A Model for Converting Dilatometric Strain Measurements to the Fraction of Phase Formed during the Transformation of Aus
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INTRODUCTION
QUENCH dilatometry is used extensively to study the transformation behavior of steels, and the majority of the time-temperature-transformation and continuous cooling transformation (CCT) diagrams that are available today were generated using quench dilatometry. The method is based on the principle that, during heating and cooling of steels, dimensional changes occur as a result of thermal expansion and phase transformation. Sensitive high-speed dilatometers detect, measure, and record these changes in dimensions as functions of time and temperature during a defined thermal cycle. The changes in dimensions are then converted to strains, which in turn are used to determine the start and completion of phase transformations. Historically, the conversion of the measured dilatometric strain to volume fraction of phase formed during a phase transformation was performed assuming a linear relationship between the transformation strain and volume fraction of phase formed. This model is often referred to as the Lever Rule model and implicitly assumes that the transformation is essentially complete when maximum strain is reached, usually when cooling has proceeded to room temperature.[1] However, most of the phase transformations that occur in commercial steels do not reach completion upon cooling to room temperature, and often a residual amount of austenite is retained in the steel. More accurate models for VIRENDRA S. WARKE, formerly Graduate Student, Materials Science and Engineering Program, Department of Mechanical Engineering, Worcester Polytechnic Institute, is with Bodycote North America, Andover, MA 01810. RICHARD D. SISSON, Jr., Professor of Mechanical Engineering and Head of the Materials and Science and Engineering Group, and MAKHLOUF M. MAKHLOUF, Professor of Mechanical Engineering and Director of the Advanced Casting Research Center, are with the Materials Science and Engineering Program, Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609. Contact e-mail: [email protected] Manuscript submitted February 25, 2008. Article published online January 24, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
converting the transformation strain obtained from dilatometric measurements to volume fraction of phase formed have been recently developed.[2–6] Most of these models are based on converting the measured dilatometric strain to a volume change that is assumed to be caused entirely by the difference in crystal structure between the parent phase (e.g., fcc austenite-c) and the product phase (e.g., bct martensite-a¢). These models, although more accurate than the Lever Rule model, were developed for the transformation of c to proeutectoid ferrite or pearlite, and specifically for wrought steels. Therefore, they cannot account for the significant effect that porosity, which may exist in powder metallurgy steels, has on the magnitude of the measured dilatometric strain. In this publication, we describe a model that allows converting dilatometric strains that occur during the CCT
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