Heat treatment is the most common way of altering the mechanical, physical, and even chemical properties of steels. This chapter describes the changes that occur in carbon and low-alloy steels during conventional heat treatments. It explains how austenite decomposition largely defines the final microstructure, and how the associated phase transformations are driven by nucleation and growth processes. It describes diffusionless and diffusive growth mechanisms and provides detailed information on the properties, structure, and behaviors of the transformation products produced, namely martensite and bainite. It also discusses the formation of austenite, the control and measurement of austenitic grain size, the characteristics of ferritic microstructures, and the methods used to classify ferrite morphology.

The decomposition of austenite, during controlled cooling or quenching, produces a wide variety of microstructures in response to such factors as steel composition, temperature of transformation, and cooling rate. This chapter provides a detailed discussion on the isothermal transformation and continuous cooling transformation diagrams that characterize the conditions that produce the various microstructures. It discusses the mechanism and process variables of quenching of steel, explaining the factors involved in the mechanism of quenching. In addition, the chapter provides information on the causes and characteristics of residual stresses, distortion, and quench cracking of steel.

This chapter explains how the presence of intermediate phases affects the melting behavior of binary alloys and the transformations that occur under different rates of cooling. It begins by examining the phase diagrams of magnesium-lead and copper-zinc, noting some of the complexities associated with intermediate phases. It then discusses the difference between ordered and disordered phases and how they are accounted for on phase diagrams. It describes how the atoms in a disordered solution may arrange themselves into an ordered array, forming a superlattice in the process of cooling, and goes on to identify the most common superlattice structures and their corresponding alloy phases. It also discusses the factors that limit the formation of superlattices along with the kinetics of spinodal decomposition and its effect on microstructure development.

This chapter describes the two types of Time-Temperature-Transformation (TTT) diagrams used and outlines the methods of determining them. As a precursor to the examination of the decomposition of austenite, it first reviews the phases and microconstituents found in steels. This includes a presentation of the iron-carbon phase diagram and the equilibrium phases. The chapter also covers the common microconstituents that form in steels, including the nomenclature used to describe them. The chapter provides a comparison of isothermal and continuous cooling TTT diagrams. These diagrams are affected by the carbon and alloy content and by the prior austenite grain size, and the way in which these factors affect them is examined.

The properties of steel are affected markedly as the percentage of carbon varies. This chapter describes the properties of alloys of iron and carbon, including a review of the iron-carbon phase diagram and, in particular, the portion of the diagram relevant to carbon steels. It addresses the processes involved in the transformation (decomposition) of austenite to achieve various microstructures.

This chapter describes the effects that can be observed by light microscopy when a steel in the hardened condition, consisting of martensite and possibly some retained austenite, is heated at subcritical temperatures. It includes micrographs that illustrate the effect of carbide precipitation, the decomposition of retained austenite, and recovery and recrystallization. It also includes images that reveal the characteristic structures produced by tempering medium-carbon hypoeutectoid and hypereutectoid steels as well as the effects of plastic deformation, austenitic grain size, and temper brittleness.

This chapter provides a short introduction to phase transformations, namely, the liquid-to-solid phase transformations that occur during solidification and the solid-to-solid transformations that are important in processing, such as heat treatment. It also introduces the concept of free energy that governs whether or not a phase transformation is possible, and then the kinetic considerations that determine the rate at which transformations take place. The chapter also describes important solid-state transformations such as spinodal decomposition and martensitic transformation.