Nitrogen solubility in liquid Fe and Fe-Mn alloys
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HARUE WADA, SEOUNG WON LEE, and ROBERT D. PEHLKE The nitrogen solubility in liquid Fe-Mn alloys was measured by the levitation melting method at 3.8, 8.5, and 15.8 wt pct Mn. One atmosphere nitrogen pressure was used to minimize the vaporization of Mn from the melts. The temperature range was 1762 to 2271 K. Figure 1 shows a melting unit arrangement. A sample of weight ranging from 0.7 to 2 g was held from a glass hook with an Fe wire. The sample was melted under nitrogen gas and held at constant temperature for 1 minute, then quenched in a water-cooled copper mold. The nitrogen content in the quenched sample was analyzed either by a LECO TN-144 nitrogen analyzer or by the Kjeldahl method. High purity nitrogen gas was further purified by passing through heated Cu gauze and P205 columns. The melting time required to reach equilibrium was determined to be 1 minute from preliminary experiments. The sample temperatures were measured with a two-color radiation pyrometer, which was calibrated at the melting points of Fe, Ni, and Cu. Three Fe-Mn alloys were prepared by induction melting Ferrovac E iron (99.97 pct Fe, 0.00015 pct N, and 0.0012 pct O) and Mn (99.8 pct Mn, max. 0.007 pct N, and max. 0.5 pct O) under Ar. The shape of the watercooled Cu mold was chosen from several configurations because this shape resulted in the lowest segregation of nitrogen in Fe. 1 The experimental results for nitrogen solubility in liquid Fe at the standard 1 atmosphere nitrogen pressure were determined by a regression analysis of the data as: - 1.125
[1]
The free energy of solution of nitrogen in Fe is then AG~ = (8230 -
820)
+ (21.54 --- 0.25)T
J/g-atom N
[2]
HARUE WADA, Associate Research Scientist, and ROBERT D. PEHLKE, Professor, are with the Department of Materials and Metallurgical Engineering, University of Michigan, Ann Arbor, MI 48109. SEOUNG WON LEE is Assistant Professor, ChoongnamNational University, Tae Jeon, Korea 300-31. Manuscript submittedMay 24, 1985. 238--VOLUME 17BIMARCH1986
f
~ b EO0as
Water Fig. 1--Arrangement of levitation melting unit.
Nitrogen Solubility in Liquid Fe and Fe-Mn Alloys
log (wt pct N)Fe = - 4 3 0 / T
a- Quartz Reaction Tube b- Molten Sample c- Levitation Coil d- Copper Mold e- Two- Color Pyrometer f- Glass Holding Hook
The nitrogen solubility in Fe at 1873 K shows reasonable agreement among previous measurements: 2-t~from 0.044 to 0.046 wt pct N. However, the temperature dependencies of the solubility are different with no clear reason; they are lower in the measurements by the Sieverts' method than those by the sampling method as summarized in Table I. The nitrogen solubilities in Fe-Mn alloys are shown in Figure 2 as a function of temperature. Increasing Mn content increases the nitrogen solubility and changes its temperature dependence from positive to negative. The solubility lines were determined by regression analysis for each Mn composition. The standard heat of solution of nitrogen in Fe-Mn alloys is calculated as AH~ = 8230 - 1770(wt pct Mn)
J/g-atom N [3]
Figure 3 summariz
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