The main solid phase transformation of steel during heating is the transformation from non austenitic phase to austenitic phase, that is, the process of austenitizing. The whole process is related to carbon diffusion. Among alloy elements, non carbide forming elements reduce the activation energy of carbon in austenite and increase the rate of austenite formation; However, the strong carbide forming elements strongly hinder the diffusion of carbon in steel and significantly slow down the austenitizing process.
The phase transformation during steel cooling refers to the decomposition of undercooled austenite, including pearlite transformation (eutectoid decomposition), bainite transformation and martensite transformation. To take only the effect of alloy elements on the isothermal transformation curve of undercooled austenite as an example, most alloy elements, except cobalt and aluminum, play a role in slowing down the isothermal decomposition of austenite, but each element plays a different role. The influence of non carbide forming elements (such as silicon, phosphorus, nickel, copper) and a small amount of carbide forming elements (such as vanadium, titanium, molybdenum, tungsten) on the transformation of austenite to pearlite and bainite is not different, so the transformation curve moves to the right.
If the content of carbide forming elements (such as vanadium, titanium, chromium, molybdenum, tungsten) is large, the transformation of austenite to pearlite will be significantly delayed, but the transformation of austenite to bainite will not be significantly delayed. Therefore, the isothermal transformation curves of these two transformations will be separated from the "nose" and form two C-shapes.
