Formation mechanism of bainitic ferrite and carbide
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I.
INTRODUCTION
D U R I N G an investigation of the various morphologies of proeutectoid ferrite in plain carbon steels, Dub6 et al. ~j devised a system for morphologies, which is known as the Dub6 morphological classification system. The system was then extended by Aaronson t21 and applied to describe the different morphologies of proeutectoid cementite. 13j Many of the precipitates involved in this classification scheme have a plate morphology.141 In addition to proeutectoid ferrite and cementite, which form at relatively high temperatures in steel, martensite and bainite are also formed in the shape of a plate. After more than 50 years of debate, tSl considerable disagreement remains as to whether a bainite plate forms by shear or diffusion. [6'7] Note that the term "shear "lSj is used here to define the "military "m,~~ process in which the movement of atoms on one side of the interphase boundary is coordinated simultaneously or precisely by the sequential glide-type motion of both substitutional and interstitial atoms across the boundary. This behavior of wellcoordinated motion requires glissile interphase boundaries, t9J The term "diffusion" is used in this article to describe a nonmilitary process in which atoms migrate toward or away from the interphase boundary individually, i.e., a diffusion process is accomplished by the biased diffusional random walk of substitutional and interstitial atoms, t~zj We still lack the methods needed to study directly the movement of individual atoms across the phase boundary during transformations. Thus, we must rely on the analysis of some experimental results, which provide us with some indirect but very important clues to the atomic mechanism of growth. This article will deal with the roles of superledges in the formation mechanism of ferrite plates in steel. Although bainitic carbides are not plates, their formation HONG-SHENG FANG and YAN-KANG ZHENG, Professors, and JIA-JUN WANG, Ph.D., are with the Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, the People's Republic of China. 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
mechanism is also discussed because bainitic ferrite plate precipitation is usually accompanied by that of carbides. The formation mechanism of bainitic carbides is closely associated with that of ferrite plates. II.
EXPERIMENTAL METHODS
Specimens of an Fe-2.17C-12.18Cr-0.26Mn-0.31Si (wt pct) steel were homogenized for 69 hours at 1423 K, austenitized for 10 minutes at 1373 K in evacuated quartz capsules, and isothermally reacted for 22 or 29 hours at 573 K in a deoxidized salt bath after the capsules were broken. The alloy composition and heattreatment process were designed to avoid martensite formation. The Ms temperature in
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