The microstructure of carbon steel is largely determined by the transformation of austenite to ferrite, cementite, and pearlite. This chapter focuses on the microstructures produced by diffusion-controlled transformations that occur at relatively low cooling rates. It describes the conditions that promote such transformations and, in turn, how they affect the structure of various phases and the rate at which they form. The chapter also discusses the concepts of transformation kinetics, minimum free energy, and nucleation and growth, and provides information on alloying, interphase precipitation, and various types of transformations.

This appendix includes two annotated iron-carbon (Fe-C) phase diagrams. One is a poster-size diagram showing iron-carbon phases up to 7 wt% C along with representative microstructures. The other diagram is close-up view showing the phases that occur from 0 to 1.2 wt% C. It also includes labels identifying the microconstituents that form in plain carbon steels under rapid quenching conditions.