Influence of nitrogen on oxidation resistance of low-alloy steels

Abstract

The oxidation resistance of mild steel is shown to be considerably enhanced by additions of nitrogen which result in the formation of an adherent fine-grain-size oxide scale. A mechanism is proposed by which improved protection results from enhanced nucleation of oxide at incoherent nitride particles in the surface metal layer. A suitable nitrided surface is forme by constant activity ageing in an NH₃ : H₂ gas mixture to form austenite at nitriding temperature. The resulting austenite + martensite micro-structure tempers rapidly during oxidation at 420°C forming incoherent Fe₄N precipitates which act as oxide nucleation sites. Similar results have been obtained from studies of iron alloys containing small additions of titanium. Formation of a surface layer containing a fine dispersion of titanium nitride particles in a ferrite matrix is shown to enhance oxidation resistance.

The proposed mechanism is similar to that of the development of improved oxidation resistance in superalloys by subsurface dispersion of incoherent stable oxide particles.