The nature of lower bainite midrib
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I.
INTRODUCTION
M I D R I B S are frequently observed in lenticular plate martensite and have been considered to be one of the substructural features differentiating martensite of this type. m Though a complete understanding of the nature of the martensite midrib has not been achieved, it is generally agreed that the thin plate midrib is the first formed region of the product phase, and the midrib plane is the one on which the nucleation of the shear transformation occurs ( i . e . , the habit plane). I2,3,41Recently, several authors announced observations of a bainite midrib in ferrous alloys t5-81 and gave different suggestions to its origin. Liu e t al. ESI considered the midrib as the nucleus of bainitic ferrite formed along the habit plane by a shear mechanism; Okamoto and Oka t7,81 suggested that the midrib is actually a thin plate of isothermal martensite formed in advance of the adjacent lower bainitic ferrite. Yu e t a/. 191 interpreted the midrib as the interface between two groups of bainitic ferrite subunits. Ohmori t~~ and Spanos e t al. ltll observed a single, nearly carbide-free ferrite thin plate formed in association with the lower bainite sheaf containing high density carbides. This thin ferrite plate, termed as "spine" by Spanos e t al. ,[11] is in many aspects similar to the midrib except that instead of locating at the mid part of the bainite sheaf, it lies mainly along one side of the sheaf. Again, this spine was thought to be the initial element of a lower bainite sheaf, but now it was supposed to be precipitated by purely diffusional ledgewise migration of partially coherent ferrite/austenite boundaries. Controversies over the nature of the bainite midrib seem unavoidable, since this is a newly encountered phenomenon and the investigations of it are at present far from complete. The present authors have been investigating the growth mechanism of lower bainitic ferrite and the precipitation sources of lower bainitic carbides in silicon contained medium carbon steels 16,12A31and found in these steels that the midrib is a generally observed feature in the lower bainite structure. 16,121In the present article, more JIALIN SUN, Lecturer, is with the Department of Materials Engineering, Kunming Institute of Technology, Kunming, Yunnan, People's Republic of China. HONGXIU LU, Engineer, is with the Department of Metal Physics, Central Iron and Steel Research Institute, Ministry of Metallurgical Industry, Beijing, People's Republic of China. MOKUANG KANG, Professor, is with the Department of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, Shaanxi, People's Republic of China. Manuscript submitted May 18, 1991. METALLURGICALTRANSACTIONSA
detailed examinations were made in an attempt to shed further light on the nature of the lower bainite midrib.
II.
EXPERIMENTAL PROCEDURE
The chemical compositions and experimentally determined Ms temperatures of the steels used are listed in Table I. Massive ingots (5 kg) of the materials were homogenized at 1473 K for 60 hours b
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