attained is called the eutectoid carbon content. The ferrite-cementite phase mixture of this composition formed during cooling has a characteristic appearance and is called pearlite and can be treated as a microstructural entity or microconstituent. It is an aggregate of alternating ferrite and cementite particles dispersed with a ferrite matrix after extended holding close to A1. The Fe-C diagram is of experimental origin. The knowledge of the thermodynamic principles and modern thermodynamic data now permits very accurate calculations of this diagram. This is particularly useful when phase boundaries must be extrapolated and at low temperatures where the experimental equilibria are extremely slow to develop. If alloying elements are added to the carbon-alloy, the position of the A1, A3, and Acm boundaries and the eutectoid composition are changed. Classical diagrams show the variation of A1 and the eutectiod carbon content with increasing amount of a selected number of alloying elements. If suffices here to mention that all important alloying elements decrease the eutectoid carbon content, the austenite-stabilizing elements manganese and nickel decrease A1, and the ferrite-stabilizing elements chromium, silicon, molybdenum, and tungsten increase A1. Modern thermodynamic calculations allow accurate determinations of these shifts that affect the driving force for phase transformation. Transformation DiagramsThe kinetic aspects of phase transformations are as important as the equilibrium diagrams for the heat treatment of steels. The metastable phase martensite and the morphologically metastable microconstituent bainite, which are of extreme importance to the properties of steels, can generally form with comparatively rapid cooling to ambient temperature, when the diffusion of carbon and alloying elements is suppressed or limited to a very short range. Bainite is a eutectoid decomposition that is a mixture of ferrite and...
