This chapter discusses the fundamentals of heat treating of alloy steels. It begins with an overview of the designations of AISI-SAE grades of alloy steels, followed by a description of the purposes served by alloying elements. The effects of specific alloying elements on the heat treatment of alloy steels and of boron on hardenability of alloy steels are then discussed. Procedures for heat treating four specific alloy steels (4037, 4037H; 4140, 4140H; 4340, 4340; and E52100) are subsequently presented. The chapter concludes with a brief account of austempering and martempering treatments.

Alloy steels are alloys of iron with the addition of carbon and one or more of the following elements: manganese, chromium, nickel, molybdenum, niobium, titanium, tungsten, cobalt, copper, vanadium, silicon, aluminum, and boron. Alloy steels exhibit superior mechanical properties compared to plain carbonsteels as a result of alloying additions. This chapter describes the beneficial effects of these alloying elements in steels. It discusses the mechanical properties, nominal compositions, advantages, and engineering applications of various classes of alloy steels. They are low-alloy structural steels, SAE/AISI alloy steels, high-fracture-toughness steels, maraging steels, austenitic manganese steels, high-strength low-alloy steels, dual-phase steels, and transformation-induced plasticity steels.

This chapter describes the designations of carbon and low-alloy steels and their general characteristics in terms of their response to hardening and mechanical properties. The steels covered are low-carbon steels, higher manganese carbon steels, boron-treated carbon steels, H-steels, free-machining carbon steels, low-alloy manganese steels, low-alloy molybdenum steels, low-alloy chromium-molybdenum steels, low-alloy nickel-chromium-molybdenum steels, low-alloy nickel-molybdenum steels, low-alloy chromium steels, and low-alloy silicon-manganese steels. The chapter provides information on residual elements, microalloying, grain refinement, mechanical properties, and grain size of these steels. In addition, the effects of free-machining additives are also discussed.

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 article discusses the role of alloying in the production and use of carbon and low-alloy steels. It explains how steels are defined and selected based on alloy content and provides composition and property data for a wide range of designations and grades. It describes the effect of alloying on structure and composition and explains how alloy content can be controlled to optimize properties and behaviors such as ductility, strength, toughness, fatigue and fracture resistance, and resistance to corrosion, wear, and high-temperature creep. It also examines the effect of alloying on processing characteristics such as hardenability, formability, weldability, machinability, and temper embrittlement. In addition, the article provides an extensive amount of engineering data with relevance in materials selection.

This appendix is a collection of tables listing coefficients of linear thermal expansion for carbon and low-alloy steels, presenting a summary of thermal expansion, thermal conductivity, and heat capacity; and listing thermal conductivities and specific heats of carbon and low-alloy steels.

Steels contain a wide range of elements, including alloys as well as residual processing impurities. This chapter describes the chemical composition of low-alloy AISI steels, which are classified based on the amounts of chromium, molybdenum, and nickel they contain. It explains why manganese is sometimes added to steel and how unintended consequences, such as the development of sulfide stringers, can offset the benefits. It also examines the effect of alloying elements on the iron-carbon phase diagram, particularly their effect on transformation temperatures.

This chapter describes important requirements for ferrous and nonferrous alloys used for gears. Wrought surface-hardening and through-hardening carbon and alloy steels are the most widely used of all gear materials and are emphasized in this chapter. The processing characteristics of gear steels and the bending fatigue strength and properties of carburized steels are reviewed. In addition to wrought steels, the chapter provides information on the other iron-base alloys that are used for gears, namely cast carbon and alloy steels, gray and ductile cast irons, powder metallurgy irons and steels, stainless steels, and tool steels. In terms of nonferrous alloys, the chapter addresses copper-base alloys, die cast aluminum alloys, zinc alloys, and magnesium alloys.

This chapter describes the classification of steels and the various compositional categories of commercial steel products. It explains how different alloying elements affect the properties of carbon and low-alloys steels and discusses strength, toughness, and corrosion resistance and how to improve them.

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.

Most steels that are hardened are subjected to a subcritical heat treatment referred to as tempering. Tempering improves the toughness of as-quenched martensitic microstructures but lowers strength and hardness. This chapter describes the microstructural changes that occur during tempering and their effect on the mechanical properties of steel. It also discusses the effect of alloying elements and the formation of oxide colors.

This chapter describes the causes of cracking, embrittlement, and low toughness in carbon and low-alloy steels and their differentiating fracture surface characteristics. It discusses the interrelated effects of composition, processing, and microstructure and contributing factors such as hot shortness associated with copper and overheating and burning as occur during forging. It addresses various types of embrittlement, including quench embrittlement, tempered-martensite embrittlement, liquid-metal-induced embrittlement, and hydrogen embrittlement, and concludes with a discussion on high-temperature hydrogen attack and its effect on strength and ductility.

The properties of martensite and the mechanisms that govern its formation are the key to understanding hardness and the hardenability of carbon steel. Martensite is a transformation product of austenite that requires rapid cooling to suppress diffusion-dependent transformation pathways. This chapter describes the conditions that must be met for martensite to form. It discusses the role of quenching and the factors that affect cooling rate, including heat transfer, thermal diffusivity, emissivity, and section size. It defines hardenability and explains how to quantify it using the Grossmann-Bain approach or Jominy end-quench testing. It also explains how hardenability can be improved through the addition of boron, phosphorus, and other alloys.

This chapter reviews the causes and cases associated with the problems originated by tempering of steels. To provide background on this phenomenon, a brief description of the martensite reactions and the steel heat treatment of tempering is given to review the different stages of microstructural transformation. A section describing the types of embrittlement from tempering, along with mechanical tests for the determination of temper embrittlement (TE), is presented. Various factors involved in the interaction of the TE phenomenon with hydrogen embrittlement and liquid-metal embrittlement are also provided. The cases covered are grinding cracks on steel cam shaft and transgranular and intergranular crack path in commercial steels.

This chapter discusses the compositions, metallurgical phases, microstructure, heat treatment, grades, and structure-property relationships of steels. In addition, it describes the mechanisms involved in deformation, strengthening, and annealing of steels.

Steels for hot-work applications, designated as group H steels in the AISI classification system, have the capacity to resist softening during long or repeated exposures to high temperatures needed to hot work or die cast other materials. These steels are subdivided into three classes according to the alloying approach: chromium hot-work steels, tungsten hot-work steels, and molybdenum hot-work steels. This chapter discusses the composition, characteristics, applications, advantages, and disadvantages of each of these steels.

Microstructures can be altered intentionally or unintentionally. In some cases, metallographers must diagnose what may have happened to the steel or cast iron based on the microstructural details. This chapter discusses how microstructure in steels and cast irons can be intentionally altered during heat treatment, solidification, and deformation (hot and cold working). Some specific examples are then shown to illustrate what can go wrong through unintentional changes in microstructure, for example, the loss of carbon from the surface of the steel by the process known as decarburization or the buildup of brittle carbides on the grain boundaries of an austenitic stainless steel by the process known as sensitization.

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.