2918. RUER, R. [Iron-Carbon Alloys.] Ztschr. anorg. Chem., vol. 117, 1921, pp. 249-261; Chem. Abs., vol. 16, 1922, p.702.
(1) Solubility of cementite in molten iron. A quenched melt of an Fe-C alloy containing 6.5% C showed a structure consisting entirely of cementite. Cementite is precipitated directly from the melt and is not formed by the reaction between 2 phases, 1 solid and the other liquid. The solubility curve of Fe3C in molten Fe rises from the eutectic temperature to a maximum at 6.67% C corresponding tot he compound Fe3C. the melting point of Fe3C in the metastable system Fe-C cannot be determined because Fe3C dissociates at temperatures above 1,100°. The slope of the solubility curve of Fe3C in the metastable system is less than that of graphite in the stable system because with increasing C content the suppression of the separation of graphite becomes more difficult. Small cubes of Swedish Fe containing 2.5-4% C and no Mn were heated in a vacuum for 10 min. at temperatures 1,100°-1,150°. The velocity of the dissociation of Fe3C on the surface of the cubes was higher than the velocity at the center; no explanation is offered.
(2) Complete equilibrium between austenite, ferrite, and graphic corresponding tot he pearlite equilibrium. A sample of Fe containing 3.5% C and no Mn or Si was solidified slowly. After the solid had cooled to 1,000° it was heated to just above the eutectic temperature and again slowly solidified. After a repetition of this latter procedure, the sample was cooled to room temperature. Repeated heating and cooling curves for the sample were then made. The maximum temperature reached in each experiment was 772°. The first curve showed an arrest at 734° on heating and at 720° on cooling. The second curve showed arrests at 736° and 746° on heating and at 720° on cooling. The third curve showed arrests at 736° and 746° on heating and at 720° and 707° on cooling. all these arrests were recognized on subsequent heating and cooling curves up to the twelfth. The twelfth curve showed an arrest at 736° on heating and 712° on cooling. The arrest at 736° corresponds to the transformation of pearlite into austenite. The rate of cooling was so slow that Fe and graphite rather than Fe3C precipitated from the austenite. The arrest at 746° corresponds in the stable system Fe-C to the solution of ferrite and graphite in austenite. The gradual disappearance of the 746° arrest from the heating curves is caused by the coalescence of the graphite particles into large masses, which, under the experimental conditions, would not dissolve in austenite. The double arrest on the cooling curves is not easily explained. The upper arrest at 718° cannot be attributed to the transformation in the stable system and the lower to the transformation in the metastable system because white cast Fe shows the austenite-pearlite transformation at 718°. The simultaneous precipitation of pearlite and the graphite eutectic may occur. Since the arrest at 746° ascribed to the transformation of the eutectic in the stable system and the lower to the transformation in the metastable system because white cast FE shows the austenite-pearlite transformation at 718°. The simultaneous precipitation of pearlite and the graphite eutectic may occur. Since the arrest at 746° ascribed to the transformation of the eutectic in the stable system lies 12° higher than the arrest at 734° ascribed to the transformation of pearlite in the metastable system, then the equilibrium temperature of the former transformation should occur 12° higher than the equilibrium temperature of the latter transformation, that is, at 721+12=733. A horizontal should be drawn across the Fe-C diagram from 0-100% C at 733° to represent the eutectic transformation in the stable system. The concentration of C in the eutectic in the stable system is 0.7%.