This chapter describes the ferritic microstructures that form in carbon steels under continuous cooling conditions. It begins with a review of the Dubé classification system for crystal morphologies. It then explains how cooling-rate-induced changes involving carbon atom diffusion and the associated rearrangement of iron atoms produce the wide variety of morphologies and microstructures observed in ferrite. The chapter also describes a classification system developed specifically for ferritic microstructures and uses it to compare common forms of ferrite, including polygonal or equiaxed ferrite, Widmanstatten ferrite, quasi-polygonal or massive ferrite, acicular ferrite, and granular ferrite.

This chapter discusses the stress-strain response of ferritic microstructures and its influence on tensile deformation, strain hardening, and ductile fracture of carbon steels. It describes the ductile-to-brittle transition that occurs in bcc ferrite, the effects of aging and grain size on strength and toughness, continuous and discontinuous yielding behaviors, and dispersion and solid-solution strengthening processes.

Heat treatment is the most common way of altering the mechanical, physical, and even chemical properties of steels. This chapter describes the changes that occur in carbon and low-alloy steels during conventional heat treatments. It explains how austenite decomposition largely defines the final microstructure, and how the associated phase transformations are driven by nucleation and growth processes. It describes diffusionless and diffusive growth mechanisms and provides detailed information on the properties, structure, and behaviors of the transformation products produced, namely martensite and bainite. It also discusses the formation of austenite, the control and measurement of austenitic grain size, the characteristics of ferritic microstructures, and the methods used to classify ferrite morphology.

Bainite is an intermediate temperature transformation product of austenite. This chapter describes the conditions under which bainite is likely to form. It discusses the effects of alloying on bainitic transformation, the difference between upper and lower bainite, and the influence of solute drag on bainite formation mechanisms. It also discusses the development of ferrite-carbide bainites and their effect on toughness, hardness, and ductility.

This chapter discusses various alloying and processing approaches to increase the strength of low-carbon steels. It describes hot-rolled low-carbon steels, cold-rolled and annealed low-carbon steels, interstitial-free or ultra-low carbon steels, high-strength, low-alloy (HSLA) steels, dual-phase (DP) steels, transformation-induced plasticity (TRIP) steels, and martensitic low-carbon steels. It also discusses twinning-induced plasticity (TWIP) steels along with quenched and partitioned (Q&P) steels.

This chapter describes heat treatments that produce uniform grain structures, reduce residual stresses, and improve ductility and machinability. It also discusses spheroidizing treatments that improve strength and toughness by promoting dispersions of spherical carbides in a ferrite matrix. The chapter concludes with a brief discussion on the mechanical properties of ferrite/pearlite microstructures in medium-carbon steels.

The microstructure of carbon steel is largely determined by the transformation of austenite to ferrite, cementite, and pearlite. This chapter focuses on the microstructures produced by diffusion-controlled transformations that occur at relatively low cooling rates. It describes the conditions that promote such transformations and, in turn, how they affect the structure of various phases and the rate at which they form. The chapter also discusses the concepts of transformation kinetics, minimum free energy, and nucleation and growth, and provides information on alloying, interphase precipitation, and various types of transformations.

Medium-carbon steels are typically hardened for high-strength, high-fatigue-resistant applications by austenitizing, quenching to martensite, and tempering. This chapter explains how microalloying with vanadium, niobium, and/or titanium provides an alternate way to improve the mechanical properties of such steels. It also addresses microalloyed forging steels and explains how nontraditional bainitic microstructures can be produced by direct cooling after forging.