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 concernin g the overall transformation kinetics of ferrous alloys. Considerable confusion exists because of the mixup of the continuous cooling kineti cs with the isothermal transformation kinetics. Therefore, these two t opics are discussed separately. For the continuous cooling process, th e following topics are examined: (1) the suppressibility of the marten site transformation at high cooling rates; (2) the cooling rate depend ence of M(s) (martensite transformation-start temperature) and B-s (ba inite transformation-start temperature); (3) the formation conditions of lath martensite and twinned martensite; and (4) the various feature s of continuous cooling transformation (CCT) diagrams. For the isother mal transformation process, the following issues are examined: (1) the isothermal transformation kinetics of martensite; (2) the relationshi p between athermal transformation of martensite and isothermal transfo rmation of martensite; (3) the general features of time-temperature-tr ansformation (TTT) diagrams; (4) the validity of the ''isothermal mart ensite'' concept; and (5) the definition of M(s) and B-s for isotherma l transformations. Among the main conclusions are: (1) twinned martens ite can be formed in all steels, even in pure iron and low-carbon and/ or low-alloy steels; (2) isothermal transformation of martensite alway s follows C-curve kinetics; and (3) B-s and M(s) for isothermal transf ormations are different from those obtained from cooling transformatio ns. Comparison of literature results with the present assessment of is othermal B-s and M(s) is made and good agreement is observed. The weak ness of using TTT diagrams to analyze the continuous cooling kinetics is also discussed. Moreover, (metastable) product diagrams for austeni te decomposition are established for both the continuous cooling proce ss and the isothermal transformation process in order to develop a cle arer paradigm for both processes.