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.

All superalloys, whether precipitation hardened or not, are heated at some point in their production for a subsequent processing step or, as needed, to alter their microstructure. This chapter discusses the changes that occur in superalloys during heat treatment and the many reasons such changes are required. It describes several types of treatments, including stress relieving, in-process annealing, full annealing, solution annealing, coating diffusion, and precipitation hardening. It discusses the temperatures, holding times, and heating and cooling rates necessary to achieve the desired objectives of quenching, annealing, and aging along with the associated risks of surface damage caused by oxidation, carbon pickup, alloy depletion, intergranular attack, and environmental contaminants. It also discusses heat treatment atmospheres, furnace and fixturing requirements, and practical considerations, including heating and cooling rates for wrought and cast superalloys and combined treatments such as solution annealing and vacuum brazing.

This chapter discusses the effect of heat treating on titanium alloys and the influence of time and temperature on critical properties and behaviors. It explains how heat treatments are used to make titanium stronger, tougher, more ductile, and easier to machine as well as more resistant to the effects of corrosion and thermal and mechanical fatigue. It describes accepted practices for stress relieving, aging, annealing, and post-treatment processing along with associated challenges and concerns.

This chapter covers the fundamentals of heat treating. It begins with a review of the composition, classification, and properties of iron and steel, the phases of the iron-carbon system, and the basic types of heat treatments. It then discusses the topics of hardness and hardenability, the role of carbon in the hardening of steels, the process of austenitization, and the influence of cooling rate on subsequent transformations. The chapter also explains how induction heating affects residual stress, distortion, and grain size.

Steel is an important material because of its tremendous flexibility in metal working and heat treating to produce a variety of mechanical, physical, and chemical properties. The purpose of this chapter is to present the metallurgical principles of heat treatment of steel in a generalized manner. The chapter provides a discussion on the constitution of commercially pure iron, subsequently leading to discussion on the iron-carbon alloy system. The chapter also describes the effect of carbon on the constitution of iron and of the solubility of carbon in iron. It provides information on transformations and on the classification of steels by carbon content. The chapter ends with a discussion on the effect of time on transformation and on the use of time-temperature-transformation diagrams.