Tungsten-nitrogen interactions in iron

Abstract

Precipitation in iron-tungsten alloys containing nitrogen as the predominant interstitial constituent is studied by a variety of techniques including X-ray methods, optical metallography and electron microscopy. Investigations into the tungsten-nitrogen system show the marked effect of oxygen on the formation of tungsten nitrides. "Constant activity nitriding" of iron-tungsten alloys containing up to 9.3 wt.% produces as many as six different equilibrium tungsten or iron-tungsten nitrides and oxynitrides. The phase produced depends upon the tungsten, nitrogen and oxygen potential as well the the temperature. Alloys containing more than about 4.0 wt.% tungsten nitrided at 590-640 C in appropriate nitrogen potentials exhibit a precipitation sequence GP zones -intermediate ---- equilibrium precipitat precipitate The transition from zones to intermediate precipitate occurs by a continuous ordering process in which tungsten atoms replace those of iron in certain sites. The final stage occurs continuously.Tungsten concentrations or nitrogen potentials lower than the critical values prevent rapid homogeneous precipitation and the intermediate phase is precipitated heterogeneously. Metallographic studies of quenoh-aged tungsten nitrogen-ferrites demonstrate that tungsten has a negligible effect on the Para-preoipitation of iron nitrides.