The existence of austenite and ferrite, along with carbon alloying, is fundamental in the heat treatment of steel. In view of the importance of structure and its formation to heat treatment, this chapter describes the various microstructures that form in steels, the various factors that determine the formation of microstructures during heat treatment processing of steel, and some of the characteristic properties of each of the microstructures. The discussion also covers the constitution of iron during heat treatment and the phases of heat-treated steel with elaborated information on iron phase transformation, hysteresis in heating and cooling, ferrite and austenite as two crystal structures of solid iron, and the diffusion coefficient of carbon.

This chapter describes some of the most effective tools for investigating boiler tube failures, including scanning electron microscopy, optical emission spectroscopy, atomic absorption spectroscopy, x-ray fluorescence spectroscopy, x-ray diffraction, and x-ray photoelectron spectroscopy. It explains how the tools work and what they reveal. It also covers the topic of image analysis and its application in the measurement of grain size, phase/volume fraction, delta ferrite and retained austenite, inclusion rating, depth of carburization/decarburization, scale thickness, pearlite banding, microhardness, and hardness profiles. The chapter concludes with a brief discussion on the effect of scaling and deposition and how to measure it.

This chapter describes the phases and constituents present in iron-carbon steels in near-equilibrium conditions. It explains how to use phase diagrams to predict and manage the development of ferrite, austenite, cementite, and pearlite through controlled cooling. It discusses the transformations, grain structure, and properties associated with each phase and identifies the primary stabilizing elements. It includes several micrographs revealing various microstructural features and describes the processing route by which they were achieved. It explains how to estimate the volume fraction of iron-carbon phases in equilibrium and how to determine the amount of each phase that must be present to reach a desired composition. The chapter also discusses the phases associated with hypo- and hyper-eutectoid steels and presents more than a dozen micrographs, identifying important structural features along with cooling conditions and sample preparation procedures.

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.

This chapter describes the iron-carbon phase diagram, its modification by alloying elements, and the effect of carbon on the chemistry and crystallography of austenite, ferrite, and cementite found in Fe-C alloys and steels. It also lays the groundwork for understanding important metallurgical concepts, including solubility, critical temperature, dislocation defects, slip, and diffusion, and how they affect the microstructure, properties, and behaviors of steel.

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.

Bainite is an intermediate temperature transformation product of austenite. This chapter describes the conditions under which bainite is likely to form. It discusses the effects of alloying on bainitic transformation, the difference between upper and lower bainite, and the influence of solute drag on bainite formation mechanisms. It also discusses the development of ferrite-carbide bainites and their effect on toughness, hardness, and ductility.

This chapter describes the ferritic microstructures that form in carbon steels under continuous cooling conditions. It begins with a review of the Dubé classification system for crystal morphologies. It then explains how cooling-rate-induced changes involving carbon atom diffusion and the associated rearrangement of iron atoms produce the wide variety of morphologies and microstructures observed in ferrite. The chapter also describes a classification system developed specifically for ferritic microstructures and uses it to compare common forms of ferrite, including polygonal or equiaxed ferrite, Widmanstatten ferrite, quasi-polygonal or massive ferrite, acicular ferrite, and granular ferrite.

This chapter discusses the stress-strain response of ferritic microstructures and its influence on tensile deformation, strain hardening, and ductile fracture of carbon steels. It describes the ductile-to-brittle transition that occurs in bcc ferrite, the effects of aging and grain size on strength and toughness, continuous and discontinuous yielding behaviors, and dispersion and solid-solution strengthening processes.

This chapter describes heat treatments that produce uniform grain structures, reduce residual stresses, and improve ductility and machinability. It also discusses spheroidizing treatments that improve strength and toughness by promoting dispersions of spherical carbides in a ferrite matrix. The chapter concludes with a brief discussion on the mechanical properties of ferrite/pearlite microstructures in medium-carbon steels.

Metallurgy, as discussed in this chapter, focuses on phases normally encountered in stainless steels and their characteristics. This chapter describes the thermodynamics and the three basic phases of stainless steels: ferrite, austenite, and martensite. Formation of the principal intermetallic phases is also covered. In addition, the chapter provides information on carbides, nitrides, precipitation hardening, and inclusions.

This chapter discusses the composition and structure of low-carbon irons and steels, particularly those used in the production of hot-rolled strip. It describes the manufacturing process from the production of ingots to coiling, and it explains how finishing and coiling temperatures affect ferritic grain size and the distribution of cementite particles. It also discusses subsequent processing, including cold rolling and annealing, and the parameters with the greatest impact on grain size and microstructure. In addition, it describes the production of enameling irons, the benefits of high-temperature heat treatments, and the effects of quench and strain aging.

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.

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.

Structural steels are used for components such as I-beams and automobile frames. This chapter focuses on processing these steels to attain a fine primary ferrite grain size to develop high strength. It first reviews the concepts and principles of recrystallization in plastically deformed metals. The chapter reviews the concepts of annealing of cold worked metals. It then looks at hot working and the grain size associated with it. Additionally, the chapter reviews the methods of strengthening in the steels that rely mainly on reduction in the primary ferrite grain size. It discusses basic methods used to develop a small austenite grain size, and hence a small primary ferrite grain size. Then, the chapter covers the processes involved in the precipitation hardening of the ferrite. Finally, it examines some commercial thermomechanical processes used on structural steels, namely hot deformation and controlled cooling.