Effects of aluminum, silicon, and boron on the dissolution rate of nitrogen into molten iron
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I. INTRODUCTION
NITROGEN dissolves into molten iron during its refining processes and even a small amount of nitrogen in steel causes harmful effects on the mechanical properties. Accordingly, evaluation of the nitrogen dissolution rate into molten iron is important and many kinetic studies[1–21] have been conducted. Nitrogen dissolution is known to take place through three steps, a mass transfer in molten iron, a chemical reaction at the surface, and a mass transfer in the gas phase, respectively. Most of the research has been performed by the sampling method,[1–12] in which it is difficult to focus only on the chemical reaction rate especially when the concentration of surface active elements is low. In order to know the maximum rate of nitrogen removal, it is important to investigate 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. This measurement technique has been developed by Belton[13] and Byrne and Belton,[14] and a few studies on the effect of surface active elements[14–17] have been carried out. We have also employed the technique in which the rate of change in the ratio of 30N2/28N2 is monitored and the effects of some elements have already been reported.[18,19,20] We found that the rate of nitrogen dissolution into molten iron increases by adding an element such as Ti, Zr, V, and Cr with stronger affinity for nitrogen than Fe due to the increase in the activity of vacant site at the metal surface. In the present study, we focused on the alloying elements for steels that form stable nitride, and the effects of Al, Si, and B addition have been studied.
KAZUKI MORITA, Associate Professor, and TARO HIROSUMI, Graduate Student, are with the Department of Metallurgy, The University of Tokyo, Tokyo 113-8656, Japan. NOBUO SANO, Emeritus Professor, Department of Metallurgy, The University of Tokyo, is Executive Advisor, Nippon Steel Corporation, Chiba 293-8511, Japan. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS. METALLURGICAL AND MATERIALS TRANSACTIONS B
II. EXPERIMENTAL Since nitrogen dissolves into molten iron according to Reaction [1], the rate of nitrogen dissolution, v (mol/cm2?s), is expressed by Eq. [2]. N2 (gas) 5 2N (in Fe)
[1]
v 5 kPN2
[2]
where PN2 (atm) is the partial pressure of nitrogen and k (mol/cm2?s?atm) is the rate constant of nitrogen dissolution reaction. Since three types of N2 gas molecules, 28N2, 29N2, and 30N2, exist in the gas phase, the following reactions can be considered as the nitrogen dissolution processes: N2 5 2 14N
[3]
N2 5 14N 1 15N
[4]
N2 5 2 N
[5]
28 29 30
15
30
Considering a mass balance of N2, Eq. [6] holds on the assumption that the composition of absorbed gas is not affected by desorbed gas, and the fraction of 30N2 in the total absorbed nitrog
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