Austenite instability and precipitation behavior of high nitrogen stainless steels

Nitrogen is a strong austenite stabilizer and improves the mechanical properties and the corrosion resistance of austenitic stainless steels. For these reasons, nitrogen addition to steel has been extensively studied in the last 40 years. This chapter presents a literature review since 1926, when a pioneer study on the effects of nitrogen in iron-based alloys was reported. The maximum solubility of nitrogen in stainless steels is high, although it decreases considerably at temperatures below 1000ºC. Therefore, depending on the composition, high nitrogen austenitic stainless steels can undergo different phase transformations during exposure to temperatures ranging between 500 and 1000oC. Continuous and discontinuous precipitations of chromium nitride, as well as ferrite and sigma phase formation, have been observed and, as a result of these phase transformations, a loss of toughness and lower corrosion resistance are frequently detected. The aim of this chapter is to present and discuss the austenite stability and illustrate the precipitation behavior of high nitrogen austenitic stainless steels. Three different high nitrogen stainless steels are used in order to highlight stabilization/destabilization effects and microstructural features associated with the presence/depletion of nitrogen. The nucleation of discontinuous precipitation of chromium nitride and its growth kinetics are discussed and transformation models are presented. \nKeywords: high nitrogen steels, austenite stability, chromium