This chapter discusses the isothermal transformation of austenite to pearlite, bainite, martensite, proeutectoid ferrite, and proeutectoid cementite. It describes the transformation mechanisms in eutectoid, hypoeutectoid, and hypereutectoid steels, the factors that influence nucleation and growth, and the characteristic features of the various microstructures. It also describes the transformation of austenite during continuous cooling.

A spheroidized structure, which consists of spherically shaped cementite in a matrix of ferrite, is often desired in the production of steel, whether to improve properties, such as machinability and ductility, or accommodate subsequent hardening treatments. This chapter discusses the spheroidization of normalized and annealed steels by heating at subcritical temperatures. It explains how lamellar pearlite and proeutectoid cementite transform when heated and how deformation prior to heating affects both the mechanism and kinetics of spheroidization. It also explains how austenitizing contributes to the production of spheroidal transformation products and why secondary graphitization sometimes occurs.

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 chapter examines the microstructure and properties of annealed and normalized steels containing more than 0.25% carbon. It shows, using detailed micrographs, how incrementally higher levels of carbon affect the structure and distribution of pearlite and how it intermingles with proeutectoid ferrite and cementite. It explains how ferrite and pearlite respond to deformation and how related features such as slip lines, dislocations, shear bands, and kinking can be detected as well as what they reveal. It also describes the structure of patented wires, cast steels, and sintered steels and the morphology of manganese sulfide inclusions in castings.

Isothermal and continuous cooling transformation (CT) diagrams help users map out diffusion-controlled phase transformations of austenite to various mixtures of ferrite and cementite. This chapter discusses the application as well as limitations of these engineering tools in the context of heat treating eutectoid, hypoeutectoid, and proeutectoid steels. It also provides references to large collections of transformation diagrams and includes several diagrams that plot quenching and hardening transformations as a function of bar diameter.

This chapter discusses the characteristics of eutectoid transformations, a type of solid-state transformation associated with invariant reactions, focusing on the iron-carbon system of steel. It describes the compositions, characteristics, and properties of ferrite, eutectoid, hypoeutectoid, and hypereutectoid structures and how they are affected by the addition of various alloying elements. The chapter also discusses the formation of peritectoid structures in the uranium-silicon alloy system.

This appendix contains sample problems with worked solutions pertaining to the use of binary phase diagrams. The problems require the determination of favorable temperatures and compositions, the amount and composition of phases in an alloy at a given temperature, the amount of a certain phase in different steels, and the microstructure developed in different alloys.