The decomposition of austenite, during controlled cooling or quenching, produces a wide variety of microstructures in response to such factors as steel composition, temperature of transformation, and cooling rate. This chapter provides a detailed discussion on the isothermal transformation and continuous cooling transformation diagrams that characterize the conditions that produce the various microstructures. It discusses the mechanism and process variables of quenching of steel, explaining the factors involved in the mechanism of quenching. In addition, the chapter provides information on the causes and characteristics of residual stresses, distortion, and quench cracking of steel.

Isothermal and continuous cooling transformation (CT) diagrams help users map out diffusion-controlled phase transformations of austenite to various mixtures of ferrite and cementite. This chapter discusses the application as well as limitations of these engineering tools in the context of heat treating eutectoid, hypoeutectoid, and proeutectoid steels. It also provides references to large collections of transformation diagrams and includes several diagrams that plot quenching and hardening transformations as a function of bar diameter.

This appendix is a collection of isothermal diagrams for carbon steels, chromium-molybdenum steels, nickel-chromium-molybdenum steels, nickel-molybdenum steels, and chromium steels.

This chapter addresses the concept of hardenability by first describing the basic hardening process for steel, starting with austenitization followed by quenching and tempering. The context also serves to clarify the difference between hardenability and hardness, which are often confused. Most of the information in the chapter is of a practical nature, covering application-oriented topics such as isothermal transformation (IT) and continuous transformation (CT) diagrams which are used to predict and control the rate of formation of ferrite, pearlite, and bainite. The chapter also discusses the effect of grain size and alloying elements and explains how Jominy end quench testing is used to evaluate the hardenability of steel.

This chapter describes some of the most essential tools in metallurgy and what they reveal about the structure, composition, and processing requirements of steel. It begins by identifying important details in the constitutional diagram of iron-cementite. It then explains how to read isothermal transformation and continuous-cooling diagrams and how to recognize the effect of various alloying elements.