NITROGEN UPTAKE BY AUSTENITIC STAINLESS-STEELS ON ANNEALING IN N-2-H-2 ATMOSPHERES

Laboratory simulation of bright annealing of types 301 and 304L stainl ess steel has been performed in N2-H-2 atmospheres. Nitrogen profiles have been quantified by glow discharge optical emission spectroscopy a nd electron microprobe analysis, whereas transmission electron microsc opy has been utilised to characterise the surface phase status after a nnealing. Systematic variation in the water content of the annealing a tmosphere showed that at low water vapour levels, Cr2N precipitates co uld coexist with a surface film of silicon oxide or be inhibited by th e presence of a metal aluminium silicate, whereas at higher water vapo ur levels, nitride precipitation was completely suppressed by the form ation of a Cr-Mn spinel type oxide. The results are discussed in therm odynamic terms, and kinetic limitations caused by the development of a n oxidised surface are considered in some detail. It was found that, i n addition to the more obvious variables of alloy content and nitrogen activity, surface reactions and cooling rate are critical parameters controlling the surface nitrogen composition.