Morphology of Upper and Lower Bainite with 0.7 Mass Pct C
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MEHL introduced the terms upper and lower bainite. He illustrated upper bainite with micrographs of feathery bainite, which originates from grain boundaries. It is composed of very many parallel plates of ferrite with cementite particles in the interspaces. He illustrated lower bainite with acicular units, initiated by intragranular nucleation of ferrite plates. It seems that Mehl, in reality, simply distinguished between bainite nucleated either at grain boundaries or intragranularly. Hultgren[2] presented a sketch of the start of upper bainite where the plates were nucleated on a grain boundary and somewhat elongated cementite particles formed in the interspaces. Aaronson and Wells[3] introduced the term ‘‘sheaf’’ for intragranular groups of closely packed parallel plates and proposed that they form by repeated sympathetic nucleation, starting from an initial plate of ferrite. Hillert[4] observed that the outermost plate of ferrite in feathery bainite was sometimes covered with a more intimate mixture of ferrite and cementite. It could also form in a wider interspace between ferrite plates. He did not identify it as pearlite but proposed that it was a eutectoid mixture [1]
JIAQING YIN, MATS HILLERT, and ANNIKA BORGENSTAM are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, Brinellva¨gen 23, 100 44 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted April 12, 2017. Article published online July 6, 2017 4006—VOLUME 48A, SEPTEMBER 2017
that had formed under cooperation between ferrite and cementite although the ferritic constituent came from the bainitic ferrite. He predicted that this mixture should be more common at lower temperatures. Ko and Cottrell[5] discovered that bainite, similarly to martensite, forms with a surface relief. That inspired Matas and Hehemann[6] to propose that bainitic ferrite grows very fast, without time for diffusion of carbon. Cementite in lower bainite would then form by a subsequent precipitation of carbon from the supersaturated ferrite. At higher temperatures, there would instead be time for carbon to escape to austenite in the interspaces where it would precipitate as coarse particles of cementite. However, they had measured the lengthening rate of bainite[7] and Kaufman, Radcliffe and Cohen[8] reported that their and similar growth rates[9,10] were slow enough to be accounted for by carbon diffusion during growth. Goodenow and Hehemann[11] then explained this apparent discrepancy by proposing that the lengthening of a bainite unit occurs by rapid, diffusionless formation of subunits of limited length and the slower macroscopic growth rate is controlled by slow nucleation of a succession of subunits. Oblak and Hehemann[12] published micrographs of subunits and a sketch illustrating the morphological differences of Widmansta¨tten ferrite, upper bainite, and lower bainite. The proposal by Matas and Hehemann,[6] that cementite precipitates from austenite in upper bainite and from supersaturated ferrite in lower bainite, is
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