Search Results for transformation products

A spheroidized structure, which consists of spherically shaped cementite in a matrix of ferrite, is often desired in the production of steel, whether to improve properties, such as machinability and ductility, or accommodate subsequent hardening treatments. This chapter discusses the spheroidization of normalized and annealed steels by heating at subcritical temperatures. It explains how lamellar pearlite and proeutectoid cementite transform when heated and how deformation prior to heating affects both the mechanism and kinetics of spheroidization. It also explains how austenitizing contributes to the production of spheroidal transformation products and why secondary graphitization sometimes occurs.

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.

The properties of martensite and the mechanisms that govern its formation are the key to understanding hardness and the hardenability of carbon steel. Martensite is a transformation product of austenite that requires rapid cooling to suppress diffusion-dependent transformation pathways. This chapter describes the conditions that must be met for martensite to form. It discusses the role of quenching and the factors that affect cooling rate, including heat transfer, thermal diffusivity, emissivity, and section size. It defines hardenability and explains how to quantify it using the Grossmann-Bain approach or Jominy end-quench testing. It also explains how hardenability can be improved through the addition of boron, phosphorus, and other alloys.

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.

Mechanical treatments such as grinding and shot peening are often employed in the production of case-carburized parts. Grinding, besides restoring precision, removes carbide films, internal oxidation, and high-temperature transformation products. Shot peening strengthens component surfaces and induces a stress state that increases fatigue resistance. This chapter describes both processes as well as roller burnishing. It explains how these treatments are applied and how they influence the microstructure, properties, and behaviors of case-hardened components. It also addresses process challenges, particularly in regard to grinding.

Gas carburizing is known to promote internal oxidation in steel which can adversely affect certain properties. This chapter discusses the root of the problem and its effect on component lifetime and performance. It explains that gas-carburizing atmospheres contain water vapor and carbon dioxide, providing oxygen that reacts with alloying elements, particularly manganese, chromium, and silicon. It examines the composition and distribution of oxides produced in different steels and assesses the resulting composition gradients. It describes how these changes influence the development of high-temperature transformation products as well as microstructure, hardenability, and carbon content and properties such as fatigue and fracture behaviors, hardness, and wear resistance. It also explains how to manage internal oxidation through material design, process control, and other measures.