The solubility of the liquid oxysulfide phase in liquid Fe-O-S alloys
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I.
INTRODUCTION
THE
solubility of "FeO" and of the iron-oxysulfide in liquid iron has been measured by a novel method employing levitation melting. The method was selected because it avoided the need for a crucible to contain the melt, which in turn avoided possible contamination of the nonmetallic phase with the crucible material and possible loss of sulfur and oxygen to the crucible from the iron melt. In addition, electromagnetic stirring assured complete mixing of the liquid. A small pellet of iron that contained specified concentrations of oxygen and sulfur was levitated and melted in a stream of argon, and then brought to a high temperature where these nonmetallic elements were completely dissolved in the liquid iron. The specimen was then cooled slowly, and its temperature was monitored. At some temperature, the liquid became saturated with respect to the nonmetallic phase, and a bright fleck of the oxide, or oxysulfide, phase appeared. The temperature at which this occurred was the saturation temperature for the initial composition of the alloy. The method was particularly interesting because it permitted the melts being studied to be undercooled up to 150 ~ below the monotectic temperature in the Fe-O system (1528 ~ Thus, it was possible to study the behavior of the melts over a range of 360 ~
II.
EXPERIMENTAL
The apparatus employed was the levitation system described by Yarwood, Flemings, and Elliott. 1 When levitated, the metal pellet is suspended within a vertical Vycor tube; the levitation coil is positioned outside around this tube. The upper end of the tube is closed with an opticalquality sight glass on which is positioned a glass prism
M. NDUAGUBA was formerly Graduate Research Assistant, Department of Materials Science and Engineering, Massachusetts Institute of Technology. J. F. ELLIOTT is AISI Distinguished Professor and Professor of Metallurgy, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted March 1, 1983. METALLURGICAL TRANSACTIONS B
through which a two-color pyrometer (Milltron) can be sighted down on the upper portion of the levitated specimen. The stream of purified argon passes into this tube through a side tubulation and then flows down past the specimen. A sealed brass box that is attached to the lower end of the Vycor tube contains a turntable. Mounted on the turntable are five boron nitride cups to hold specimens to be levitated and five split copper cups into which the melted specimens can be dropped and quenched. A retractable lifting device that projects through the bottom of the box is utilized to lift a specimen and cup from the turntable and to place the specimen at the center of the levitation coil. The boron nitride cup is then retracted to the turntable once the specimen is levitated.
A. Preparation of Materials The metal specimens for levitation were small samples (1.5 to 2 gm) of iron which were prepared from 0.5-inch diameter rod stock (Ferrovac-E) by swaging the rod to a diameter of 5 mm. The composition of the iron is shown in Tab
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