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 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.

Heat treatment of steel involves a number of processes to condition the microstructure and obtain desired properties. This includes various methods namely, annealing, normalizing, and hardening by quenching and tempering. This chapter focuses on general heat treatment procedures and the applications of particular types or grades of carbon and low-alloy steels. The discussion covers carbon steel classification for heat treating, tempering of quenched carbon steels, and austempering of steel. In addition, the chapter discusses the effects of alloying and hardenability on steel and provides information on martempering of steel.

This chapter provides an overview of the fundamental material- and process-related parameters of quenching on residual stress, distortion control, and cracking. It begins with a description of phase transformations during heating and quenching of steel. This is followed by a section on the effects of materials and quench process design on distortion of steel during heat treating. Details on stress raisers and their role in quench cracking are then presented. The chapter ends with various selected case histories of failures attributed to the quenching process.

Quenching is a critical step in the production of hardened steel. This chapter untangles some of the complexities of the quenching process and its effect on the microstructure and properties of various steels. Making extensive use of cooling curves, it sheds light on the transformations that occur at different cooling rates and the extent to which they can be changed by adjusting quench parameters. It discusses the role of quenching in martempering and austempering along with related problems such as cracking and distortion and the challenges posed by low-hardenability steels. It also discusses the use of various quenchants, including oil, polymer, and molten salt, and explains how to measure and compare their performance using a standard (ISO 9950) test.

Induction heating, in most applications, is used to selectively heat only a portion of the workpiece that requires treatment. This chapter covers the basic principles, features, and metallurgical aspects of induction heating. The discussion includes the conditions required for induction heating and quenching, the use of magnetic flux concentrators to improve the efficiency of surface heating, and the quenching systems used for induction hardening. The discussion also provides information on time-temperature dependence in induction heating, workpiece distortion in induction surface hardening, residual stresses after induction surface hardening and finish grinding, and input and output control of steel for induction surface hardening of gears.

This chapter introduces the principal heat treating processes, namely normalizing, annealing, stress relieving, surface hardening, quenching, and tempering. An overview of four of the more popular surface hardening treatments, namely carburizing, carbonitriding, nitriding, and nitrocarburizing, is provided.

This chapter is a detailed account of heat treating techniques for cast irons (gray and ductile), providing the reader with a basic understanding of the differences among various types of cast irons and the concept of carbon equivalent. The types of heat treatments discussed are stress relieving, annealing, normalizing, surface hardening, quenching, martempering, austempering, and flame and induction hardening.

The hardenability of steel is governed almost entirely by the chemical composition (carbon and alloy content) at the austenitizing temperature and the austenite grain size at the moment of quenching. This article introduces the methods to evaluate hardenability and the factors that influence steel hardenability and selection. The discussion covers processes involved in Jominy end-quench test for evaluating hardenability. The effect of carbon on hardenability data and the effect of alloys on hardenability during quenching and on the tempering response (after hardening) are also discussed. In addition, the article provides information on the hardenability limits of H-steels after a note on hardenability correlation curves and Jominy equivalence charts.

This chapter addresses the concept of hardenability by first describing the basic hardening process for steel, starting with austenitization followed by quenching and tempering. The context also serves to clarify the difference between hardenability and hardness, which are often confused. Most of the information in the chapter is of a practical nature, covering application-oriented topics such as isothermal transformation (IT) and continuous transformation (CT) diagrams which are used to predict and control the rate of formation of ferrite, pearlite, and bainite. The chapter also discusses the effect of grain size and alloying elements and explains how Jominy end quench testing is used to evaluate the hardenability of steel.

Most quenched steels are tempered because the toughness of as-quenched steels is generally very poor. The tempering operation sacrifices strength for improvements in ductility and toughness. This chapter discusses the tempering process, the challenge of tempered martensite embrittlement, and the effect of wt% carbon on toughness. It also explains how alloying elements improve the hardenability and tempering response of plain carbon steels.

This chapter, a detailed account of furnaces and related equipment for heat treating, begins by describing three basic modes of heat transmission, namely conduction, convection, and radiation, followed by a discussion on the working principle, applications, advantages, and disadvantages of furnaces classified based on the heat transfer medium employed. The types of furnaces covered are batch-type, continuous-type, liquid bath, fluidized bed, and vacuum. The subsequent sections provide information on furnace parts, fixtures, quenching mediums, and quenching systems. The final section of the chapter describes the types of atmospheres available, emphasizing their applications and limitations.

This chapter discusses the processes used in manufacturing to thermally alter the properties of metals and alloys. It begins with a review of the iron-carbon system, the factors that affect hardenability, and the use of continuous cooling transformation diagrams. It then explains how various steels respond to heat treatments, such as annealing, normalizing, spheroidizing, tempering, and direct and interrupted quenching, and surface-hardening processes, such as flame and induction hardening, carburizing, nitriding, and carbonitriding. It also addresses the issue of temper embrittlement and discusses the effect of precipitation hardening on aluminum and other alloys.

This chapter describes the general characteristics of major types of steel annealing, including the process of normalization, which is a process that refines or normalizes the microstructure of steel. The first part of the chapter begins with an overview of the three-stage process of recovery, recrystallization, and grain growth. This is followed by discussions on annealing processes, namely subcritical annealing, critical-range annealing, full annealing, isothermal annealing, annealing for microstructure, and solution or quench annealing. Next, the chapter describes two undesirable reactions that occur during annealing: decarburization and scaling. Information on the gases and gas mixtures used for controlled atmospheres is then provided. The second part of the chapter focuses on the processes involved in normalizing, along with information on furnace equipment for normalizing. In addition, the chapter includes information on processes involved in induction heating of steel.

This chapter discusses the evolution of engineering alloy steels, namely chromium, nickel, and nickel-chromium alloy steels. The discussion includes the automotive demand and development of specifications for the alloy steels. It also covers various research on heat treatment of alloy steels, providing information on hardening, transformation of austenite, hardenability testing, and tempering of as-quenched martensite.

This chapter provides information on various contributors to failure of carburized and carbonitrided components, with the primary focus on carburized components. The most common contributors covered include component design, selection of proper hardenability, increased residual stress, dimensional stability, and generation of quenching and grinding cracks. They also include insufficient case hardness and improper core hardness, influence of surface carbon content and grain size, internal oxidation, structure of carbides, and inclusion of noncarbide. Details on micropitting, macropitting, case crushing, pitting corrosion, and partial melting are also provided.

This chapter discusses the various factors pertinent to gravity permanent mold (GPM) castings, along with their advantages, limitations, and significance. The discussion covers the geometric factors, process and manufacturing elements, gating practices, and feeding principles of and pouring systems in GPM. The influences of mold coatings on GPM and low pressure permanent mold castings are described. The chapter also discusses various processes involved in the engineering of core boxes and cooling of GPM for casting integrity and cycle time control. It provides information on some of the processes involved in post-casting operations, namely de-coring and de-gating. The key design aspects for consideration in water quenching during the T6 heat treatment are reviewed. The chapter also provides information on two critical cycle events important in engineering at the manufacturing facility: tipper cycle planning and table or cell cycle planning.

The mechanical properties of steel are strongly influenced by the underlying microstructure, which is readily observed using optical microscopy. This chapter describes common room-temperature steel microstructures and how they are achieved via heat treatment. It discusses the production of hypo- and hypereutectoid steels and the effect of cooling rate on microstructure. It also examines quenched steels and the phase transformations associated with rapid cooling. It describes the development of lath and plate martensite, retained austenite, and bainite and how to identify the various phases. The chapter concludes with a brief review of spheroidized microstructures.

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.