This chapter discusses the factors that govern the mechanical properties of titanium, beginning with the morphology of the alpha phase. It explains that the shape of the alpha phase has a significant effect on many properties, including hardness, tensile strength, toughness, and ductility as well as creep, fatigue strength, and fatigue crack growth rate. It also discusses the influence of other titanium phases and the properties of titanium-based intermetallic compounds, metal-matrix composites, and shape-memory alloys.

Casting is the most economical processing route for producing titanium parts, and unlike most metals, the properties of cast titanium are on par with those of wrought. This chapter covers titanium melting and casting practices -- including vacuum arc remelting, consumable electrode arc melting, electron beam hearth melting, rammed graphite mold casting, sand casting, investment casting, hot isostatic pressing, weld repair, and heat treatment -- along with related equipment, process challenges, and achievable properties and microstructures. It also explains how titanium parts are produced from powders and how the different methods compare with each other and with conventional production techniques. The methods covered include powder injection molding, spray forming, additive manufacturing, blended elemental processing, and rapid solidification.

Modern gears are made from a wide variety of materials. Of all these, steel has the outstanding characteristics of high strength per unit volume and low cost per pound. Although both plain carbon and alloy steels with equal hardness exhibit equal tensile strengths, alloy steels are preferred because of higher hardenability and the desired microstructures of the hardened case and core needed for the high fatigue strength of gears. This chapter provides an overview of the key considerations involved in the selection and application of heat treating processes for alloy steel gears and serves as an introduction to the subsequent chapters in this book.

The properties of steel are affected markedly as the percentage of carbon varies. This chapter describes the properties of alloys of iron and carbon, including a review of the iron-carbon phase diagram and, in particular, the portion of the diagram relevant to carbon steels. It addresses the processes involved in the transformation (decomposition) of austenite to achieve various microstructures.

This chapter introduces the fundamental considerations involved in heat treating of gears and the primary processes used, namely, through-hardening, carburizing and hardening, nitriding, carbonitriding, and induction hardening.

Through-hardening heat treatment is generally used for gears that do not require high surface hardness. In through hardening, gears are first heated to a required temperature and then cooled either in the furnace or quenched in air, gas, or liquid. Four heat treatment methods are primarily used for through-hardened gears: annealing, normalizing and annealing, normalizing and tempering, and quenching and tempering. This chapter begins with a discussion of these through-hardening processes. This is followed by sections providing some factors affecting the design and hardness levels of through-hardened gears. Next, the chapter reviews the considerations related to distortion of through-hardened gears. It then discusses the applications of through-hardened gears. Finally, the chapter presents a case history of the design and manufacture of a through-hardened gear rack.

The primary objective of carburizing and hardening gears is to secure a hard case and a relatively soft but tough core. For this process, low-carbon steels (up to a maximum of approximately 0.30% carbon), either with or without alloying elements (nickel, chromium, manganese, molybdenum), normally are used. The processes involved in hardening, tempering, recarburizing, and cold treatment of carburized and quenched gears are discussed. Next, the chapter reviews the selection of materials for carburized gears and considerations related to carbon content, core hardness, and microstructure. This is followed by sections discussing some problems that can be experienced in the carburizing process and how these can be addressed, including a section on shot peening to induce compressive residual stress at and below the surface. It then discusses the applications of carburized gears and finally presents a case history of distortion control of carburized and hardened gears.

Nitriding is a case-hardening process used for alloy steel gears and is quite similar to case carburizing. Nitriding of gears can be done in either a gas or liquid medium containing nitrogen. This chapter discusses the processes involved in gas nitriding. It reviews the effects of white layer formation in nitrided gears and presents general recommendations for nitrided gears. The chapter describes the microstructure, overload and fatigue damage, bending-fatigue life, cost, and distortion of nitrided gears. Information on nitriding steels used in Europe and the applications of nitrided gears are also provided. The chapter presents case studies on successful nitriding of a gear and on the failure of nitrided gears used in a gearbox subjected to a load with wide fluctuations.

Several limitations in achieving optimal gear performance with conventional nitriding have led researchers to work on a variety of novel and improved nitriding processes. Of these, ion/plasma nitriding offers some promising results, which are reviewed in this chapter. The chapter concludes with a case history describing the application of ion nitriding to an internal ring gear of an epicyclic gearbox.

Carbonitriding is a process in which carbon and alloy steel gears are held at a temperature above the transformation range in a gaseous atmosphere of such composition that the steel absorbs carbon and nitrogen simultaneously. The gears are then cooled at a specific rate to room temperature that produces the desired properties. Carbonitriding is generally regarded as a modified gas carburizing process, rather than a form of nitriding. This chapter briefly describes the case depth that can be achieved with carbonitriding, how case depth is measured, and the materials and applications that are suitable for carbonitriding.

Some gears may need to be hardened only at the surface without altering the chemical composition of the surface layers. Induction hardening may be a suitable processing choice in these cases. This chapter provides information on the wide variety of materials that can be induction hardened and on process details involved in induction hardening gears. It discusses the processes involved in heating, quenching, and tempering of gears. Information on surface hardness and case depth after induction hardening, induction hardening problems, the applications of induction hardening gears, and the advancements in induction hardening are also provided.

The successful design and manufacture of gears are influenced largely by design requirements, material selection, and proper heat treatment. This chapter addresses the cost factors and tradeoffs involved in selecting a material, design features, and a heat treating process to optimize gear performance for a particular application.

This chapter contains a list of technical papers and books related to the heat treatment of gears.

Mechanical treatments such as grinding and shot peening are often employed in the production of case-carburized parts. Grinding, besides restoring precision, removes carbide films, internal oxidation, and high-temperature transformation products. Shot peening strengthens component surfaces and induces a stress state that increases fatigue resistance. This chapter describes both processes as well as roller burnishing. It explains how these treatments are applied and how they influence the microstructure, properties, and behaviors of case-hardened components. It also addresses process challenges, particularly in regard to grinding.