temperature. The yield strength was observed to increase with decreasing grain size, consistent with the Hall-Petch relationship. When the grain size effect was excluded, and in the absence of Z-phase, a higher N content led to an increase in strength due to solid solution strengthening. On the other hand, an increase in Nb content resulted in reduced strength, even though there was little change in the austenite N content. Thermodynamic analysis suggested that Z-phase precipitation may replace MX particles, which could account for the observed decrease in strength. While Z-phase precipitation promotes a finer grain size, coarse Z-phase particles did not contribute effectively to precipitation hardening. Therefore, to achieve higher strength in high-nitrogen austenitic stainless steels, alloy design must be optimized, particularly when Nb addition is considered.