The crystallography of bainite in a medium-carbon steel containing Si, Mn, and Mo
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I.
INTRODUCTION
THE nature of the bainite transformation has engendered more controversy than all other phase transformations in metallic systems. The reader interested in the history of this subject and the origin of the continuing debate is referred to two recent reviews by Bhadeshia and Christian [1] and Aaronson et al. E2"31Our intention in this article is to present what we hope is an even-handed account of a transmission electron microscopy (TEM) study of bainite formed by normalizing a Si-Mn-Mo medium carbon steel. One of the advantages of studying this steel is that its high Si content prevents carbide formation on air-cooling so that the retained austenite-bainite interface structure and orientation relationship are readily studied. We shall demonstrate that many of the crystallographic aspects of the first stages of the transformation can be explained by the phenomenological theory of martensite. In this sense, our approach follows the pioneering study of Srinivasan and W a y m a n [41 and later studies of Bhadeshia and Edmonds, t51 Ohmori, t6j Hoekstra, r7j Sandvik, tS~ and Servant and Cizeron. E91 On the other hand, the transformation also displays other morphological characteristics, which are evidence of a diffusion-controlled transformation, and it therefore cannot meet all the requirements of a martensitic transformation, t2,31 (Indeed there appears to be no experimental study of bainite in steels which satisfies all of the predictions of the phenomenological theory.) The Si-Mn-Mo medium-carbon steel possesses excellent fatigue properties in the normalized state and is often employed in the manufacture of pneumatic drills. In a previous study, t~~ the steel was found to contain a complex microstructure of upper and lower bainite, martensite, and approximately 20 vol pct of retained austenite in the normalized condition. The upper bainite laths were C.P. LUO and ZHENG-YI LIU, Professors, are with the Department of Mechanical Engineering (I1), South China University of Technology, Guangzhou, People's Republic of China 510641. G.C. WEATHERLY, Professor, is with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada L8S 4M1. Manuscript submitted March 15, 1991. METALLURGICAL TRANSACTIONS A
always separated by thin films of retained austenite and showed clear evidence of a midrib. The advantages a midrib offers in determining the habit plane have been clearly demonstrated for both martensite tn,121 and balnite tTl transformations, although the origin of the midrib contrast in TEM images itself is obscure. For the purposes of this study, the midrib afforded two reasonably accurate, complementary methods for determining the habit plane, as used in a prior investigation of bodycentered cubic (bcc) Widmanst~itten plate precipitation in a Ni-Cr alloy. 031 II.
EXPERIMENTAL PROCEDURE
A commercial medium-carbon alloy steel containing (in weight percent) 0.55C, 1.32Si, 0.78Mn, and 0.45Mo was supplied as slabs measuring approximately 100 • 50 x 20 nun. Bars with a cross
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