A Thermodynamic Analysis of Isothermal Transformation Diagrams

H. K. D. H. Bhadeshia

Abstract

A thermodynamic model has been developed to allow the prediction of isothermal transformation diagrams for steels, based on their chemical composition.

This research investigates the thermodynamic principles governing how steel transforms at constant temperatures, specifically focusing on the incubation periods required for new phases to begin forming. The author proposes that Time-Temperature-Transformation (TTT) diagrams should be viewed as a combination of two distinct curves representing diffusional and shear reactions. By utilizing a phenomenological approach and adjusting for temperature-dependent variables like activation enthalpy, the study attempts to predict these transformations across various alloy compositions. While the model shows high correlation with experimental data from several low-alloy steels, the author notes that certain complexities in nucleation theory still limit the absolute accuracy of real-time predictions. Ultimately, the work provides a mathematical framework intended to assist in the theoretical design of steels by better understanding the chemical free energy changes during phase transitions.

Metal Science, Vol. 16, 1982, pp. 159-165

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Computer Programs

MAP_STEEL_MTTTDATA: A powerful suite of software for modelling of the thermodynamics and kinetics of solid-state transformations in steels. Calculates Widmanstatten, Bainite and Martensite start temperatures.This program is to be used with MTDATA.; Language: FORTRAN, C, executable
MAP_STEEL_MUCG46: A powerful suite of software for modelling the thermodynamics and kinetics of solid-state transformations in steels containing Fe, C, Si, Mn, Ni, Mo, Cr and V.; Language: FORTRAN
MAP_STEEL_MUCG46_90: A Fortran 90 version of MAP_STEEL_MUCG46.; Language: FORTRAN 90
MAP_STEEL_MUCG73: An enhanced version of MAP_STEEL_MUCG46 but with the four additional emements: Co, Cu, Al and W.; Language: FORTRAN
MAP_STEEL_MUCG83: An enhanced version of MAP_STEEL_MUCG46 but with the four additional emements: Co, Cu, Al and W. Corrects an inconsistency in MUCG73.; Language: FORTRAN