The toughness of a material is its ability to absorb energy in the form of plastic deformation without fracturing. It is thus a measure of both strength and ductility. This chapter describes the fracture and toughness characteristics of metals and their effect on component lifetime and failure. It begins with a review of the ductile-to-brittle transition behavior of steel and the different ways to measure transition temperature. It then explains how to predict fracture loads using linear-elastic fracture mechanics and how toughness is affected by temperature and strain rate as well as grain size, inclusion content, and impurities. It also presents the theory and use of elastic-plastic fracture mechanics and discusses the causes, effects, and control of temper embrittlement in various types of steel.

Inclusions and chemical segregation are factors in many process-induced failures involving steel parts. Inclusions are nonmetallic compounds introduced during production; segregation is a type of chemical partitioning that occurs during solidification. This chapter discusses the origins of segregation and inclusions and their effect on the mechanical properties and microstructure of steel. It explains how to identify various types of inclusions and characteristic segregation patterns, such as banding. It also describes the effect of hot work processing on solidification structure and the chemical variations produced by interdendritic segregation.

This chapter is a study of the microstructure of case-hardened steels. It explains what can be learned by examining grain size, microcracking, nonmetallic inclusions, and the effects of microsegregation. It identifies information-rich features, describing their ideal characteristics, the likely cause of variations observed, and their effect on mechanical properties and behaviors. The discussions throughout the chapter are aided by the use of images, diagrams, data plots, and tables.

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.

Analyzing the structure of composite materials is essential for understanding how the part will perform in service. Assessing fiber volume variations, void content, ply orientation variability, and foreign object inclusions helps in preventing degradation of composite performance. This chapter describes the optical microscopy and bright-field illumination techniques involved in analyzing ply terminations, prepreg plies, splices, and fiber orientation to provide the insight necessary for optimizing composite structure and performance.

This chapter covers a broad range of low-carbon steels optimized for structural applications. Low-carbon structural steels are generally considered the highest-strength steels that can be welded without undue difficulty, even in the field. They include mild steels, carbon-manganese and niobium- and vanadium-containing steels, and high-strength low-alloy steels. Chapter 5 discusses the composition, microstructure, and properties of these workhorse materials and explains how to identify the cause of production-related issues such as lamellar tearing and ferrite-pearlite banding. It also describes some of the alloying variations that have been developed to improve machinability and the mechanisms by which they work.

Porosity in aluminum is caused by the precipitation of hydrogen from liquid solution or by shrinkage during solidification, and more usually by a combination of these effects. Nonmetallic inclusions entrained before solidification influence porosity formation and mechanical properties. This chapter describes the causes and control of porosity and inclusions in aluminum castings as well as the combined effects of hydrogen, shrinkage, and inclusions on the properties of aluminum alloys. In addition, it discusses the applications of radiography to reveal internal discontinuities in aluminum.

This chapter covers the emerging practice of quantitative microscopy and its application in the study of the microstructure of metals. It describes the methods used to quantify structural gradients, volume fraction, grain size and distribution, and other features of interest. It provides examples showing how the various features appear, how they are measured, and how the resulting data are converted into usable form. The chapter also discusses the quantification of fracture morphology and its correlation with material properties and behaviors.

This chapter describes cast steel features that may be identified or attributed to component failure during heat treatment or subsequent processing or service, namely porosity (generated by the presence of gas as well as by shrinkage pores), decarburization, cold joint, and inclusions.

This chapter examines the microstructure and properties of annealed and normalized steels containing more than 0.25% carbon. It shows, using detailed micrographs, how incrementally higher levels of carbon affect the structure and distribution of pearlite and how it intermingles with proeutectoid ferrite and cementite. It explains how ferrite and pearlite respond to deformation and how related features such as slip lines, dislocations, shear bands, and kinking can be detected as well as what they reveal. It also describes the structure of patented wires, cast steels, and sintered steels and the morphology of manganese sulfide inclusions in castings.

This chapter provides an overview of the fundamental material- and process-related parameters of quenching on residual stress, distortion control, and cracking. It begins with a description of phase transformations during heating and quenching of steel. This is followed by a section on the effects of materials and quench process design on distortion of steel during heat treating. Details on stress raisers and their role in quench cracking are then presented. The chapter ends with various selected case histories of failures attributed to the quenching process.