A Review of the Data on the Interlamellar Spacing of Pearlite
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I.
INTRODUCTION
PEARLITE is a lamellar eutectoidal decomposition product which may form in steels and various nonferrous alloys during transformation under isothermal, continuous cooling or forced velocity (directional) growth conditions. Figure 1 taken from the work of Villela, Guellich, and Bain I shows the well-defined lamellar structure of pearlite formed during isothermal transformation of a plain carbon eutectoid steel. The interlamellar spacing is reflected by the diffusion kinetics at the transformation front and, although not the concern of this review, is also an important parameter in determining the mechanical properties of pearlitic steels. The present review will consider methods for measuring pearlite interlamellar spacing, the effects of recalescence on spacing, optimization criteria, and the relationship between interlamellar spacing and undercooling for isothermally transformed eutectoids or between spacing and imposed velocity for directional growth conditions. Attention will be concentrated on experimental studies which have been made since the 1960 symposium on the Decomposition of Austenite by Diffusional Processes, 2 although the pearlite reaction, including theoretical and experimental aspects of interlamellar spacing, was the subject of a comprehensive review by Puls and Kirkaldy in 1972.3 To provide a background for the review, some of the more important features of the pearlite reaction in isothermally transformed eutectoid steels will be outlined. A. Pearlite Reaction in Steels
In plain carbon steels pearlite is formed from the parent austenite (3') phase by the cooperative growth of ferrite (a) and cementite (Fe3C). The establishment of cooperative growth has been discussed by Hillert. 4 Nucleation occurs N. RIDLEY is Senior Lecturer, Department of Metallurgy and Materials Science, University of Manchester/UMIST, Grosvenor Street, Manchester M1 7HS, England. This paper is based on a presentation made at the symposium "Establishment of Microstructural Spacing during Dendritic and Cooperative Growth" held at the annual meeting of the AIME in Atlanta, Georgia on March 7, 1983 under the joint sponsorship of the ASM-MSD Phase Transformations Committee and the TMS-AIME Solidification Committee. METALLURGICALTRANSACTIONS A
Fig. 1 - - Lamellar pearlite formed isothermally at 705 ~ in a plain carbon eutectoid steel (Ref. 1).
heterogenously, primarily on austenite grain boundaries, and before impingement growth takes place on a spherical front. However, in alloys of high purity nucleation at high temperatures occurs mainly on free surfaces. The appropriate section of the Fe-Fe3C phase diagram is shown in Figure 2(a), with a schematic representation of the austenitepearlite transformation front in Figure 2(b). The important growth parameters are V, the radial velocity, and A, the interlamellar spacing. It can be seen from Figure 2(a) that carbon has a low solubility in the a-phase ( - 0 . 0 2 wt pct), whereas Fe3C contains a substantial amount of carbon ( - 6 . 6 8 wt pct). Hence, it is clear
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