Measurement of Nitrogen Dissolution Rate into Molten Alloys by Isotope Exchange Technique

Abstract

An isotope exchange technique is introduced as a method for measuring the nitrogen dissolution rate into molten metal, and the results [H. Ono et al., Metall. Mater. Trans. B, 26, 991 (1995); ibid., 27, 848 (1996)] (refs. 1 and 2) for molten iron-M (M: Ti, Zr, V, Cr, O, Se, and Te) alloys at temperatures from 1873 to 2023 K are summarized.
The rate of nitrogen dissolution into molten iron is shown to increase by adding an element with a stronger affinity for nitrogen than Fe, such as Ti, Zr, V, and Cr. Among these elements, Ti increases the reaction rate most strongly and the rate constant for an Fe-0.08 mass pct Ti alloy is 1.5 times as large as that for pure iron at 1873 K. On the other hand, the addition of surface active elements, such as O, Se, and Te, retards the nitrogen dissolution into molten iron, and the degree of the retarding effect is in the order of Te, Se, and O.
For the addition of non-surface active elements, the correlation of rate constant with thermodynamic interaction parameter with nitrogen has been observed, and this effect is discussed in terms of the change in the activity of the vacant site on the surface of the molten alloy. In case of the addition of surface active elements, the adsorption coefficients of each elements are estimated by fitting the concentration dependence of the nitrogen dissolution rate constant. It can be suggested that surface active elements and nitrogen are adsorbed on the same site at the interface and that the dissociation reaction of nitrogen molecule on the site represented by the equation, N₂^ad+∅=2N^ad, is the rate determining on the assumption that all sites at the metal surface have a uniform adsorption energy for each solute.