Analyses of Transformation Kinetics of Carbide-Free Bainite Above and Below the Athermal Martensite-Start Temperature
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THERE is a continuing interest in increasing the strength of steels without compromising formability or weldability. Bainitic steels are increasingly seen as promising candidates. The bainite transformation occurs between the temperatures of pearlite and athermal martensite formation in plain carbon and low alloy steels. Bainitic ferrite is frequently classified as upper or lower, depending on whether carbide formation has occurred at the ferrite-austenite interfaces or within the bainitic ferrite laths. Increasing the amount of silicon suppresses the development of carbides in these steels such that their final microstructures are mixtures of ferrite and retained austenite, often termed ‘‘carbide-free bainite.’’* In this regard, the work of Bhadeshia and *This term is in current use within the metallurgical community; it refers to a structure that is similar to true bainite, but which contains no carbides.
co-workers[1,2] has elucidated the range of desirable properties that can be obtained from dispersions of ferrite and retained austenite. Electrical resistivity measurement is a well-established method of continuously tracking phase transformations in steels;[3] it provides information very similar to that obtained through other volume-averaging methods, such as dilatometry and magnetic saturation techniques. I.A. YAKUBTSOV, formerly Research Associate, with the Department of Materials Science and Engineering, McMaster University, is now R&D Scientist, with the Integrity Testing Laboratory Inc., Toronto, ON, Canada. Contact e-mail: [email protected] G.R. PURDY, University Professor, is with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7. Manuscript submitted January 13, 2011. Article published online September 30, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
A significant acceleration of the onset of austenite decomposition occurs at temperatures just above the athermal martensite-start temperature MS (for hypereutectoid compositions) and immediately below MS in hypoeutectoid steels.[4–10] This anomalous kinetic behavior was termed a ‘‘swing-back’’ phenomenon. The fast onset of the bainite reaction in hypoeutectoid steels below MS was considered due to the effect of martensite plates on the nucleation of bainite in adjacent regions of untransformed austenite.[4] Swing-back phenomena were elucidated by means of the study of the formation of lower bainite and isothermal martensite in hypereutectoid steels.[11] The acceleration of reaction kinetics of lower bainite in medium carbon steel near the MS temperature was attributed to stress created by the growth of lower bainite stimulating the formation of martensite and vice versa.[12] The idea has been advanced that austenite decomposition in plain carbon low alloy steels can be described as a continuum of bainitic ferrite and lath martensite formation in the appropriate temperature range.[13] With decreasing temperature, thermodynamic conditions may develop that are conducive to the stimulation of mar
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