The hardenability of steel is governed almost entirely by the chemical composition (carbon and alloy content) at the austenitizing temperature and the austenite grain size at the moment of quenching. This article introduces the methods to evaluate hardenability and the factors that influence steel hardenability and selection. The discussion covers processes involved in Jominy end-quench test for evaluating hardenability. The effect of carbon on hardenability data and the effect of alloys on hardenability during quenching and on the tempering response (after hardening) are also discussed. In addition, the article provides information on the hardenability limits of H-steels after a note on hardenability correlation curves and Jominy equivalence charts.

The results of certain heat treating processes must be verified for case quality and case depth by destructively sectioning a part or parts that were subjected to the process. Test coupons or test pins are often used for diffusion processes such as carburizing, carbonitriding, nitriding, and ferritic nitrocarburizing to provide an accurate heat treating process evaluation. This appendix briefly describes the advantages and selection and design considerations of test coupons. A typical example of the use of test pins for monitoring carburizing and hardening of gears is provided.

This chapter addresses the concept of hardenability by first describing the basic hardening process for steel, starting with austenitization followed by quenching and tempering. The context also serves to clarify the difference between hardenability and hardness, which are often confused. Most of the information in the chapter is of a practical nature, covering application-oriented topics such as isothermal transformation (IT) and continuous transformation (CT) diagrams which are used to predict and control the rate of formation of ferrite, pearlite, and bainite. The chapter also discusses the effect of grain size and alloying elements and explains how Jominy end quench testing is used to evaluate the hardenability of steel.

This chapter contains problems that illustrate the calculation or determination of such items as ideal critical diameter, the Jominy curve, and the severity of quench by methods. It presents solutions for the calculation of the effect of prior austenite grain size, carbon content, chromium content, and molybdenum content on ideal critical diameter. The chapter also contains solutions for calculation of Jominy curves and determination of minimum hardness of quenched steels, tempered hardness, ideal critical diameter, severity of quench, heat treatment, and effect of tempering during heat-up to tempering temperature.

This chapter describes the processes involved in heat treatment of carbon and low alloy steel, high strength low alloy steels, austenitic manganese steels, martensitic stainless steels, and austenitic stainless steels. In addition, precipitation hardening and quench hardening of carbon steel is also covered.

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.

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 provides a practical understanding of heat treatments and how to employ them to optimize the properties and structures of cast irons and steels. It discusses annealing, normalizing, quenching, tempering, patenting, carburizing, nitriding, carbonitriding, and nitrocarburizing. It describes the primary objectives of each treatment along with processing sequences, process parameters, and related phase transformations. The chapter contains more than 100 images, including time-temperature diagrams, transformation curves, data plots, and detailed micro- and macrographs. It also discusses the concepts of hardenability, critical diameter, quench severity, and Jominy testing.

The decomposition of austenite, during controlled cooling or quenching, produces a wide variety of microstructures in response to such factors as steel composition, temperature of transformation, and cooling rate. This chapter provides a detailed discussion on the isothermal transformation and continuous cooling transformation diagrams that characterize the conditions that produce the various microstructures. It discusses the mechanism and process variables of quenching of steel, explaining the factors involved in the mechanism of quenching. In addition, the chapter provides information on the causes and characteristics of residual stresses, distortion, and quench cracking of steel.

This chapter discusses the processes involved in the heat treatment of steel, namely austenitizing, hardening, quenching, and tempering. It begins with an overview of austenitizing of steels by induction heating, followed by a discussion on the processes involved in transformation of the soft austenite into martensite or lower bainite in the hardening operation. The chapter provides information on various quenching systems and a description of quenching techniques, namely austempering, martempering, and patenting. Difficulties associated with hardening of steel are discussed. Further, the chapter describes the equipment used for and principal variables of tempering. It discusses the causes for various forms of embrittlement due to tempering. Information on multiple tempering, protective-atmosphere tempering, and selective tempering are also provided, along with processes involved in selection of tempering temperature. The chapter ends with a section discussing various effects, advantages, and disadvantages of precipitation hardening.

The appendix provides a glossary of practical heat treating terms.

Heat treatment of steel involves a number of processes to condition the microstructure and obtain desired properties. This includes various methods namely, annealing, normalizing, and hardening by quenching and tempering. This chapter focuses on general heat treatment procedures and the applications of particular types or grades of carbon and low-alloy steels. The discussion covers carbon steel classification for heat treating, tempering of quenched carbon steels, and austempering of steel. In addition, the chapter discusses the effects of alloying and hardenability on steel and provides information on martempering of steel.

This chapter first examines the tempering behavior of plain carbon steels and then that of alloy steels. Next, some correlations are examined which allow estimations of the tempered hardness from the chemical compositions, tempering temperature and tempering time. The chapter then describes the effect of tempering on the mechanical properties of plain carbon steels and the microstructure of plain carbon steels. It shows examples of the structure of plain carbon steels. Additionally, the chapter explains the stages and kinetics of tempering in alloy steels and plain carbon steels. It also describes some methods of estimating the hardness. Finally, the chapter discusses the important problem of temper embrittlement.

Hardness tests provide valuable information about the quality of materials and how they are likely to perform in different types of service. This chapter covers some of the most widely used hardness testing methods, including Vickers, Rockwell, and Brinell tests, Shore scleroscope and Equotip hardness tests, and microindentation tests. It describes the equipment and procedures used, discusses the factors that influence accuracy, and provides hardness conversion equations for different types of materials. It also explains how hardness testing sheds light on anisotropy, machinability, wear, fracture toughness, and tensile strength as well as temperature effects, residual stress, and quality control.

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.

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.