Purpose of Text, Microstructure and Analysis, Steel Definitions, and Specifications

Steels—Processing, Structure, and Performance, Second Edition Copyright © 2015 ASM International® G. Krauss All rights reserved asminternational.org CHAPTER 1 Introduction— Purpose of Text, Microstructure and Analysis, Steel Definitions, and Specifications Purpose of This Book THE PURPOSE OF THIS BOOK is to describe the physical metallurgy, i.e., the processing- structure- property relationships, of steels. Pro- cessing refers to the manufacturing steps used to produce a finished steel product and includes casting, hot and cold work (mechanical and thermo- mechanical processing), and all sorts of heat treatment (thermal processing), some of which involves changes in surface chemistry (thermochemical processing). Steelmaking is the important first step in processing and has evolved over centuries to produce today huge tonnages of high- quality steel. Thus steelmaking, its history, and its effect on the structure of solid steel are discussed briefly in subsequent chapters. Microstructure of Steels Size Scales Together with steel chemistry, processing steps create the many microstructures that may form in each of the great variety of steels. The term 5441G_Steels.indb 1 01/29/2015 3:37:57 PM 2 / Steels—Processing, Structure, and Performance, Second Edition microstructure derives its meaning from the fact that microscopy is re- quired to resolve characteristic features of steel internal structures that range in size from those resolvable with the unaided eye to features resolvable only by light and electron microscopy. The unaided eye can resolve 0.1 mm (0.004 in.), and more closely spaced features require microscopy of some sort. The most appropriate unit for many microstructural features of steel, for example, grain or crystal size, is the micron or micrometer (μm), 10–6 m, or 0.001 mm (0.00004 in.), well below features that are resolvable by eye. The light microscope has a resolution on the order of 0.5 μm and therefore is quite adequate for the characterization of many features of steel microstructures. However, many features that affect performance are too fine to be resolved in the light microscope, for example, fine precipitates and crystal defects, and for the characterization of such features, electron microscopy must be used. In view of the fact that light microscopy was the only technique initially available, finer features now resolvable are often referred to as substructures. The electron microscope can resolve features down to the order of atomic dimensions, around one nanometer (nm), 10–9 m, or 0.001 μm, and therefore effectively covers the size range of structures below that resolvable in the light microscope. Instrumentation The above discussion relates to the size scales of the structural compo- nents that make up a given microstructure. This section briefly describes the many approaches and instruments now available to characterize not only sizes, morphology, and distribution of microstructural features but also the crystallography and the chemistry of the features. The availability of these instruments and how and what they reveal makes possible more and more complete characterization of steel structures. Examples of the structures shown by the various techniques are given throughout this book, and the techniques used to produce the images are identified in the figure captions. Scanning Electron Microscopy . Microstructures on polished and etched steel surfaces, shown by variations in reflected light within the resolution limits of the light microscope, are well characterized (Ref 1.1, 1.2). Scanning Electron Microscopes (SEM) raster electron beams over surface features and are capable of zooming up from macroscopic features through structures on the order of size covered by light microscopy through features finer than resolvable in the light microscope. Good depth of field is provided in SEM, and therefore not only features on polished and subsequently etched surfaces but also very rough surfaces, as produced by fracture, can be evaluated (Ref 1.3). As will be noted in the next section of this Chapter, steels are composed of many chemical elements, both beneficial and detrimental, and therefore the distribution of these elements in steel microstructures is extremely 5441G_Steels.indb 2 01/29/2015 3:37:57 PM Chapter 1: Introduction—Purpose of Text, Microstructure and Analysis, Steel Definitions, and Specifications / 3 important. In electron microscopes chemical compositions of selected microstructural features are determined by high energy electron beam inter- actions that cause inner shell electrons of the various atoms to be ejected with the release of X- ray energies and wavelengths characteristic of the atoms (Ref 1.4). In the scanning electron microscope the characteristic energy spectra are typically measured by solid state detectors in the process referred to as Energy Dispersive Spectroscopy (EDS). In Electron Probe Microanalyzers the spectra are resolved with better resolution by diffraction of the characteristic Xrays from single crystals in a process referred to as Wavelength Dispersive Spectroscopy (WDS) (Ref 1.3). Auger Electron Spectroscopy . The X-ray spectra generated from the atom inner shell electrons in SEM come from volumes relatively deep in specimens, distances on the order of one micron from the specimen surfaces. However, there are electron instruments that are designed to mea- sure spectra produced by ejection of more loosely bound outer shell electrons, electrons termed Auger electrons (Ref 1.5). These low energy electrons come from very close to specimen surfaces, on the order of a few nanometers, and therefore Auger Electron Spectroscopy (AES) is capable of showing thin concentrations of low atomic number elements ex- posed at fracture surfaces in specimens broken under high vacuum in Auger electron microscopes. The latter experimental technique has been very important in showing impurity atom segregation on austenitic grain boundaries, segregation phenomena that are responsible for various types of brittle grain boundary fracture in steels as discussed in Chapter 19, “Low Toughness and Embrittlement Phenomena in Steel.” Transmission Electron Microscopes . The analytical techniques discussed above all involve examination of specimen surfaces. In contrast, Transmission Electron Microscopes (TEM) make possible the evaluation of fine microstructural features within volumes of steel specimens made thin enough to permit the passage of incident high energy electron beams. Images are produced by electron diffraction from the crystal structures of the features, and diffraction patterns that identify crystal types and orien- tation are generated (Ref 1.6). TEM is the only analytical technique that makes possible the direct imaging of crystal defects termed dislocations. Traditionally thin foil specimens have been made by sectioning of bulk specimens and electropolishing. More recently specimens from selected small areas have been removed from bulk samples in Focused Ion Beam (FIB) instruments that use Liquid Metal Ion Sources (LMIS) of gallium to remove and thin specimens for examination in TEM (Ref 1.7). Electron Backscatter Diffraction (EBSD) . More recently, scanning electron microscopes have been developed to characterize variations in crystal orientations between microstructural features and substructures in steels. The technique used is referred to as Electron Backscatter Dif- fraction (EBSD) and is based on precise computer indexing of diffraction patterns produced by backscattered electrons generated by stepping the incident electron beam across specimen surfaces (Ref 1.8). Differences in 5441G_Steels.indb 3 01/29/2015 3:37:57 PM 4 / Steels—Processing, Structure, and Performance, Second Edition orientations of areas as close as 50 nm can be measured in new field emis- sion SEMs. Although EBSD can be applied to thin foil analysis in the TEM, specimen preparation for the application of the technique in the SEM is not as demanding. Atom Probe Tomography (APT) . Atom Probe Tomography (APT) is a powerful technique now widely used to establish atomic level distributions of various types of atoms. Atoms are evaporated from thin needle-shaped specimens by pulsed electric fields or lasers, identified by differences in time of flight of the atoms based on their atomic weight differences, and assigned to locations in three-dimensional, high-m agnification reconstruc- tions of microstructures (Ref 1.9, 1.10) All of the above analytical techniques and instruments have been de- scribed very briefly, primarily to indicate the type of information and how that information from each technique and instrument is obtained. Each technique and the specimen preparation for that technique are based on considerable other published theoretical and practical information, as de- scribed in detail in the listed references. Integration of Microstructure into the Physical Metallurgy of Steel This book begins by describing the phases or crystals of unique chemistry and structure that most commonly form in steels. These phases are arranged by processing to produce characteristic microstructures. The microstructures produced by solidification, and the solid- state transforma- tions that produce microstructures consisting of ferrite, pearlite, bainite, and martensite, are then considered, followed by chapters that describe types of steels that are based on the production of the various types of microstructures. Properties and performance depend directly on microstructure, and therefore,

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