Predicting the onset of transformation under noncontinuous cooling conditions: Part II. Application to the austenite pea
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I.
INTRODUCTION
THE temperature at which the transformation begins is a critical parameter in the genesis of the final microstructure. It dictates the amount of driving force available for nucleation and governs the rate of growth and coarsening of the new phase, tl41 Despite the considerable amount of research and development which has been conducted to establish a method for estimating the onset of the austenite decomposition, the prediction methods available today are still semiempiricaltS.6.7] and limited to continuous cooling situations in which the cooling rate does not change abruptly. Continuous cooling transformation (CCT) diagrams have long been used to evaluate the metallurgical response of different steels; they are traditionally generated from tests using natural or linear cooling conditions. This restricts their applicability to processes which involve comparable cooling situations. However, modem steel processing schedules can become quite sophisticated, involving a range of continuous and step cooling situations, t8,9,~~ Continuous cooling transformation diagrams appropriate to these more complex processes would have to be generated using comparable cooling conditions, which would require sophisticated instrumentation and complex trials. The additivity principle provides a mathematical relationship between the transformations that occur under nonisothermal conditions and those that occur at constant temperatures. As such, it offers a potential procedure for using isothermal data to describe a process involving a
T.T. PHAM, Research Scientist, is with BHP Research, Melbourne Laboratories, Mulgrave, Victoria, Australia. E.B. HAWBOLT, Professor, and J.K. BRIMACOMBE, Alcan Chair in Materials Process Engineering and Director, are with the Centre for Metallurgical Process Engineering, The University of British Columbia, Vancouver, BC, Canada V6T IZ4. Manuscript submitted November 21, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
complex thermal path. However, the success of the additivity principle in estimating the transformation start has not been rigorously tested. In Part I of this series, the additivity principle and its application to predict the incubation of a transformation were reviewed. The concept of an ideal TTT curve was introduced, which could be used to describe transformation behavior occurring under nonisothermal conditions. Mathematical relationships between the ideal and nonisothermal conditions were quantified. This second part describes the results of a series of experimental tests used to determine the ideal TTT curve for the austenite-to-pearlite transformation and its use in predicting continuous cooling behavior.
II.
EXPERIMENTAL
PROCEDURES
A. Material and Apparatus In this study, an industrial eutectoid carbon steel having the composition shown in Table I was used for measuring the incubation time associated with the start of the austenite-to-pearlite transformation. The eutectoid grade was chosen because of its inherent high hardenability which enables the incubation
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