The oxidation resistance of nitrided iron alloys

PhD ThesisUsing gas-metal equilibration, a nitrogen martensite case can be produced on iron and mild steel and tempering of this nitrogen martensite produces a fine dispersion of iron nitrides in a ferrite matrix. Under oxidising conditions the finely distributed nitrides provide sites for oxide nucleation and also act as vacancy sinks for inwardly diffusing cation vacancies during the subsequent growth of the oxide scale, thus giving improved metal-oxide adherence. The fine-grained oxide layer produced on nitrided iron and mild steel inhibits the outward diffusion of cations along dislocations and enhances the inward diffusion of anions along oxide grain boundaries. An oxide forming reaction thus takes place at the metal/oxide interface as well as at the oxide/gas interface, further enhancing metal-oxide adherence. Nitriding of binary Fe-X alloys (where X is a strong nitride forming substitutional alloying element) occurs by the formation of metastable , substitutional-interstitial clusters which subsequently transform to stable nitrides. The dispersion of stable nitrides formed on Fe-Cr, Fe-Ti and Fe-Mo alloys behaves similarly to the iron nitrides in nitrided iron and mild steel in affecting the " subsequent formation and growth of oxide. Coherent clusters are less effective in improving oxide adherence. There is no significant increase in oxidation Iresistance of nitrided alloys in isothermal oxidation (450°-500°C) in air for exposures up to 500h. The oxidation rates of nitrided alloys thermally cycled in air for similar times can be greater than those of un-nitrided alloys.The Central Electricity Generating Board