Kinetics of austenite-ferrite and austenite-pearlite transformations in a 1025 carbon steel |
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Authors: | E B Hawbolt B Chau J K Brimacombe |
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Affiliation: | (1) The Centre for Metallurgical Process Engineering, Department of Metallurgical Engineering, The University of British Columbia, V6T 1W5 Vancouver, BC, Canada |
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Abstract: | Isothermal and continuous-cooling transformation kinetics have been measured dilatometrically for the γ → α+ γ′ and γ′→ P reactions in a 1025 steel. The isothermal transformation of austenite for each reaction was found to fit the Avrami equation
after the fraction transformed was normalized to unity at the completion of the reaction and a transformation-start time was
determined. The transformation kinetics under isothermal conditions therefore were characterized in terms of then andb parameters from the Avrami equation together with the transformation-start times. The parametern was found to be independent of temperature over the range studied (645 to 760 ‡C) and to have values of 0.99 and 1.33 for
the ferrite and pearlite reactions, respectively. This indicates that the nucleation condition is essentially constant and
site saturation occurs early in the transformation process. The continuous-cooling experiments were conducted at cooling rates
of 2 to 150 ‡C per second to determine the transformation-start times for the ferrite and pearlite reactions and the completion
time for transformation to pearlite under CCT conditions. Both reactions were found to obey the Additivity Principle for continuous
cooling provided that the incubation (pre-transformation) period was not included in the transformation time. The isothermal
transformation data and CCT transformation-start times have been incorporated in a mathematical model to predict continuous-cooling
transformation kinetics that agree closely with measurements made at three cooling rates. |
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