This chapter concentrates on very low-temperature martensitic transformations, which are of great concern for cryogenic applications and research. The principal transformation characteristics are reviewed and then elaborated. The material classes or alloy systems that exhibit martensitic transformations at very low temperatures are discussed. In particular, the martensitic transformations and their effects in austenitic stainless steels, iron-nickel alloys, practical superconductors, alkali metals, solidified gases, and polymers are discussed.

This chapter provides a short introduction to phase transformations, namely, the liquid-to-solid phase transformations that occur during solidification and the solid-to-solid transformations that are important in processing, such as heat treatment. It also introduces the concept of free energy that governs whether or not a phase transformation is possible, and then the kinetic considerations that determine the rate at which transformations take place. The chapter also describes important solid-state transformations such as spinodal decomposition and martensitic transformation.

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.

This chapter discusses the quenching process and its adaptation to induction heat treating. It describes the three stages of quenching, the cooling characteristics of various types of quenchants, and the details of nearly a dozen compatible quenching methods. It also explains how to verify whether a quenchant can cool a workpiece fast enough to achieve martensitic transformation without cracking or distortion.

The formation of martensite is characterized by its athermal transformation kinetics, crystallographic features, and development of fine structure. This chapter describes the diffusionless, shear-type transformation of austenite to martensite and how it affects the morphology and microstructure of heat-treatable carbon steels. It also provides information on lath and plate martensite and how they differ in structure and deformation properties.