Carbon partitioning into austenite after martensite transformation
Introduction
Carbon partitioning between ferrite and austenite during high temperature diffusional transformations is relatively well understood. These reactions are frequently referred to as reconstructive transformations, because of the short-range diffusional movements of iron (and substitutional) atoms that accomplish a change in crystal structure between bcc and fcc [1]. In contrast, the details of carbon partitioning during or after displacive or martensitic transformations are somewhat more controversial, particularly with respect to the growth of bainite. In martensite, the displacive transformation (i.e. a process that involves coordinated rather than diffusional movements of the iron atoms) is usually believed to occur without diffusion of carbon or other interstitials [2], and thus the body-centered martensite phase can be substantially supersaturated with carbon. Subsequent carbon partitioning between martensite and any retained austenite is usually not considered, because the temperature is too low for substantial amounts of diffusion to occur after quenching, and also because carbon supersaturation is usually eliminated by competing processes, e.g. carbide precipitation during tempering. There is, however, evidence that carbon partitioning from martensite to (retained) austenite does occur, to thin interlath films during cooling [3] or by isothermal holding in a Si-containing steel after transformation [4]. Carbide-free bainite microstructures may also form by diffusionless martensitic growth, followed by, or along with, carbon partitioning into austenite [1]. (It should be noted that the bainite transformation is not universally accepted to involve a shear mechanism, and also that it has been suggested that the term “bainite” should not be applied to carbide-free microstructures [5]). Carbon partitioning is one means of stabilizing austenite against further transformation at lower temperatures, and is likely to be especially important in these steels containing alloying additions (e.g. silicon) that suppress formation of iron carbides.
Carbon migration after martensite transformation is most often considered in the context of carbide precipitation reactions during tempering. Consequently, the thermodynamics of carbon partitioning between martensite and retained austenite have not been examined completely. Thus, the present work is focused on developing a model to address carbon partitioning from as-quenched martensite into austenite, under conditions where competing reactions are suppressed. (Such reactions include cementite or transition carbide formation, and decomposition of retained austenite by other processes such as bainite transformation.) The model does not address partitioning kinetics, but rather predicts the “endpoint” where partitioning is complete at a given partitioning temperature. The starting microstructure (i.e. initial fractions of martensite and retained austenite) is an input to the carbon partitioning model developed below, and may be controlled, for example, by the martensite transformation behavior (particularly the Ms temperature of the austenite in relation to the quenching temperature during heat treatment).
Section snippets
Thermodynamics of carbon partitioning
Under equilibrium conditions, mixtures of quenched martensite plus retained austenite in binary Fe-C alloys are expected to decompose to ferrite and iron carbide [6]. The phase compositions at equilibrium are given by the phase boundaries on the usual Fe-C phase diagram, and the phase fractions may be determined from the lever rule. In the presence of substitutional (“X”) alloying additions, such as in a ternary Fe-C-X system, long-range diffusional processes at low temperature are limited
The constrained paraequilibrium condition in Fe-C alloys
The constrained paraequilibrium condition can be calculated for Fe-C alloys using published thermodynamic data, and knowledge of the as-quenched microstructure. Carbon activities in ferrite and austenite have been determined by Lobo and Geiger [10], [11], with reference to graphite as the standard state. For the purposes here, it is convenient to use the data in the simplified form [12]:where ΓCγ and ΓCγ represent the Henrian activity coefficients for
Example calculations of constrained paraequilibrium
Determination of the CPE condition using the formulation presented in , , , requires only the steel composition (i.e. carbon content), and the initial microstructure (mole fractions of martensite and retained austenite) to be specified. For example, we can consider an iron-carbon alloy containing 0.5 wt. pct. carbon, quenched to an initial condition where 75% (molar basis) martensite is present, along with 25% retained austenite. At a partitioning temperature of 400 °C, the calculations yield
Application of CPE partitioning
It is important to consider applications of CPE carbon partitioning and implications of the model results, although some further alloy and process design considerations are also reported elsewhere [15]. It is envisioned that CPE partitioning will be most applicable for processing conditions where austenite is formed at elevated temperature (either during full austenitization or intercritical heat treatment), followed by cooling to a temperature carefully selected to control the fractions of
Conclusions
Consideration of carbon partitioning from quenched martensite to retained austenite in the presence of a stationary α/γ interface, under conditions where carbide precipitation is precluded, indicates that the metastable orthoequilibrium condition (or paraequilibrium in multicomponent alloys) between ferrite and austenite cannot be achieved. Consequently, a “constrained paraequilibrium” or CPE model was developed here to predict the endpoint of carbon partitioning in the presence of a stationary
Acknowledgements
Two authors (Speer and Matlock) gratefully acknowledge the support of the sponsors of the Advanced Steel Processing and Products Research Center at the Colorado School of Mines. Prof. H.K.D.H. Bhadeshia, University of Cambridge, is gratefully acknowledged for helpful discussions.
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