Direct Observation that Bainite can Grow Below M S
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INTRODUCTION
THE question whether bainite can grow below the MS temperature is of fundamental importance. The background is that there are two conflicting hypotheses[1] regarding the growth mechanism of the acicular ferrite that forms during the first stage of the formation of bainite. For clarity, this ferrite can be called bainitic ferrite. According to one hypothesis, bainitic ferrite grows with a sufficient velocity to prevent carbon from escaping by diffusion into the interior of the parent austenite. The carbon will thus be trapped in the virgin ferrite, but most of it will soon escape either by precipitating as fine carbide particles inside the bainitic ferrite or by diffusion into the remaining austenite. This has been described as the diffusionless hypothesis. Zener[2] described how the start temperature of bainite BS can be predicted from this hypothesis by considering the driving force for diffusionless growth of ferrite. He thus considered ferrite formed in this way as bainite even before cementite had formed. Today, his definition of the term bainite is in common use and may be exemplified by the term carbide-free bainite, which is applied to a microstructure formed at fairly low temperatures in steels with high contents of silicon or aluminum, which prevent the formation of cementite. The term bainite in the title is defined in this way. According to the other hypothesis, bainitic ferrite is identical to Widmansta¨tten ferrite, and it grows with a velocity controlled by the rate of carbon diffusion in the parent austenite away from the moving ferrite/austenite
PETER KOLMSKOG, Ph.D. Student, ANNIKA BORGENSTAM, Associate Professor, MATS HILLERT, Professor Emeritus, and PETER HEDSTRO¨M, Researcher, are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden. Contact e-mail: [email protected] SUDARSANAM SURESH BABU, Associate Professor, is with the Department of Materials Science and Engineering, Ohio State University, Columbus, OH 43221. HIDENORI TERASAKI, Associate Professor, and YU-ICHI KOMIZO, Professor, are with the Joining and Welding Research Institute, Osaka University, Ibaraki, Osaka 567-0047, Japan. Manuscript submitted April 13, 2012. Article published online August 23, 2012 4984—VOLUME 43A, DECEMBER 2012
interface. This has been described as the diffusional hypothesis. These two hypotheses have existed as long as bainite has been studied, but their definitions have become sharper over the years. The hypothesized high growth velocity has not been observed experimentally but was easily accepted by the proponents of the diffusionless hypothesis when it was reported that bainite forms with a surface relief similar to that of martensite.[3] However, when an increasing number of reports of slow growth of bainite units appeared,[4–6] it was necessary to modify the diffusionless hypothesis by suggesting that the first nucleus of bainite starts to grow rapidly in a martensitic fashion, but it only results in a short subunit because its growth is stopped by the
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