Microstructural engineering applied to the controlled cooling of steel wire rod: Part II. Microstructural evolution and
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INTRODUCTION
T H E qualitative link between the microstmcture and mechanical properties of steel has long been known. Numerous studies have appeared in the literature concerning the relative effect of changing microstructure on properties, but little work has been directed at systematically developing a quantitative relationship between the two. By definition, microstructural engineering demands a quantitative characterization of the evolution of phases during the processing of any material and valid relationships among the material composition, microstructure, and mechanical properties. It is the aim of Part II of this three-part article to present such relationships for steel rod undergoing a continuous cooling operation on a Stelmor line. II.
PREVIOUS WORK
these experiments, whereas industrial processing is usually nonisothermal. Among the equations utilized for the characterization of isothermal phase transformation kinetics is that originally proposed by Avrami: I1,2,31 X = 1 - exp ( - b t n)
[1]
where X is the fraction transformed, t is the transformation time, and b and n are empirically determined constants. In general, b is a kinetic parameter that represents the combination of nucleation and growth rates, whereas n is related to the geometry of the growing phase and the conditions of nucleation. The following modified form of the Avrami equation has been proposed by Umemoto et al. [4] to include a term for the effect of prior austenite grain size on the kinetics of the phase transformation: X = 1 - exp \ dr /
[21
A. Phase Transformations and Additivity The prediction of microstructural evolution during nonisothermal processing of metals is complicated by the variation in both the driving force for the transformation and the diffusivity of the rate-controlling species. Thus, nucleation and growth rates for the new phase are independent functions of temperature. A majority of the literature published on phase transformations in metals has been concerned with isothermal measurements, owing to the relative simplicity of conducting and interpreting
P.C. CAMPBELL, formerly Graduate Student, The University of British Columbia, is with BHP Central Research Laboratories, Wallsend, New South Wales 2287, Australia. E.B. HAWBOLT, Professor, Department of Metals and Materials Engineering and The Centre for Metallurgical Process Engineering, and J.K. BRIMACOMBE, Stelco/NSERC Professor and Director, The Centre for Metallurgical Process Engineering, are with The University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Manuscript submitted February 14, 1990. METALLURGICAL TRANSACTIONS A
where m is an empirically determined constant and d~ is the prior austenite grain diameter. Equation [2] predicts that as the prior austenite grain size increases, the rate of transformation decreases. Recognizing the limitations in applying the isothermal Eqs. [1] and [2] to nonisothermal events, the additivity principle has been proposed to enable the application of the Avrami equation over a range of temperatures. The pri
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