Nitrogen Effect in Microstructural Parameters of a Cast Duplex Stainless Steel
Duplex stainless steels (d2s) are intended to present a microstructure composed of similar volume fractions of austenite and ferrite; this is the phase composition leading to the best compromise of mechanical and corrosion properties. To stabilize the austenite phase, nitrogen is added as an alloying element in the so-called second generation d2s; nitrogen is a cheap element (compared to nickel, the most common austenite stabilizer) that simultaneously increases the strength by a solid solution hardening effect; usually nitrogen contents in the second generation d2s vary from 0.15 to 0.25 weight %. The effect of a 0.14 % nitrogen addition to a cast d2s with a nominal composition corresponding to EN 10283 has been studied to evaluate its austenite stabilizing effect and its influence on the partition intensity of the elements contributing to the pitting corrosion resistance (chromium and molybdenum) between the two phases composing the microstructure. Two cast d2s have been prepared for this study: one without any nitrogen addition and another with 0.14% nitrogen; both had the same nominal composition. Specimens of the two cast d2s have been heat treated in a range of temperatures from 1050 to 1400°C, every 50°C; after a 2 hour soaking the specimens have been cooled in water. It is known that increasing the solution temperature increases the volume fraction of ferrite in the cast d2s microstructure. Both the volume fraction of each constitutional phase (austenite and ferrite) and the respective chromium and molybdenum content have been measured. The results of the volume fraction measurements showed an intense austenite stabilizing effect of the nitrogen. The results of the microanalysis of chromium and molybdenum showed that both elements present an ability to concentrate in the ferrite, although the molybdenum partition is clearly more intense than the chromium partition.
Paula Maria Vilarinho
H. Santos et al., "Nitrogen Effect in Microstructural Parameters of a Cast Duplex Stainless Steel", Materials Science Forum, Vols. 514-516, pp. 524-528, 2006