Precipitation in iron-chromium-nitrogen alloys

Abstract

Precipitation reactions in iron-chromium alloys containing nitrogen are studied by X-ray methods and by optical and electron microscopy. By experimental control of reaction conditions, seven different phases are identified in iron alloys containing up to 40wt. 1% chromiwa nitrided at 450 - 10500C. The precipitation of chromium nitrites is shown to be consistent with the thermodynamic properties of these phases. Quench-aging observations on nitrogen-ferrites shcw that precipitation of iron nitrides below 4OO°C is markedly affected by the presence of chromium in solution. The results are explicable when the effect of chromium in decreasing, the activity coefficient of dissolved nitrogen is taken into account. These observations are compared with recent work on precipitation of iron nitrides in iron-molydenum-nitrogen and iron-silicon-nitrogen alloys. Previous observations at Newcastle, of homogeneous precipitation in the iron-nitrogen system and in ternary iron-alloy element-nitrogen alloy's containing molybdenum, niobium, titanium or silicon are extended to the technologically important Fe-Cr-N system. The precipitation reactions, which go through the stalres well-recognised in face-centred cubic alloys i.e. Guinier-Preston zones > metastable intermediate precipitate > equilibrium precipitate, are discussed and compared with the observations in the present investigation. In addition, the effects of chromium content and nitrogen potential on the kinetics of nitriding are reported. The crystal structure and X-ray line broadening of the manganese carbide, M22C6, are discussed in an appendix.