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

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

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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

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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

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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 described 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 described 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 transformations 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, microstructure-property interrelationships are incorporated into the descriptions of the various types of steel. Because steels are designed primarily for structures or load-bearing applications, attention is also paid to atomic-scale strengthening, deformation, and fracture mechanisms in the microstructural systems designed for specific applications.

Steels—Definitions Steels are defined primarily by chemical composition, namely, that they are alloys composed of iron and other elements. For the structural and heat treatable steels of major interest in this book, carbon is an essential alloying element; thus steel may be defined as an alloy of iron and small amounts of carbon and other elements. Carbon steels are traditionally bracketed in carbon content, from negligible to about 2 wt%. Alloys with- out carbon are traditionally termed irons, but this boundary is challenged

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by the ability of modern steelmaking to produce ultra-low carbon or interstitial-free steels, with carbon levels in the parts per million. Iron alloys containing more than 2 wt% C are called cast irons because of their dominant iron content, low melting points, and good castability. However, cast irons historically were brittle, a characteristic that differentiated them from steels with good combinations of strength and ductility. Again, the basis for this historical differentiation is challenged by modern technol- ogy: good foundry practice produces nodular and austempered ductile cast irons with good combinations of strength and toughness. Carbon steels fall into two groups: plain carbon steels and alloy steels. Plain carbon steels, for bar and forging applications, are defined as alloys with definite ranges of carbon and a maximum of 1.65 wt% Mn, a maximum of 0.60 wt% Si, a maximum of 0.60 wt% Cu, and maxima in sulfur and phosphorus (Ref 1.11). Immediately, the latter definition shows that elements other than iron and carbon are important for the commercial characterization of steel. Alloy steels also have definite ranges of carbon and limits on manganese, silicon, copper, phosphorus, and sulfur but may also contain definite ranges or minimum quantities of aluminum, chromium, cobalt, niobium, molybdenum, nickel, titanium, tungsten, vanadium, zirco- nium, or any other element added to obtain a desired alloying effect (Ref 1.11). The maximum values of the ranges for the various alloying elements more accurately describe the alloy steels as low-alloy steels. Important alloy systems covered in this book in addition to carbon and alloy steels are the stainless steels and tool steels, each much more heavily alloyed than carbon or alloy steels described previously. The ranges of chemical composition for stainless and tool steels are described in later chapters. The preceding discussion shows the great importance of defining steels by their chemistry and makes the tacit assumption that the chemistry of a steel component is uniform throughout the component. While the latter assumption may be true in the liquid state, in the solid state, alloying and residual elements are distributed non-uniformly throughout the microstructure. Solid steels consist of crystals of iron, ferrite, and/or austenite as described in Chapter 3, “Phases and Structures,” and crystals of other elements incorporated into the matrix of iron crystals to produce unique microstructures. Thus, another view of steels is that they are alloys that consist of crystals of iron and other elements. Non-uniformity in microstructure may be a result of solidification or diffusion-controlled solid- state phase transformations, as described in subsequent chapters. All of the many chemical elements present in liquid steels produced by steelmaking are incorporated somewhere into the crystalline solid microstructure, sometimes by design for beneficial purposes, sometimes causing detrimental effects on performance and fracture. The beneficial effects are attributed to alloying elements (for example, the carbon, manganese, and silicon in carbon steels), and the detrimental effects in carbon steels are attributed to residual or impurity elements (for example, sulfur, phosphorus, and copper), depending on amount and distribution.

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Steel Specifications A widely used system for designating carbon and alloy steel grades has been developed by the American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE). Because AISI does not write specifications, currently only SAE designations are used (Ref 1.12). The SAE system consists of a four-digit AISI/SAE numbering system for the various chemical grades of carbon and alloy steels. The first two digits specify the major alloying elements, and if none are present, as for plain carbon steels, the first two digits are 10. The second two digits specify nominal carbon contents in hundredths of a percent. Table 1.1 presents the SAE system for carbon and alloy steels. Much more information about the chemistry, processing, properties, and quality of the various grades of steel is necessary than just the nominal compositions listed in Table 1.1. For example, because exact amounts

Table 1 1. SAE-AISI designations and alloying elements in carbon and low-alloy carbon steels

Numerals Type of steel and nominal Numerals Type of steel and nominal Numerals Type of steel and and digits alloy content, % and digits alloy content, % and digits nominal alloy content, % Carbon steels Nickel-chromium-molybdenum Chromium (bearing) steels steels 10xx(a) Plain carbon (Mn 1.00 50xx Cr ∆0.50 max) 43xx Ni 1.82; Cr 0.50 and 51xx Cr 1.02 C 1.00 min 11xx Resulfurized 0.80; Mo 0.25 52xxx Cr 1.45 } 12xx Resulfurized and 43BV.xx Ni 1.82; Cr 0.50; Mo Chromium-vanadium steels rephosphorized 0.12 and 0.25; V 15xx Plain carbon (max Mn 0.03 min 61xx Cr 0.60, 0.80, and range: 1.00–1.65) 47xx Ni 1.05; Cr 0.45; Mo 0.95; V 0.10 and 0.20 and 0.35 0.15 min Manganese steels 81xx Ni 0.30; Cr 0.40; Mo Tungsten-chromium steel 13xx Mn 1.75 0.12 86xx Ni 0.55; Cr 0.50; Mo 72xx W 1.75; Cr 0.75 Nickel steels 0.20 Silicon-manganese steels 23xx Ni 3.50 87xx Ni 0.55; Cr 0.50; Mo 25xx Ni 5.00 0.25 92xx Si 1.40 and 2.00; Mn 88xx Ni 0.55; Cr 0.50; Mo 0.65, 0.82, and 0.85; Nickel-chromium steels 0.35 Cr 0 and 0.65 31xx Ni 1.25; Cr 0.65 and 93xx Ni 3.25; Cr 1.20; Mo High-strength low-alloy steels 0.80 0.12 32xx Ni 1.75; Cr 1.07 94xx Ni 0.45; Cr 0.40; Mo 9xx Various SAE grades 33xx Ni 3.50; Cr 1.50 and 0.12 Boron steels 1.57 97xx Ni 0.55; Cr 0.20; Mo 34xx Ni 3.00; Cr 0.77 0.20 xxBxx B denotes boron steel 98xx Ni 1.00; Cr 0.80; Mo Molybdenum steels Leaded steels 0.25 40xx Mo 0.20 and 0.25 xxLxx L denotes leaded steel Nickel-molybdenum steels 44xx Mo 0.40 and 0.52 46xx Ni 0.85 and 1.82; Mo Chromium-molybdenum steels 0.20 and 0.25 41xx Cr 0.50, 0.80, and 48xx Ni 3.50; Mo 0.25 0.95; Mo 0.12, 0.20, Chromium steels 0.25, and 0.30 50xx Cr 0.27, 0.40, 0.50, and 0.65 51xx Cr 0.80, 0.87, 0.92, 0.95, 1.00, and 1.05 (a) The xx in the last two digits of these designations indicates that the carbon content (in hundredths of a percent) is to be inserted. Source: Ref 1.13

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Table 1 .2 UNS Designations for ferrous metals and alloys

UNS designation Description Ferrous metals Dxxxxx Specified mechanical properties steels Fxxxxx Cast irons Gxxxxx SAE and Former AISI carbon and alloy steels (except tool steels) Hxxxxx AISI H-steels Jxxxxx Cast steels Kxxxxx Miscellaneous steels and ferrous alloys Sxxxxx Heat and corrosion resistant (stainless) steels Txxxxx Tool steels Welding filler metals Wxxxxx Welding filler metals, covered and tubular electrodes classified by weld deposit composition

Source: Ref 1.14.

of elements cannot be produced commercially, acceptable ranges of carbon and other elements for a given grade must be specified. Such specifications are written not only by SAE but also by other organizations that represent various user groups of steels. Such organizations and specification systems include the American Petroleum Institute (API), the Steel Founders Society of America (SFSA), Aerospace Materials Specifications (AMS), the American National Standards Institute (ANSI), the American Society of Mechanical Engineers (ASME), the American Society for Test- ing and Materials (ASTM), the American Welding Society (AWS), and Military Specification (MIL) (Ref 1.12, 114). Many countries throughout the world have their own unique specification organizations and designation systems (Ref. 1.15). There is considerable overlap, as well as differences, among the steels in the specifications written by various organizations, not only inthe United States but also in Europe and Asia. As a result, the Unified Num- bering System (UNS) has been developed to cross reference various numbering systems used to identify similar grades of steel. The UNS system is alphanumeric, with the prefix letter describing classes of alloys, and the digits may incorporate SAE digits and other alloy characteristics. Table 1.2 lists UNS designations for ferrous metals and alloys. Cross-references for American and international specifications for similar grades of steel are available in references 1.12 to 1.16.

REFERENCES 1.1 L. E. Samuels, Light Microscopy of Carbon Steels, ASM Interna- tional, 1999 1.2 G. F. Vander Voort, Metallography: Principles and Practice, ASM International, 1984. 1.3 J. Goldstein, D. E. Newbury, D. C. Joy, and C. E. Lyman, Scanning Electron Microscopy and X-ray Microanalysis, Third Edition, Ple- num Press, New York, 2007

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1.4 B. D. Cullity and D. R. Stock, Elements of X-ray Diffraction, Third Edition, Prentice Hall, Upper Saddle River, NJ, 2001 1.5 J. C. Vickerman, Surface Analysis-The Principle Techniques, John Wiley & Sons, 1997 1.6 D. B. Williams and C. B. Carter, Transmission Electron Microscopy, A Text for Materials Science, Second Edition, Springer, 2009 1.7 L. A. Giannuzzi and F. A. Stevie, Introduction to Focused Ion Beams: Instrumentation, Theory, Techniques and Practice, Springer, 2005. 1.8 Electron Backscatter Diffraction in Materials Science, A. J. Schwartz, M. Kumar, and B. L. Adams, Editors, Kluwer Academic/ Plenum Publishers, New York, 2000. 1.9 M. K. Miller, A. Cerezo, M. G. Hetherington, and G. D. W. Smith, Atom Probe Field Ion Microscopy, Oxford University Press, Ox- ford, 1996. 1.10 M. K. Miller, Atom Probe Tomography: Analysis at the Atomic Level, Kluwer Academic/Plenum Publishers, New York, 2000. 1.11 Steel Bar Product Guidelines,AIST, Warrendale, PA, 2010 1.12 Metals and Alloys in the Unified Numbering System, 12th Ed., SAE International, 2012 1.13 Properties and Selection: Irons, Steels, and High-Performance Al- loys, Vol 1, ASM Handbook, ASM International, 1990 1.14 Handbook of Comparative World Steel Standards, 2nd ed., John E. Bringas, Ed., ASTM International, 2002 1.15 Worldwide Guide to Equivalent Irons and Steels, 5th ed., ASM In- ternational, 2006 1.16 C. W. Wegst, Stahlschlüssel (Key to Steel), ASM International, 2013

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