The existence of austenite and ferrite, along with carbon alloying, is fundamental in the heat treatment of steel. In view of the importance of structure and its formation to heat treatment, this chapter describes the various microstructures that form in steels, the various factors that determine the formation of microstructures during heat treatment processing of steel, and some of the characteristic properties of each of the microstructures. The discussion also covers the constitution of iron during heat treatment and the phases of heat-treated steel with elaborated information on iron phase transformation, hysteresis in heating and cooling, ferrite and austenite as two crystal structures of solid iron, and the diffusion coefficient of carbon.

This chapter provides a detailed discussion on the Brownian motion, diffusion coefficient , and Fick's diffusion laws. It presents a review of solid phase diffusion and discusses the mechanism of diffusion and diffusion coefficient.

Diffusion is the movement of atoms through the crystalline lattice. This chapter discusses the two main types of diffusion that can occur in solids: interstitial diffusion and substitutional diffusion. It describes Fick's first and second laws of diffusion, with emphasis on several applications of the latter. The chapter also provides information on the temperature dependence of diffusion, intrinsic diffusion coefficients (Kirkendall effect), and high diffusion paths.