The Fe-rich corner of the metastable C-Cr-Fe liquidus surface
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I.
INTRODUCTION
40
THE C-Cr-Fe
system is a most important ternary system in metallurgy and the phase relationships in the Fe-rich corner are of great commercial significance. A considerable amount of work has been reported on the liquidus reactions 1-~° so that a diversity of derived liquidus projections has resulted. With regard to the extent of the M23C~ liquidus field, for example, the area of primary crystallization of M23C 6 starts above 58 wt pct Cr according to Forgeng and Forgeng.~° while in a recent review by Rivlin ~ a much lower Cr value is proposed, In view of this lack of agreement between previously published diagrams, it was decided that an extensive experimental program should be carried out, using differential thermal analysis (DTA) during heating and cooling, optical and scanning electron microscopy, and X-ray diffraction, on alloys in the region of commercial interest (up to 40 wt pct Cr and up to 6 wt pct C). The results of these experiments were then combined with previously determined data on the binary Cr-Fe ~2 and C-Fe ~3"~4 edges, to produce a selfconsistent liquidus projection.
30 ~_ z "' O cc a_ w "r
20
w
d o 10
II. THE DETERMINED C-Cr-Fe LIQUIDUS SURFACE Figure 1 shows the derived liquidus surface, based on the results listed in the Appendix, with liquidus isotherms at 50 °C intervals. The invariant reactions shown in the figure are given in Table I. In this table, and in the text following, the carbides will be referred to by the usual abbreviations: (Fe,Cr)3C-M3C, (Cr,Fe)TC3-MTC3, and (Cr,Fe)23C6-M23C6. The invariant reactions p~ and el at the C-Fe binary edge are well known and have been discussed at length most recently by Chipman. ~5'~6 The two invariant reactions U~ and U2 are described by Jackson 9 and, as can be seen from Table I, are of the form L + c~ ~-/3 + y, usually referred to as quasi-peritectic equilibria. ~7 Table II, after Scheil, 18 WARREN R. THORPE, Senior Research Scientist, and BRUNO CHICCO, Experimental Scientist, are with CSIRO, Division of Manufacturing Technology, Adelaide Laboratory, Leeds Crescent, Woodville North, South Australia 5012, Australia. Manuscript submitted December 3, 1984.
METALLURGICAL TRANSACTIONS A
Fe
Pl 1
2
3
4
e1
5
6
C, WEIGHT PERCENT Fig. 1
Fe-rich comer of metastabte C-Cr-Fe liquidus surface,
gives the reaction sequence in the composition range of interest, including the monovariant eutectlc and peritectic liquidus field boundaries. The freezing sequence of typical alloys is illustrated in Figure 2, where the code numbers are those used in the Appendix. Solidification commences with the formation of the primary phase at the temperature indicated by the isotherrns, and the arrows approximate the subsequent path of the liquid composition until solidification is complete. The solidification of these alloys will be discussed in detail later in conjunction with the reaction sequences described in Tables I and II and the data given in the Appendix.
VOLUME 16A, SEPTEMBER 1985-- 1541
Table I. Invariant Liquidus Reactions in
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