The kinetics of nitrogen absorption and desorption from a plasma arc by molten iron
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I. I N T R O D U C T I O N
IT was first observed while arc welding in nitrogencontaining atmospheres that a weld could attain a nitrogen content in excess of the equilibrium nitrogen solubility. Since then, nitrogen solubilities and kinetics during arc welding and plasma arc melting have been extensively investigated. With the development of the plasma torch as a melting device, researchers in the United States and later in Japan and Russia studied nitrogen uptake into iron melts. Death tl] at Union Carbide found that the nitrogen content increased with pN2 until about 0.27 atm before leveling out at a maximum of 0.20 wt pct. In addition, Death studied the effect of metallic and nonmetallic alloying elements and gas mixtures such as argon-hydrogen-nitrogen and argon-helium-nitrogen in addition to argon-nitrogen. A plasma is defined as a highly ionized gas containing an electrically balanced number of positive ions and electrons. Consequently, a plasma is electrically conducting while at the same time electrically neutral. During plasma arc melting, nitrogen absorption is thought to take place only in the area of the melt where impingement of the plasma arc occurs, since all nitrogen outside this region is fully recombined. In contrast, desorption occurs over the entire melt surface. When the apparent rate of absorption of nitrogen is equal to the apparent rate of desorption, a steady-state nitrogen content is established. The steady-state nitrogen content is always greater than the equilibrium nitrogen solubility. As might be expected, the steady-state nitrogen content is sensitive to the ratio of the area of the melt on which the plasma arc impinges to the total area of the melt. In addition, any variable that affects either the rate of abJ.D. KATZ, Staff Member, is with the Los Alamos National Laboratory, Mail Stop G771, Los Alamos, NM 87545. T.B. KING, formerly Professor of Metallurgy at the Massachusetts Institute of Technology, Cambridge, MA, is deceased. This paper is based on a presentation made in the T.B. King Memorial Symposium on "Physical Chemistry in Metals Processing" presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS. METALLURGICAL TRANSACTIONS B
sorption or desorption will alter the steady-state nitrogen content. Since the steady-state nitrogen content exceeds the equilibrium value, it is clear that absorption must occur via a reaction other than the equilibrium absorption reaction, i.e., N2(g) = 2N(pct). T w o alternative absorption reactions have been suggested, both of which are dependent on the unique properties of the plasma arc. The first reaction involves atomic nitrogen which is believed to be present in the plasma arc and in the gas boundary layer in the region of impingement. If unrecombined nitrogen atoms in the plasma arc strike the melt, the reaction N(g) = N(pct), the solution of atomic nitrogen in iron, is a possible absorption reaction. This reaction can be envisioned to oc
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