Unlike conventional quench and temper heat treatment, austempering is an iron and steel heat-treatment process that enhances mechanical properties through the isothermal transformation of austenite with a minimum amount of quenching stresses. This chapter begins with a discussion of austemperability requirements. Then outlines of austenitizing and austempering cycles and resultant microstructures are presented. This is followed by sections discussing the mechanical properties, advantages, limitations, machinability, process variants, and applications of austempered ductile iron (ADI). Information on the growth of premachined ADI components is also provided. Further, the chapter describes two slightly different systems for austempering: atmospheric-salt and salt-salt systems. Finally, it presents general guidelines for component designers, casting manufacturers, and heat treaters to apply ADI more widely and with improved success.

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.

Austenite is the key to the versatility of steel and the controllable nature of its properties. It is the parent phase of pearlite, martensite, bainite, and ferrite. This chapter discusses the importance of austenite, beginning with the influence of austenitic grain size and how to accurately measure it. It then describes the principles of austenite formation and grain growth and examines several time-temperature-austenitizing diagrams representing various alloying and processing conditions. The chapter concludes with a discussion on hot deformation and subsequent recrystallization.

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.

Tool steels are specialty steels, produced in relatively low volumes, optimized for applications requiring precise combinations of wear resistance, toughness, and hot hardness. This chapter describes the AISI classification system by which tool steels are defined. It discusses primary types, including high-speed and shock-resisting steels, and their associated subtype groups (W, L, S, O, A, D, H, M, and T series). It also discusses the types of carbides found in tool steels and their influence on mechanical properties. The chapter concludes with a discussion on heat treatment effects unique to tool steels, including two-phase effects, austenite stabilization, and the conditioning of retained austenite.

This chapter addresses the issue of retained austenite in quenched carburized steels. It explains why retained austenite can be expected at the surface of case-hardened components, how to estimate the amount that will be present, and how to effectively stabilize or otherwise control it. It presents detailed images and data plots showing how retained austenite appears and how it influences hardness, tensile properties, residual stresses, fatigue and fracture behaviors, and wear resistance.

This chapter familiarizes readers with tempering and refrigeration treatments and their effect on case-carburized parts. It explains how tempering makes such parts easier to machine, more structurally and dimensionally stable, and more durable in certain applications. It identifies key process parameters and provides test data showing how they affect hardness, yield strength, bending and contact fatigue, and fracture toughness. It also addresses potential problems stemming from process-related factors such as the presence of hydrogen and the effects of aging and grinding. In regard to refrigeration, the chapter explains that it is not uncommon for subzero treatments to be included in the production of carburized parts whether as a standard procedure or optional step. Subzero cooling promotes the transformation of retained austenite to martensite, thereby increasing surface hardness and reducing the propensity of quenched carburized steels to burn and crack during surface grinding. The chapter includes numerous data plots and tables showing how the various treatments influence hardness, wear resistance, tensile properties, and fatigue and fracture behaviors.

This chapter describes how the phases are arranged into desired microstructures during the heat treatment of tool steels. It describes the microstructural changes that are the objectives of the austenitizing, quenching, and tempering steps of tool steel hardening. The chapter covers austenite composition, retained austenite, and austenite grain size and grain growth. It provides information on the hardness and hardenability of tool steel. The chapter reviews some of these concepts and describes the microstructural appearance of the products of diffusion-controlled transformation of austenite. The role that diffusion-controlled phase transformations play relative to the hardenability of high-carbon and alloy tool steels is then emphasized. It presents general considerations of transformation diagrams, Jominy curves, and the hardenability of tool steels. The factors related to the kinetics and stabilization of martensite transformation are also covered. It briefly reviews selected aspects of the changes that evolve during tempering.

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 crux of this chapter is to develop a method to quantitatively define hardenability. The chapter includes the empirical methods to estimate the hardenability knowing the chemical composition, describes prior austenite grain size, and examines their utility. It then reviews the Jominy end-quench test and explains its relation to hardenability. The chapter outlines the concepts of the critical diameter and the ideal critical diameter, leading to establishing a quantitative measure of hardenability. Next, it examines methods that have been developed which allow estimation of the ideal critical diameter from the chemical composition and the austenite grain size. The chapter reviews the methods which allow calculation of the Jominy curve from a value of the ideal critical diameter. Additionally, it describes the selection and application of H-band steels. Finally, the chapter describes the effect of boron on the hardenability of steels.

This chapter examines the cooling of steels from the austenite region. It describes the processes of determining the severity of quench. The chapter examines the methods to estimate the quench required if the size and shape of the part are known and the required cooling rate is known. The cooling rate correlation is used to calculate the hardness distribution across the diameter of cylinders. The calculations are used to illustrate the sensitivity of the hardness distribution to the severity of quench and the hardenability. The chapter discusses the methods of determining cooling rates in quenched steel components. It describes the formation of residual stresses in materials in which no phase change occurs on cooling. The chapter also examines the effect on the residual stresses of the phase changes in austenite. It provides information on two types of quench cracks in quenched steels, namely, microcracking and gross cracking during quenching.

Austenitization is the heat treatment of steel in the austenite region, and it is conducted for two reasons. One is to obtain austenite as a necessary precursor for heat treatment, and this is the main emphasis of this chapter. The other is to chemically homogenize steel, so that concentration gradients formed during solidification upon casting are minimized; this is briefly described in this chapter. Austenitization topics covered in this chapter are dendritic segregation in steels, austenitization to remove coring, ingot segregation, grain growth behavior, formation of austenite, austenite grain size, heating in the austenite region, and practical austenitizing temperatures.

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.

This chapter discusses the processes involved in the calculation of cooling and heating curves and the estimation of austenitizing time. The chapter discusses the transformation of austenite, describing nucleation and growth rates. The chapter examines the calculation of the amount of martensite and retained austenite, and hardenability calculations. It reviews models for tempering of steels, estimation of mechanical properties, the hardness of martensite, and factors to consider to be able to calculate the residual stresses. It provides information on Kirkaldy’s model used for calculating Jominy curves. Tables for calculating the temperature distribution in steels as a function of time during heating and cooling are also included.

This chapter contains problems that illustrate the calculation or determination of such items as ideal critical diameter, the Jominy curve, and the severity of quench by methods. It presents solutions for the calculation of the effect of prior austenite grain size, carbon content, chromium content, and molybdenum content on ideal critical diameter. The chapter also contains solutions for calculation of Jominy curves and determination of minimum hardness of quenched steels, tempered hardness, ideal critical diameter, severity of quench, heat treatment, and effect of tempering during heat-up to tempering temperature.