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.

This chapter discusses the formation of free carbides and their effect on case-carburized components. It explains how alloying elements influence the composition and structure of carbide phases produced at cooling rates typical of carburizing process. It describes the morphology and distribution of the various types of carbides formed and explains how they affect mechanical properties such as hardness, residual stresses, fatigue and fracture behaviors, and wear resistance. It also provides guidance for determining what processing conditions to avoid and when and why parts should be rejected.

This chapter focuses on gas carburizing. It includes a history of carburizing and explains the objectives of the carburizing process. The chapter discusses atmospheres, sources of carbon, reactions in the gas carburizing process, and effects of process variables such as temperature and time. Furnaces for gas carburizing are described and potential microstructure problems are introduced.

This chapter discusses the thermally induced changes that occur on the surface of steel exposed to different environments. It explains how oxide scales form during heat treating and how factors such as temperature, composition, and surface finish affect growth rates, grain structure, and uniformity. It provides examples of oxides that form beneath the surface of steel and explains why it occurs. It describes the conditions associated with decarburization and explains how to determine the depth of decarburized layers in eutectoid, hypoeutectoid, and hypereutectoid steels. It also discusses the carburizing process, the factors that determine the depth and gradient of the carburized case, the effect of post-process treatments, and a variation on the process known as ferritic carbonitriding.

This chapter contains a list of technical papers and books related to the heat treatment of gears.

This chapter describes important requirements for ferrous and nonferrous alloys used for gears. Wrought surface-hardening and through-hardening carbon and alloy steels are the most widely used of all gear materials and are emphasized in this chapter. The processing characteristics of gear steels and the bending fatigue strength and properties of carburized steels are reviewed. In addition to wrought steels, the chapter provides information on the other iron-base alloys that are used for gears, namely cast carbon and alloy steels, gray and ductile cast irons, powder metallurgy irons and steels, stainless steels, and tool steels. In terms of nonferrous alloys, the chapter addresses copper-base alloys, die cast aluminum alloys, zinc alloys, and magnesium alloys.

Mechanical components often require surface treatments to meet application demands. This chapter describes several surface hardening treatments for steel and their effect on microstructure, composition, and properties. It discusses flame hardening, induction heating, carburizing, nitriding, carbonitriding, and nitrocarburizing. The discussion on carburizing addresses several interrelated factors, including processing principles, alloying, surface oxidation, residual stresses, bending fatigue, contact fatigue, and fracture.

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.

This chapter addresses the issue of retained austenite in quenched carburized steels. It explains why retained austenite can be expected at the surface of case-hardened components, how to estimate the amount that will be present, and how to effectively stabilize or otherwise control it. It presents detailed images and data plots showing how retained austenite appears and how it influences hardness, tensile properties, residual stresses, fatigue and fracture behaviors, and wear resistance.

This chapter explains how decarburization can occur during carburizing processes and how to limit the severity of its effects. It describes the reactions and conditions that result in a loss of carbon atoms and how they vary with changes in the physical metallurgy of the affected material and the processing environment. It examines the characteristic features of decarburized microstructures and assesses their influence on hardness, residual stresses, and fatigue and fracture behaviors. It also discusses corrective measures and practical considerations regarding their use.

This chapter provides a brief but practical overview of the case carburizing process. It discusses the benefits and challenges of the process and compares and contrasts it with other hardening methods. It explains how design allowables and safety factors compensate for unknowns and familiarizes readers with the steps involved in determining case depth and verifying that case carbon requirements have been met.

The design of case-hardened components is an iterative process, requiring the consideration of multiple interrelated factors. This chapter walks readers through the steps involved in selecting an appropriate material and assessing the influence of alloy composition and cooling rate on core properties including hardenability, microstructure, tensile and yield strength, ductility, toughness, and fatigue resistance. It likewise explains how carbon affects case hardenability, surface hardness, and case toughness and how case depth influences residual stresses and bending and contact fatigue. It also discusses the effect of quenching methods and addresses the issue of distortion.