The time-temperature-transformation diagram within the medium temperature range in some alloy steels
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I.
INTRODUCTION
THE bainitic reactions have been extensively studied since the 1930s. Nonetheless, many contradictions exist concerning the mechanism of bainite formation. There are still many different understandings concerning the time-temperatures-transformation (TTT) diagrams of bainite and pearlite transformations. For example, one may raise the following questions about the C-curves of bainite and pearlite formation: whether bainite and pearlite have the same C-curve and the C-curve is separated at the bay temperature due to the solute drag-like effect, t',2] whether upper bainite and lower bainite have the s a m e C-curve [3,4,5]or their own C-curves,J6.7] e t c . Bainite in the heat treatment of steels is also an important structure. Therefore, a detailed investigation of the TTT diagrams of bainite reaction is necessary both in theory and in practice. II.
EXPERIMENTAL PROCESSES
Seven different alloy steels of low, medium, and high carbon content are selected for the present investigation. The chemical compositions of the steels to be studied are listed in Table I. After homogenization for 80 hours, some hot magnetic specimens 3 mm in diameter and 30 mm in length were machined from the homogenized bars and further coated with chromium to prevent oxidation. Specimens 12 mm in diameter and 1 to 2 mm in thickness for microscopic analyses were also machined from the homogenized bars. The isothermal transformation kinetics of supercooled austenite within the medium temperature range were established with a hot
M.K. K A N G and G.L. H U , Professors, and D.M. C H E N and S.P. YANG, Associate Professors, are with the Department of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, People's Republic of China. Manuscript submitted November 30, 1990. METALLURGICAL TRANSACTIONS A
magnetic instrument with a field intensity of 3000 Oe and a sensitivity of 0.2 pct. This hot magnetic instrument is linked with an X-Y recorder. The volume fraction of austenite already decomposed is determined with the following equation: P = ~
--
~0
[1]
ot I -- o/0
where a0 is a datum read from the X-Y recorder for the specimen of pure austenite (untransformed austenite), a is datum read from the X-Y recorder for the same specimen with austenite decomposed within a certain interval at the present isothermally holding temperature, and a, is a datum read from the X-Y recorder for the annealed specimen at the same holding temperature. All specimens, both for hot magnetic measurements and for microscope analyses, were austenitized in an inert BaCI2 bath at the needed temperature for 5 minutes, then quenched into another inert N a N O 2 - K N O 3 o r N a N O 3 bath at the needed temperature and held for various periods of time, and finally rapidly quenched into roomtemperature ( - 2 0 ~ water for the preparation of specimens for microscopic analyses, and the other specimens were air cooled for hot magnetic specimens. Light microscopic observations were carried out with the Neophot-I type photomicro
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