THE TIME-TEMPERATURE-TRANSFORMATION DIAGRAM WITHIN THE MEDIUM TEMPERATURE-RANGE IN SOME ALLOY-STEELS

Time-temperature-transformation (TTT) diagrams within the medium temperature range of some alloy steels have usually been found to be composed of three separate kinds of C-curves for low-carbon alloy steels. The microstructure associated with one of three kinds of C-curves is a granular structure which is nonbainite, upper bainite, or lower bainite, depending on the higher, middle, or lower location of the C-curve in the TTT diagrams. On the other hand, only two types of C-curves can be established for the alloy steels of middle and high carbon content, and the category of microstructures (upper bainite and lower bainite) corresponding to each C-curve also depends on the location of the C-curve. Meta-upper bainite, or alternatively, carbide-free bainite, and meta-lower bainite and/or the mixture of them can often be obtained in some alloy steels containing silicon. Each type of microstructure still possesses its own overall transformation activation energy, Q*, and morphological exponent, n. A considerably detailed analysis has suggested some differences among the transformation mechanisms of granular structure, upper bainite, and lower bainite.