This chapter discusses the processes involved in the heat treatment of steel, namely austenitizing, hardening, quenching, and tempering. It begins with an overview of austenitizing of steels by induction heating, followed by a discussion on the processes involved in transformation of the soft austenite into martensite or lower bainite in the hardening operation. The chapter provides information on various quenching systems and a description of quenching techniques, namely austempering, martempering, and patenting. Difficulties associated with hardening of steel are discussed. Further, the chapter describes the equipment used for and principal variables of tempering. It discusses the causes for various forms of embrittlement due to tempering. Information on multiple tempering, protective-atmosphere tempering, and selective tempering are also provided, along with processes involved in selection of tempering temperature. The chapter ends with a section discussing various effects, advantages, and disadvantages of precipitation hardening.

This chapter discusses different thermal processes applicable to the various alloy groups of stainless steels, namely austenitic, ferritic, martensitic, precipitation hardening, and duplex stainless steels. The processes discussed include soaking, annealing, stress relieving, austenitizing, tempering, aging, and conditioning.

This appendix consists of tables listing temperatures recommended for austenitizing carbon and low-alloy steels prior to hardening.

One of the primary advantages of steels is their ability to attain high strengths through heat treatment while still retaining some degree of ductility. Heat treatments can be used to not only harden steels but also to provide other useful combinations of properties, such as ductility, formability, and machinability. This chapter discusses various heat treatment processes, namely annealing, stress relieving, normalizing, spheroidizing, and hardening by austenitizing, quenching and tempering. It also discusses two types of interrupted quenching processes: martempering and austempering. The chapter concludes with a brief section on temper embrittlement.

The first step in the hardening of steel is getting it hot enough to form austenite, from which martensite can form upon quenching. Not all steels have the same austenitization requirements, however. High-carbon wear-resistant steels, such as bearing and tool steels, require the presence of carbides during austenitization; plain carbon and low-alloy steels do not. This chapter describes the austenitization process used in each of the two cases, namely single-phase austenitization (the accepted method for plain carbon low-alloy steels) and two-phase austenitization (required for high-carbon steels). It also addresses process-specific issues, explaining how the presence of carbides (in the two-phase process) produces significant changes, and how homogenization and austenite grain growth influence the single-phase process.

Tool steels represent a small, but very important, segment of the total production of steel. Their principal use is for tools and dies that are used in the manufacture of commodities. For the most part, the processes used for heat treating carbon and alloy steels are also used for heat treating tool steels, that is, annealing, austenitizing, tempering, and so forth. This chapter focuses on these heat treating processes of tool steels. Classification and approximate compositions and heating treating practices of some principal types of tool steels are provided. The steel types discussed include water-hardening; shock-resisting; oil-hardening cold-work; air-hardening, medium-alloy cold-work; high-carbon, high-chromium cold-work; low-alloy, special-purpose; mold; hot-work; and high-speed tool steels.

This chapter examines the structural changes that occur in high-carbon steels during austenitization. It describes the effect of heating time and temperature on the production of austenite and the associated transformation of ferrite and cementite in eutectoid, hypoeutectoid, and hypereutectoid steels. It discusses the factors that influence the kinetics of the process, including carbon diffusion and the morphology of the original structure. It describes the nucleation and growth of austenite grains, the effect of grain size on mechanical properties, and the difference between coarse- and fine-grained steels. The chapter also discusses grain-refinement processes and some of the effects of overheating, including sulfide spheroidization, grain-boundary sulfide precipitation, and grain-boundary liquation.

High-speed tool steels have in common the ability to maintain high hardness at elevated temperatures. High speed steels are primarily used for cutting tools that generate heat during high-speed machining. They are designated as group M or group T steels in the AISI classification system, depending on whether the major alloying approach is based on molybdenum or tungsten. This chapter describes the effects of each of the alloying elements and carbon content on the processing, microstructures, and properties of high-speed steels. It discusses the processes involved in the solidification, hot work, annealing, austenitizing for hardening, and tempering of high-speed steels. It also discusses the processes involved in controlling grain size during austenitizing and reviews the characteristics of cooling transformations and other property changes in tempered high-speed steels. Information on multipoint cutting tools is provided. The chapter discusses the applications of high-speed tool steel and factors in selecting high-speed tool steels.

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.

This chapter describes the heat treatments called annealing and normalizing for steels and examines the structures formed and the reasons for these treatments. It also provides a description of the special heat treatments, namely, martempering and austempering. Information on intercritical heat treatment is also included.

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.