The ferrite/austenite equilibrium in the Fe-Cr system (discussion of reference 1)
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(a)
The Ferrite/Austenite Equilibrium in the Fe-Cr System (Discussionof Reference 1)* GERNOT KIRCHNER AND BJORN UHRENIUS
(b) Fig. 4 - - ( a ) M i c r o s t r u c t u r e of the s t e e l aged f o r 24 h at 500~ s h o w i n g m a i n l y c o p p e r p r e c i p i t a t i o n . Magnification 102,000 t i m e s . (b) M i c r o s t r u c t u r e of the s t e e l aged f o r 24 h s h o w i n g c o l u m b i u m c a r b i d e p r e c i p i t a t i o n . M a g n i f i cation 160,000 t i m e s .
INCO, namely, IN-78711 and Hurly and Shelton' s Ni-Cu-Cb steel.' Summarizing, the studies indicate that 1) age hardening of rapidly cooled copper-columbium steels is due, in addition to the formation of c o p p e r - r i c h c l u s ters, to the formation of metastable zones or c l u s t e r s of eolumbium carbide whose nature and composition is 224-VOLUME 6A, JANUARY 1975
Chipman ~ has recently criticized our evaluation z of the thermodynamics of the bcc and fcc phases in the F e - C r system. The present authors feel that Chipman has misunderstood their intentions, misapplied their experimental data and computed results, overlooked the recent p r o g r e s s in computerized phase diagram calculations and has chosen to neglect some basic facts of thermodynamics. Thus, this communication becomes necessary. The most surprising aspect of Chipman's paper I is that he criticizes a phase diagram calculation without presenting a phase diagram of his own. The present authors have therefore calculated the ferrite/austenite equilibrium in the F e - C r system employing the p a r a m eters published by Chipman. The y loop thus obtained is presented in Fig. l(a) together with all experimental information available. It can be compared with Fig. l(b) in which the y loop by Kirchner, Nishizawa and Uhrenius z is reproduced. It is readily seen that Chipman's phase boundaries do not fit the experimental data nearly as well as those suggested by Kirehner et al. Chipman's p a r a m e t e r s " do not even yield a *JOHN CHIPMAN: Met. Trans., 1974, vol. 5, pp. 521-3. GERNOT KIRCHNER is Assistant Professor, Division of Physical Metallurgy, Royal Institute of Technology, Stockholm, Sweden. BJORN UHRENIUS is Research Metallurgist, Coromant Research Center, Sandvik AB, Stockholm, Sweden. Discussion submitted July 17, 1974. METALLURGICAL TRANSACTIONS A
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Fig. 1--Comparison between the experimental determinations and the computer generated a/~/-equtlibria in the F e - C r system according to (a) Chipman's evaluation1 and (b) Kirchner, Nishizawa and Uhrenius2. The line between the a p
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