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.

This chapter provides information on various contributors to failure of carburized and carbonitrided components, with the primary focus on carburized components. The most common contributors covered include component design, selection of proper hardenability, increased residual stress, dimensional stability, and generation of quenching and grinding cracks. They also include insufficient case hardness and improper core hardness, influence of surface carbon content and grain size, internal oxidation, structure of carbides, and inclusion of noncarbide. Details on micropitting, macropitting, case crushing, pitting corrosion, and partial melting are also provided.

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.