Isothermal transformation of austenite to pearlite and upper bainite in eutectoid steel
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I.
INTRODUCTION
THE recent
consideration 1 of experimental information accumulated by many investigators indicated that the isothermal transformation diagram for a eutectoid steel should comprise separate C-curves for pearlite, upper bainite, lower bainite, and martensite together with the Ms and MI isotherms. In this diagram the curves for pearlite and for upper bainite necessarily overlap extensively as is evident in the diagrams prepared by a few authors (e.g., References 2 to 5). Additionally, the independent isothermal formation of pearlite and upper balnite at high subcritical temperatures has been established by photomicrographs presented in several papers, notably those by Modin, 6 Modin and Modin, 7 and Hillert. 8 Experimental observations of this kind have been ignored in textbooks that show overlapping curves only schematically (e.g., References 9 to 12) or that simply suggest that pearlite and bainite may form independently (e.g., References 13 to 16). Perhaps one reason for reluctance to accept that isothermal transformation of austenite is more complex than the behavior implied by a single C-curve is the lack of a direct metallographic determination of the C-curves for the formation of pearlite and upper bainite in a plain carbon eutectoid steel. The objective of this paper is to provide such a determination.
II.
EXPERIMENTAL
A eutectoid steel containing 0.80 pct C, 0.77 pct Mn was used for the investigation. The structure at the center of a large specimen cooled at 50 ~ hour -1 after three hours at 900 ~ was completely pearlitic, thereby establishing that proeutectoid products would not occur in the structures formed during short isothermal treatment. Specimens 0.5 mm thick cut from bar stock of the steel were austenitized at 900 ~ for 30 minutes in molten salt, then quenched into another molten salt at a transformation temperature between 350 ~ and 600 ~ After elapse of predetermined times the specimens were quenched in water to transform any austenite remaining in the structure to martensite. From the heat transfer characteristics of the system it was estimated that the specimens cooled from 900 ~ to the transformation temperature in less than one second, and N. F. KENNON is with the Department of Metallurgy, The University of Wollongong, P.O. Box 1144, Wollongong, 2500, N.S.W. Australia. N.A. KAYE is with Australian Iron and Steel Pty. Ltd., Port Kembla, N.S.W. Australia. Manuscript submitted March 12, 1981. METALLURGICAL TRANSACTIONS A
that negligible thermal gradients existed in the specimens during cooling. All specimens were examined by optical metallography and by electron microscopy of two stage gold shadowed carbon replicas to identify the dark etching products of diffusional transformation. Point counting measurements were made of the amounts of the products present in the specimens and the results extrapolated or interpolated to determine the time required to form 0.01 volume fraction of each product at each temperature.
III.
RESULTS
Two morphologically different products were f
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