High nitrogen concentration in Fe-Cr-Ni alloys
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I.
INTRODUCTION
NITROGEN alloying of steels is an effective technique to improve the mechanical properties of iron alloys, especially stainless steels. Nitrogen concentrations can be controlled by metal alloy composition. Increasing chromium concentration in iron alloys increases nitrogen concentration, whereas increasing nickel concentration decreases nitrogen concentration. Previous nitrogen concentration studies in molten steel alloys have concentrated upon the effects of alloy composition at atmospheric pressure, t~-71 However, melting under nitrogen pressure can also substantially affect nitrogen concentration, ts-~4j Sievert's Law predicts that the nitrogen concentration ([N]) in molten metal should increase proportionally to the square root of the nitrogen pressure (X/PN) in pure elements, tl'lS~ Several studies have shown the [N]-V~N relationship to hold for lower pressures, 0 to 0.1 MPa for molten ironJ xm Recent studies have shown Sievert's Law for nitrogen in pure molten iron to be valid to 10 M P a . t16,17]
For iron alloys, Sievert's Law required modification which was accomplished by expressing the nitrogen concentration in a Taylor series expansion about the nitrogen concentration of Fe as a function of alloying elements. [~,~sl Thermodynamic data can be used to generate phase diagrams that predict phase relationships as a function of composition, which can be verified by experiment. Frisk and c o - w o r k e r s [19-22] have used the existing thermodynamic data to computer generate phase relationships for several Fe alloys v s [N]. To date, there J.C. R A W E R S , Metallurgist, and N.A. GOKCEN, Research Chemist, are with the United States Bureau of Mines, Albany, OR. R.D. PEHLKE, Professor, is with the University of Michigan, Ann Arbor, MI. Manuscript submitted December 27, 1991. METALLURGICAL T R A N S A C T I O N S A
have been few experimental studies capable of generating data necessary to verify Frick's predictions for high nitrogen additions to iron and iron alloys. Sievert's Law requires re-examination when existing thermodynamic analysis is extended to include high pressure, t11,16,171 One need for this extrapolation of the analysis is to study the recently introduced commercial high-nitrogen steels produced by electroslag remelting under elevated nitrogen pressures.[8] Higher nitrogen data require not only using alloy composition terms and a pressure compensating term, but also higher order interaction terms. Below, a new technique for high nitrogen alloying is described, with particular attention to massive nitrogenation of stainless steel. At lower pressure, nitrogen concentration data from iron alloys using the high-pressure melt technique compare well to existing nitrogen concentration data, thermodynamic terms, and microstructural predictions. However, new analytical correlations must be developed to evaluate the effects of pressure and alloy composition on the gas alloy concentration in molten steel. In particular, the inclusion of pressure-alloy concentration interaction terms mu
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