THERMAL STRESS AND VOLUME CHANGE DURING A COOLING PROCESS INVOLVING PHASE TRANSFORMATION
Titel:
THERMAL STRESS AND VOLUME CHANGE DURING A COOLING PROCESS INVOLVING PHASE TRANSFORMATION
Auteur:
Pan, H. H. Weng, G. J.
Verschenen in:
Journal of thermal stresses
Paginering:
Jaargang 15 (1992) nr. 1 pagina's 1-23
Jaar:
1992-01-01
Inhoud:
With the aid of a phase diagram, a theoretical principle is developed to calculate the stress in the matrix phase and the overall volume change of dual-phase metals during a cooling process involving martensitic transformation. The cooling process may start from a pure austenite phase or from a mixture of austenite and ferrite, resulting in a two- or a three-phase system during the phase transformation. The transformed martensites in the former case are taken to be randomly oriented spheroids. In the latter case, ferrite is taken as the matrix, whereas austenite will transform into martensite, both existing as randomly oriented spheroidal inclusions. The theory is developed using Eshelby's inhomogeneity and transformation principle, by which the influence of elastic heterogeneity, thermal expansion coefficients, phase-transformation strain, and inclusion shape can all be accounted for. When applied to the iron—carbon system, it is found that, for a typical cooling path, the thermal stress developed in the matrix is hydrostatically tensile in both two- and three-phase systems, with a magnitude that is strongly sensitive to the inclusion shape and carbon content. After the martensite transformation starts, the volume of the system is found to increase first, indicating the stronger contribution of the phase-transformation strain over the thermal contraction. Subsequently, the transformation rate decreases and thermal contraction becomes a more dominant factor to the volume change. Finally, it is demonstrated that the theory can be used to predict the transition of martensite morphology from the lath to the twinned structure as the carbon content increases.
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