Experimental Determination of the Austenite + Liquid Phase Boundaries of the Fe-C System
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I.
INTRODUCTION
II.
O N E of the most important binary phase diagrams in the field of physical metallurgy is undoubtedly that of the Fe-C system. It provides the basis for commercially significant ternary systems such as Fe-C-Cr, Fe-C-Si, and Fe-C-Mn, and the austenite + liquid ( y , L ) phase field is relevant to solidification studies of cast irons. For these reasons, an accurate definition of the latter region of the Fe-C diagram is obviously desirable. Hansen and Anderko 1 give an excellent history of the initial work on the Fe-C system. Following earlier studies, 2'3'4 the y solidus has been determined by Buckley and Hume-Rothery 5 and by Benz and Elliott, 6 and modified somewhat by Ban-ya, Elliott, and Chipman 7 and by Chipman. 8 The 3' liquidus was partially determined by Adcock 4 and more completely by Umino 3 and by Ruer and Goerens,9 while the most accurate data are due to Buckley and HumeRothery5 who used iron of 99.95 pct purity. These authors 5 reported the eutectic temperature for the stable system y + graphite as 1153.6 ~ (data corrected according to the 1968 International Practical Temperature Scale, IPTS). Subsequently Chipman 8 selected 1154 ~ as the best value and further proposed the metastable eutectic temperature for the system y + Fe3C as 1148 ~ Compositions for these two eutectics were taken by Chipman 8 to be 4.26 and 4.30 wt pct C, respectively, in agreement with the results of Ruth and Turpinl~ who reported corresponding values of 4.28 and 4.32 (+-0.03) wt pct C. The present work is the first stage of a project involving the determination of the liquidus surface of the Fe-C-Cr system. Differential thermal analysis (DTA) has been employed since it is particularly suitable for detecting phase changes whose thermal arrests are slight, H as in the formation of primary (Fe,Cr)3C and (Cr,Fe)7C3 from the melt. 12 To the authors' knowledge, this is the first application of the technique to the liquidus and solidus relations of the Fe-C system.
BRUNO CHICCO, Experimental Officer, and WARREN R. THORPE, Senior Research Scientist, are both with CSIRO, Division of Manufacturing Technology, Adelaide Laboratory, Leeds Crescent, Woodville North, South Australia, Australia. Manuscript submitted October 21, 1981.
METALLURGICAL TRANSACTIONS A
EXPERIMENTAL PROCEDURE
A. Alloys High purity iron ( > 99.99 wt pct) and spectroscopic carbon rods, in proportions appropriate to yield an alloy containing about 1.5 wt pct C, were melted in a recrystallized alumina crucible located in an RF induction furnace. The furnace, which was provided with a graphite susceptor, was continuously purged with argon. A pin sample was drawn from the melt with a 3.5 mm internal diameter vitreous silica tube at a temperature approximately 50 ~ above the expected liquidus. Alloys of higher carbon content were produced by adding carbon to the melt and sampling in the same manner. Specimens for chemical analysis were cut from each end of the pin samples, while the central portions were reserved for DTA determinations. Carbon content
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