The rate of absorption of nitrogen into liquid iron containing oxygen and sulfur
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IN the early 1960's several groups of investigators studied the rate of the reaction of nitrogen with liquid iron 1-3 and all obtained similar results. The most extensive work was done by Pehlke and Elliott, s who used a modified Sieverts apparatus with an inductionstirred melt. They demonstrated the interesting effect that oxygen and sulfur retard the rate of the reaction because these elements are surface-active and block surface sites on which the reactions can occur. It has been demonstrated extensively that at low oxygen and sulfur contents the rate is controlled by mass transfer in the metal boundary layer and is therefore first order with respect to nitrogen content. The earlier work of Pehlke and Elliott and of Schenck, e t aP indicated that the rate was also first order at high oxygen and sulfur contents. More recently Inouye and Chon 4 have examined the order of the reaction at high oxygen and sulfur contents and found the rate to be second order with respect to nitrogen. However the sampling technique they used may not be sensitive enough to distinguish between a first- or second-order rate. The work of Mori and Suzuki 5 and of Fuwa, e t a l 6 on the rate of nitrogen removal from liquid iron alloys also indicates the rate is second order. With the advent of the use of nitrogen in the Q-BOP and AOD processes, the order of the nitrogen reaction with liquid iron takes on added importance. To make any calculations to estimate the rate of nitrogen pickup or removal in these processes, it is necessary to know the order of the reaction. Also, all steels contain both oxygen and sulfur and no work has been done to determine the rate when both elements are present. It is therefore the purpose of the present work to determine the rate of the nitrogen reaction with iron containing oxygen or sulfur or R. J. FRUEHAN and L. J. MARTONIK are with the U. S. Steel Research Laboratory, Monroeville, PA 15146. Manuscript submitted January 31, 1980.
both, with particular emphasis on determining the order of the reaction.
EXPERIMENTAL The experiments were conducted in a conslantr volume Sieverts apparatus with induction heating. The Sieverts apparatus was similar in design to that used by Pehlke and Elliott s except that the rate was measured with a pressure transducer. The change in nitrogen content in the metal is directly proportional to the change in pressure, and the sensitivity was such that the pressure could be measured within +__5 • 10 -4 atm. During an experiment the change in pressure was small, the largest change being 0.03 atm for experiments done at one atm pressure of nitrogen. The melt usually weighed about 60 g and was contained in an alumina crucible with an internal diam of 31 mm. The melts were made up from master alloys of iron-oxygen, iron-sulfur, and high-purity vacuum-deoxidized iron. The temperature was measured with a two-color optical pyrometer calibrated against the melting point of iron and was maintained at 1600 +__2 ~ for all the experiments. The sample was heated slowly to 1600 ~ The proportion
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