Diffusion in growth of bainite
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INTRODUCTION
THE classical method to study transformations in alloys was to measure the change of some physical property during continuous heating or cooling in a furnace. By combination with microscopical studies of the resulting structures, it was possible to identify various transformations. Hultgren in 1920 [~1 extended this technique by interrupting the gradual cooli.ng by quenching from various temperatures and also by transferring the specimen to another furnace of a constant temperature and keeping it there for some time before quenching. He studied a W steel and observed a transformation occurring isothermally below the temperature range of pearlite formation and resulting in "secondary ferrite" of a Widmanst/itten appearance. For instance, he wrote, "At 525 ~ one minute is sufficient for ferrite to appear while at 550 ~ no ferrite appears in fifteen minutes." This structure would today be called carbide-free bainite and is well known from steels containing Si. Hultgren explained the formation of the secondary ferrite by referring to the extrapolated A3 line in the phase diagram (line AB in Figure 1). In particular, he stated that "the com-' position of the separated ferrite follows the extended equilibrium line AC." In another context, Hultgren t2] mentioned "needles of troostite" that sometimes occur in hardened carbon steel and more frequently in alloy steels. This is probably what today is called lower bainite, although Hultgren believed that martensite needles had first formed and had then been tempered. Robertson in 1929 t3] improved the isothermal technique used by Hultgren by immersing the specimen in a molten metal, which is necessary in order to prevent transformation in a plain carbon steel during cooling, although it was not necessary in Hultgren's W steel. However, it seems that Robertson did not intentionally MATS HILLERT, Professor Emeritus, is with the Division of Physical Metallurgy, Royal Institute of Technology, S-10044 Stockholm, Sweden. This article is based on a presentation made at the Pacific Rim Conference on the "Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products," held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL's Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
interrupt the isothermal transformation by quenching after various times, as Hultgren did. Anyway, Robertson made a very detailed study of the transformation products formed between 480 ~ and 380 ~ in a eutectoid carbon steel, i.e., bainite. He proved that it cannot be regarded as tempered martensite but forms by decomposition of austenite "initiated by the 3' ---> a change." He even claimed to have seen minute plates of ferrite in the fine structure down to 390 ~ Robertson explained the fact that bainite can form at a lower temperature than pearlite by postulating that the carbon content of ferrite increases at lower temperatures, thus decreasing the need of carbon diffusion. He even proposed that ferrite could inherit
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