This chapter describes the various phases that form in tool steels, starting from the base of the Fe-C system to the effects of the major alloying elements. The emphasis is on the phases themselves: their chemical compositions, crystal structures, and properties. The chapter also provides general considerations of phases and phase diagrams and the determination of equilibrium phase diagrams. It describes the formation of martensite, characteristics of alloy carbides, and the design of tool steels.

This chapter describes how the phases are arranged into desired microstructures during the heat treatment of tool steels. It describes the microstructural changes that are the objectives of the austenitizing, quenching, and tempering steps of tool steel hardening. The chapter covers austenite composition, retained austenite, and austenite grain size and grain growth. It provides information on the hardness and hardenability of tool steel. The chapter reviews some of these concepts and describes the microstructural appearance of the products of diffusion-controlled transformation of austenite. The role that diffusion-controlled phase transformations play relative to the hardenability of high-carbon and alloy tool steels is then emphasized. It presents general considerations of transformation diagrams, Jominy curves, and the hardenability of tool steels. The factors related to the kinetics and stabilization of martensite transformation are also covered. It briefly reviews selected aspects of the changes that evolve during tempering.

Furnaces for heat treatment of tool steels include ceramic-lined salt bath furnaces, vacuum furnaces, controlled-atmosphere furnaces, and fluidized-bed furnaces. This chapter describes the classification, operating principles, application, advantages, and disadvantages of each type of furnace.

Surface modification technologies improve the performance of tool steels. This chapter discusses the processes involved in oxide coatings, nitriding, ion implantation, chemical and physical vapor deposition processing, salt bath coating, laser and electron beam surface modification, and boride coatings that improve the performance of hot-work and high-speed tool steels.