Transformation of austenite at constant subcritical temperatures
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Transformation of Austenite at Constant Subcritical Temperatures B r E. S. DAVENPORT* AND E. C. BxIN, t KEXRNY, N. J. (Chicago Meeting, September, 1930)
WHEN annealed carbon, or low-alloy, steels are suitably heated the ferrite (alpha iron solid solution) and the carbide, of which they are composed, react together to form a single solid solution of carbon (and other elements) in gamma iron (austenite). This reaction may begin only at a certain minimum temperature called the euteetoid temperature. The first ferrite and earbide thus to react together do so in definite proportions known as the eutectoid composition. With further heating the original constituent that is present in excess is dissolved in the austenite until exhausted. Whether the excess constituent be ferrite (hypoeutectoid steels) or carbide (hypereutectoid steels) the procedure is much the same, and upon slow cooling the homogeneous austenite expels the excess of either (proeutectoid ferrite or carbide) until the eutectoid composition is reached. Thus the reaction upon heating is strictly reversed upon cooling and accordingly an equilibrium diagram of this process may be constructed. Such an equilibrium diagram concerns itself only with the temperatures involved and with the proportions and compositions of the phases developed under conditions that suffice to permit equilibrium. Specifically it has no concern for the type or mode of distribution of these constituents nor with any extra-equilibrium conditions such as undercooling, which is almost universally encountered in metals. Actually, however, of great importance is the sluggishness toward equilibration caused by the not inconsiderable time involved in diffusion. Thus instead of the theoretical transformation point AE, we find for a certain rate of heating or cooling the use of the designations Ar and Ac (transformation temperature found on cooling and upon heating). These figures obviously have no particular meaning except as measured and expressed for a definite heating or cooling rate, and represent the apparent temperature displacement caused by an inadequate time allowance. In this paper are presented the results of a study of the time required for the transformation of austenite to ferrite and carbide at a variety of temperatures and also of the time required for the reaction austenite---~ martensite at the temperatures at which this reaction occurs instead of No~n]r
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* United S t a t e s Steel C o r p n , Research L a b o r a t o r y . t H u n t Medallist, 1929. 117"
METALLURGICAL TRANSACTIONS
VOLUME 1, DECEMBER 1 9 7 0 - 3 5 0 3
Metallurgical Classics
118 TRANSFORMATION OF AUSTENITE AT SUBCRITICAL TEMPERATURES
the one first mentioned. In short, this study proposes the addition of the time factor to the iron-carbon diagram. TIME REQUIRED FOR TRANSFORMATION OF AUSTENITE Some ideas as to the time required for the transformation of austenite have been offered by earlier investigators. Portevin and Garvin, 1 Chapin 2 and Lewis* have observed the unexpectedly low rate of the react
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