Transformation of lower bainite in hypereutectoid steels
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INTRODUCTION
THE transformation mechanism of lower bainite in steels has been argued for a long time, and numerous works on it have been reported s o far.[1-7l However, it is not yet clear whether the transformation mechanism is based on a diffusionless martensitic model or on the diffusional migration of interfacial growth ledges. Bainite exhibits features common to both diffusion-controlled and diffusionless transformations. This has attracted considerable attention to this mode of austenite decomposition, although the complexity of the bainite reactions has made it difficult to elucidate the transformation mechanism and specific processes controlling the reaction rate. The present authors have been investigating the isothermal decomposition of austenite in hypereutectoid steels (0.85 to 1.87 wt pct* C) at the temperature range be*"Pct" means weight percent hereafter.
tween 623 and 333 K by means of metallographic and kinematic techniques. They observed a thin-plate isothermal martensite (TIM) tSI and two types of lower bainite, i.e., "conventional lower bainite" (CLB)and "lower bainite with midrib" (LBm). t9] Both LBm and TIM formed below about 440 K. The purposes of this paper are to examine the metallography and crystallography of CLB in hypereutectoid steels by means of microscopy and to discuss whether or not the mechanism of lower bainite formation can be described martensitically in terms of the phenomenological theory of martensitic transformation.[~~ M. OKA, Professor, and H. OKAMOTO, Research Associate, are with the Department of Mechanical Engineering, Tottori University, Koyama, Touori 680, Japan. K. ISHIDA, formerly Graduate Student at Tottori University, is with Ryobi Company Ltd., Fuchu, Hiroshima 726, Japan. This paper is based on a presentation made in the symposium "International Conference on Bainite" presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURES
Hypereutectoid carbon steels with carbon contents of 0.85, 1.10, 1.45, 1.80, and 1.87 pct were prepared by vacuum-induction melting. Square rods (15 x 15 • 50 mm) were cut from a hot forged bar of each steel. The chemical compositions and Ms temperatures of the steels used are given in Table I. They were homogenized at 1453 K for 20 hours in evacuated quartz capsules. Specimens for optical and electron microscopy were cut to sizes of 7 x 5 • 1 and 15 • 12 • 0.3 mm, respectively, from the homogenized rods. After austenitizing for 30 minutes at 1423 K under a dynamically evacuated condition, these specimens were quenched into a salt bath controlled to the required holding temperatures above 453 K and were austempered there for various holding times. They were subsequently quenched in water at room temperature to stop further transformation. In order to avoid producing the other types of isothermal products such as LBm and TIM, temp
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