Effects of Ti, Zr, V, and Cr on the rate of nitrogen dissolution into molten iron
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I.
INTRODUCTION
THE knowledge of the rate of nitrogen dissolution into molten iron is indispensable for efficient control of its nitrogen content, and many kinetic studies have been conducted. To know the maximum rate of nitrogen removal, it is important to know the rate constant of the chemical reaction and the pertinent effects of added elements. An isotope exchange technique has an advantage in that the dissolution rate can be measured without any influence of liquid phase mass transfer. Although this kind of measurement has been made extensively by Belton,Vl no studies have been done on the effect of non-surface alloying elements, except for C [2,3,4Jand Cr.tSJ Accordingly, in the present study, the effects of Ti, Zr, V, and Cr (which have stronger affinities for nitrogen than Fe) on the reaction rate have been measured using an isotope exchange technique at temperatures from 1873 to 2023 K, and the mechanism of the enhancement of reaction rate and correlations with thermodynamic properties are discussed. II.
EXPERIMENTAL
= Av(3~
-
N2 (gas) = 2N (in metal)
[2]
v = kPN2
[3]
where k (mol/cm2 9 s 9 atm) is the rate constant of nitrogen dissolution. Incorporating Eq. [3] into Eq. [1], the rate constant is represented by Eq. [4]: 3~ k =
3OF+
-
A R T 3~
-- 3~ eq
[4]
On the other hand, the rate constant equation introduced by Byme and Belton[2] was derived by assuming the complete mixing of gas at a metal surface and is represented by Eq.
[5]:
Details of the experimental principle, apparatus, and procedure have been given elsewhere, t6] Accordingly, only the subjects particular to the present study are briefly described here. The equation for calculating a rate constant in this work is not the same as that used by Byme and Belton[2] or that in the previous article, t6] It was derived as follows. Considering a mass balance of 3~ 2, Eq. [1] holds on the assumption that the composition of absorbed gas is not affected by desorbed gas, and the fraction of 3~ 2 in the total absorbed nitrogen equals that in the ingoing nitrogen. PN2V R---T-(3~ - 3~
is the surface area of molten metal, V (cm3/s) is the volume flow rate, T (K) is the temperature of gas, R [82.1 ( c m 3 9 atrn/K 9 mol)] is the gas constant, 3~ is the equilibrium fraction of 3~ and 3~ i and ~~ I are the fractions of 3~ 2 in the ingoing and outgoing gases, respectively. Since nitrogen dissolves into molten metal according to Reaction [2], the rate o f total nitrogen dissolution, v (mol/cm 2 9 s), which is a first order with respect to the partial pressure of nitrogen,t6] is expressed by the following:
3~
[1]
where PN2 (atm) is the partial pressure of nitrogen, A (cm 2)
HIDEKI ONO, Graduate Student and Research Fellow of the Japan Society for the Promotion of Science, KAZUKI MORITA, Lecturer, and NOBUO SANO, Professor, are with the Department of Metallurgy, The University of Tokyo, Bunkyo-ku, Tokyo, 113, Japan. Manuscript submitted July 13, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
"-~ k =
~
3 0 F f - 30Feq In 3~
3~
[5]
Equation
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