Abstract
Based on material constitutive models and the classic Koistinen–Marburger (KM) kinetics model, a new dilatometric analysis model was developed to extract the kinetics curve of martensitic transformation under a temperature gradient and stress from the measured dilatometric data and to determine the transformation parameters. The proposed dilatometric analysis model is generally for athermal martensitic transformation, relying only on the average atom volume of martensite and austenite. Furthermore, through theoretical calculations, the proposed model also provided a more accurate method for obtaining the martensite start temperature, which is different from the traditional method. According to the dilatometric analysis results for the martensitic transformation of a type of high-strength low-alloy steel, and the thermodynamic basis of martensitic transformation, a refined kinetics model was developed that successfully predicted the martensitic transformation kinetics curves under different stresses, taking into account the physical significance of the transformation parameter α and the driving force of stress for martensitic transformation.
Highlights
The expansion of metal is essentially a continuous or discontinuous change in atomic volume caused by temperature change or phase transformation
The product phase fraction can be extracted as a function of temperature or time from the dilatometric curve
Assuming that the fraction of the product phase is proportional to the dilatation strain, at a given temperature, the fraction of the product phase can be calculated using Equation (1), according to the relative position of the dilatometric curve between the two baselines extrapolated from the linear segments: f the lever rule model [1,4,6,7]: (1) The transformation is essentially complete when the maximum strain of the dilatometric curve is reached, usually at room temperature
Summary
The expansion of metal is essentially a continuous or discontinuous change in atomic volume caused by temperature change or phase transformation. According to Equation (7), when Tx = Ms, the martensitic transformation starts at the point with the relative position x, and the martensite-start temperature Msx measured by thermocouples in the core/middle zone can be expressed by:. Equation (21) reveals that the martensitic fraction is approximately linearly related to the difference between the measured strain and the strain due to temperature change. Ignoring the difference between the transformation parameters α in the surface/edge zone and the core/middle zone, the martensitic fraction under a temperature gradient can be calculated using the following equations. Where fC is the martensitic fraction of the core/middle zone, Msg is the equivalent transformation start temperature under a temperature gradient and can be expressed by: Materials 2021, 14, x FOR PEER REVIEW. When T < Ms, according to Equations (29) and (31), the kinetics curve of martensitic transformation without stress can be expressed as:.
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