Thermodynamic analysis of isothermal transformation diagrams

Abstract

A thermodynamic method has been developed to allow the prediction of isothermal transformation diagrams, starting simply from a knowledge of the chemical composition of the steel concerned. The method has been extensively tested and has been shown to be capable of faithfully reproducing the critical ‘bay region’ of time–temperature–transformation (TTT) curves. The TTT diagram has been treated as being composed of two overlapping ‘C’ curves, one representing the diffusional polygonalferrite and pearlite transformations, and the other representing the displacive Widmanstätten ferrite and bainite reactions. It is possible to predict relative shifts in these component curves, as a function of alloying element content, thus making the technique potentially useful in theoretical steel design. While the analysis is formally based on Russell's theory of incubation periods, it is believed that a number of difficulties prevent a fundamental interpretation of the results.