Abstract
Laboratory simulation of bright annealing of types 301 and 304L stainless steel has been performed in N2–H2 atmospheres. Nitrogen profiles have been quantified by glow discharge optical emission spectroscopy and electron microprobe analysis, whereas transmission electron microscopy has been utilised to characterise the surface phase status after annealing. Systematic variation in the water content of the annealing atmosphere showed that at low water vapour levels, Cr2N precipitates could coexist with a surface film of silicon oxide or be inhibited by the presence of a metal aluminium silicate, whereas at higher water vapour levels, nitride precipitation was completely suppressed by the formation of a Cr–Mn spinel type oxide. The results are discussed in thermodynamic terms, and kinetic limitations caused by the development of an oxidised surface are considered in some detail. It was found that, in 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.
MST/1811
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