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.

A pair of bearings mounted side by side in an aircraft engine failed in service. Photographs show that the inner rings were either broken or deformed, the balls were worn and flattened, and the cages severely damaged. The bearing races were damaged as well, but only on one side indicating a directional thrust. In addition to their examination, investigators also conducted metallographic studies and hardness tests, which indicated that the balls and inner rings reached temperatures above 825 °C (1520 °F). Based on their findings, investigators concluded that the bearings failed due to overheating, possibly as a result of misalignment compounded by insufficient lubrication and high speeds.

This chapter is a study of the microstructure of case-hardened steels. It explains what can be learned by examining grain size, microcracking, nonmetallic inclusions, and the effects of microsegregation. It identifies information-rich features, describing their ideal characteristics, the likely cause of variations observed, and their effect on mechanical properties and behaviors. The discussions throughout the chapter are aided by the use of images, diagrams, data plots, and tables.

This chapter discusses the formation of free carbides and their effect on case-carburized components. It explains how alloying elements influence the composition and structure of carbide phases produced at cooling rates typical of carburizing process. It describes the morphology and distribution of the various types of carbides formed and explains how they affect mechanical properties such as hardness, residual stresses, fatigue and fracture behaviors, and wear resistance. It also provides guidance for determining what processing conditions to avoid and when and why parts should be rejected.

This appendix discusses the general characteristics, composition, phase transformations, and properties of commercial stainless steels used for making high-quality knives.

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.

The nitriding process can be applied to various materials and part geometries. This chapter focuses on tool steels, pure irons, low-alloy steels, and maraging steels. Various considerations such as the surface metallurgy requirements of the die, including case depth, compound layer formation, and temperature, are also discussed in this chapter. The chapter also addresses steel selection and surface metallurgy of gears.

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.

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.

Mechanical components often require surface treatments to meet application demands. This chapter describes several surface hardening treatments for steel and their effect on microstructure, composition, and properties. It discusses flame hardening, induction heating, carburizing, nitriding, carbonitriding, and nitrocarburizing. The discussion on carburizing addresses several interrelated factors, including processing principles, alloying, surface oxidation, residual stresses, bending fatigue, contact fatigue, and fracture.

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 chapters reviews the various process, material, and post-treatment problems that can occur in nitriding and how to troubleshoot them. The troubleshooting methods discussed relate to gas nitriding, salt bath nitriding, and ion nitriding.

The wear caused by contact stress fatigue is the result of a wide variety of mechanical forces and environments. This chapter discusses the characteristics of four types of contact stress fatigue on mating metal surfaces: surface, subsurface, subcase, and cavitation. Features and corrective actions for these contact stress fatigue are discussed. The chapter also lists some possible ways to reduce the cavitation fatigue problem.

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.

This chapter covers the friction and wear behaviors of plastics and elastomers. It begins by describing the molecular differences between the two types of polymers and their typical uses. It then discusses the important attributes of engineering plastics and their suitability for applications involving friction, erosion, and adhesive and abrasive wear. It also discusses the tribology of elastomers and rubber along with their basic differences and the conditions under which they produce Schallamach waves. It includes information on polymer composites as well.

This chapter concerns itself with the tribology of ceramics, cermets, and cemented carbides. It begins by describing the composition and friction and wear behaviors of aluminum oxide, silicon carbide, silicon nitride, and zirconia. It then compares and contrasts the microstructure, properties, and relative merits of cermets with those of cemented carbides.

This chapter provides perspective on the physical dimensions associated with the microstructure of steel and the instruments that reveal grain size, morphology, phase distributions, crystal defects, and chemical composition, from which properties and behaviors derive. The chapter also reviews the definitions and classifications used to identify and differentiate commercial steels, including the AISI/SAE and UNS designation systems.

This chapter explains how decarburization can occur during carburizing processes and how to limit the severity of its effects. It describes the reactions and conditions that result in a loss of carbon atoms and how they vary with changes in the physical metallurgy of the affected material and the processing environment. It examines the characteristic features of decarburized microstructures and assesses their influence on hardness, residual stresses, and fatigue and fracture behaviors. It also discusses corrective measures and practical considerations regarding their use.

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 discusses the processes and procedures involved in tribotesting, the significance of test parameters and conditions, and practical considerations including test metrics and measurements and the interpretation of wear damage. It also describes the different types of erosion tests in use and common approaches for adhesive wear and abrasion testing.