Austenite has a cubic-close packed crystal structure, also referred to as a face-centred cubic structure with an atom at each corner and in the centre of each face of the unit cell.
Crystal structures of austenite, ferrite and cementite, and the Fe-C equilibrium phase diagram.
Ferrite has a body-centred cubic crystal structure and cementite has an orthorhombic unit cell containing four formula units of Fe3C. The phase diagram illustrates the domains in which particular phases or combinations of phases are stable, and contains information about their equilibrium compositions.
Equilibrium phase fractions can also be estimated from a knowledge of the carbon concentration of the steel and an application of the lever rule. We shall interpret microstructrures in the context of the iron-carbon equilibrium phase diagram, even though steels inevitably contain other solutes, whether by design or as impurities. The diagram is nevertheless useful since the transformation behaviour of austenite does not change dramatically unless the steel has a large concentration of solutes.
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