Precipitation in a nitrogen-alloyed stainless steel at 850°C

Abstract

The highly-alloyed austenitic stainless steels, with molybdenum contents typically in excess of 4 wt %, are a group of materials which are undergoing continual development due to increasing harsh user demands. Nitrogen has proved particularly attractive as an alloying element in view of its strong austenite-stabilizing tendency, beneficial effect on localized corrosion resistance and ready availability. The influence of nitrogen alloying on sensitization behavior is complex in that it increases the risk of nitride precipitation but may retard the precipitation of intermetallic phases. This has been illustrated by Thier et al for 21 Cr 25Ni 6Mo. The same has also been demonstrated to be the case for 20Cr 20-30Ni 4.5Mo steels in that the addition of 0.21 wt % nitrogen retarded the first precipitating intermetallics (principally Laves phase) but caused the appearance of Cr[sub 2]N. The present work represents an elaboration of the latter study in which the effect of various exposure times at 850 C has been investigated for the alloy containing 0.21% nitrogen in order to elucidate the precipitation reactions and equilibrium state.