Continuous cooling transformation kinetics versus isothermal transformation kinetics of steels: a phenomenological rationalization of experimental observations

The existing literature has been examined and rationalized to test the general validity of a number of generally accepted concepts concerning the overall transformation kinetics of ferrous alloys. Considerable confusion exists because of the mixup of the continuous cooling kinetics with the isothermal transformation kinetics. Therefore, these two topics are discussed separately. For the continuous cooling process, the following topics are examined: (1) the suppressibility of the martensite transformation at high cooling rates; (2) the cooling rate dependence of Ms (martensite transformation-start temperature) and Bs (bainite transformation-start temperature); (3) the formation conditions of lath martensite and twinned martensite; and (4) the various features of continuous cooling transformation (CCT) diagrams. For the isothermal transformation process, the following issues are examined: (1) the isothermal transformation kinetics of martensite; (2) the relationship between athermal transformation of martensite and isothermal transformation of martensite; (3) the general features of time-temperature-transformation (TTT) diagrams; (4) the validity of the “isothermal martensite” concept; and (5) the definition of Ms and Bs for isothermal transformations. Among the main conclusions are: (1) twinned martensite can be formed in all steels, even in pure iron and low-carbon and/or low-alloy steels; (2) isothermal transformation of martensite always follows C-curve kinetics; and (3) Bs and Ms for isothermal transformations are different from those obtained from cooling transformations. Comparison of literature results with the present assessment of isothermal Bs and Ms is made and good agreement is observed. The weakness of using TTT diagrams to analyze the continuous cooling kinetics is also discussed. Moreover, (metastable) product diagrams for austenite decomposition are established for both the continuous cooling process and the isothermal transformation process in order to develop a clearer paradigm for both processes.