National Bureau of Standards Library, N.W. Bldg NBS MONOGRAPH 18 Heat Treatment and Properties of Iron and Steel U.S. DEPARTMENT OF COMMERCE NATIONAL BUREAU OF STANDARDS THE NATIONAL BUREAU OF STANDARDS Functions and Activities The functions of the National Bureau of Standards are set forth in the Act of Congress, March 3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials, devices, and structures; advisory services to government agencies on scientific and technical problems; inven- tion and development of devices to serve special needs of the Government; and the development of standard practices, codes, and specifications. 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Astin, Dirictor Heat Treatment and Properties of Iron and Steel Thomas G. Digges and Samuel J. '^senberg National Bureau of Standards Monograph 18 Issued October 3, 1960 Supersedes Circular 495 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington 25, D.C, - Price 35 cents Contents Page Pag€ 1. Introduction 1 8. Practical considerations 21 2. Properties of iron 1 8.1. Furnaces and salt baths 21 2.1. Transformation temperatures 1 a. Protective atmospheres 2] 2.2. Mechanical properties 2 b. Temperature measurement and con- 3. Alloys of iron and carbon 2 trol 21 3.1. Iron-carbon phase diagram 2 8.2. Quenching media and accessories of mechanical properties with 3.2. Correlation 8.3. Relation of design to heat treatment microstructures of slowly cooled carbon 9. Nomenclature and chemical compositions of steels. 21 steels 4 9.1. Structural steels 2ci 4. Decomposition of ausenite 5 9.2. Tool steels 2(i 4.1. Isothermal transformation 6 9.3. Stainless and heat resisting steels 3( a. To pearlite 6 b. To bainite 7 10. Recommended heat treatments 3( c. To martensite 7 10.1. Structural steels 3( 4.2. Continuous cooling 7 10.2. Tool steels 3J 5. Heat treatment of steels 8 10.3. Stainless and heat resisting steels 3<' 5.1. Annealing 9 a. Group I—Hardenable chromium a. Full annealing 9 steels (martensitic and magnetic). 3.: b. Process annealing 9 b. Group II—-Nonhardenable chro- c. Spheroidizing 10 mium steels (ferritic and mag- 5.2. Normalizing 11 netic) 3.; 5.3. Hardening 11 c. Group III—-Nonhardenable chro- a. Effect of mass 11 mium-nickel and chromium- 5.4. Tempering 12 nickel-manganese steels (austen- 5.5 Case hardening 13 itic and nonmagnetic) .3c I a. Carburizing 13 11. Properties and uses of steels 3fe b. Cyaniding 14 11.1. Structural steels 3f c. Carbonitriding 15 a. Plain carbon structural steels 36 d. Nitriding 15 b. AUoy structural steels 3^ 5.6. Surface hardening 15 11.2. Tool steels ye. a. Induction hardening 15 11.3. Stainless and heat resisting steels 3S b. Flame hardening 16 I chromium 1 5.7. Special treatments 16 a. Group —Hardenable steels (martensitic and magnetic). 2,1 a. Austempering 16 II Nonhardenable chro- b. Martempering 16 b. Group — mium steels (ferritic and mag- c. Cold treatment 16 netic) 3f 6. Hardenability 17 c. III Nonhardenable chro- 7. Heat treatment of cast irons 18 Group — mium-nickel and chromium- 7.1. Relieving residual stresses (aging) 20 steels (austen- 7.2. Annealing 20 nickel-manganese itic nonmagnetic) a. Malleabilizing 20 and d. Semi-austenitic stainless steels 3f 7.3. Normalizing, quenching, and tempering 20 7.4. Special heat treatments 21 12. Selected references. 4( II Heat Treatment and Properties of Iron and Steel Thomas G. Digges and Samuel J. Rosenberg This ^lonograph has been prepared to give an understanding of heat treatment princi- pally to those unacquainted with this subject. To this end, the basic theoretical and practi- cal principles involved in the heat treatment of iron and steel are presented in simplified form. 1. Introduction 2. Properties of Iron The Xational Bureau of Standards receives Since iron is the basic element of steel, a knowl- many requests for general information concerning edge of some of its properties is a prerequisite to the heat treatment of iron and steel and for direc- an understanding of the fimdamental pi"inciples tions and explanations of such processes. This underlying the heat treatment of steels. Monograph has been prepared to answer such inquiries and to give in simplified form a working 2.1. Transformation Temperatures knowledge of the basic theoretical and practical principles involved in the heat treatment of iron If a molten sample of pure u"on were allowed to and steel. The effects of various treatments on cool slowly and the temperature of the iron were the structures and mechanical properties of these measured at regular intervals, an idealized (equi- materials are described. Many theoretical aspects librium) time-temperature plot of the data would are discussed only briefly or omitted entirely, and appear as shown in figure 1. The discontinuities in some instances, technical details have been (temperature arrests) in this curve are caused by neglected for sunplicity. The present Monograph physical changes in the U'on. supersedes Circular 495, which was published in The first arrest at 2,800° F marks the tempera- 1950. ture at which the iron freezes. The other arrests Heat treatment may be defined as an operation (known as transformation temperatures or criti- or combination of operations that involves the cal points) mark temperatm-es at which certain heating and cooling of a solid metal or alloy for the internal changes take place in the solid iron. purpose of obtaining certain desirable conditions Some of these temperatures are very important in ^ or properties. It is usually desired to preserve, as the heat treatment of steel. nearly as possible, the form, dimensions, and The atoms in all solid metals are arranged "I m surface of the piece being treated. in some definite geometric (or crystallographic) m Steels and cast irons are essentially alloys of pattern. The atoms in iron, immediately after Mjl iron and carbon, modified by the presence of other freezing, are arranged in what is termed the body- m elements. Steel may be defined as an alloy of u'on centered cubic system. In this crystal structure 3j[i and carbon (with or without other alloymg ele- ments) containing less than about 2.0 percent of I carbon, usefidly malleable or forgeable as initially isj cast. Cast iron may be defined as an alloy of u'on and carbon (with or without other alloying ele- ments) containing more than 2.0 percent of carbon, not usefully malleable or forgeable as initially cast. For reasons that will be apparent later, the divid- ing line between steels and cast irons is taken at 2.0 percent of carbon, even though certam special steels contain carbon in excess of this amount. In addition to carbon, fom^ other elements are nor- J maUy present in steels and in cast irons. These are mangnaese, silicon, phosphorus, and sulfiu". Steels may be broadly classified into two types, (1) carbon and (2) alloy. Carbon steels owe their properties chiefly to the carbon. They are fre- quently called straight or plain carbon steels. Alloy steels are those to which one or more alloying elements are added in sufficient amounts to modify certain properties. The properties of cast iron TIME also may be modified by the presence of alloying elements—such irons are called alloy cast irons. Figure 1. Idealized cooling curve for pure iron. the unit cell consists of a cube with an ii'on atom 3. Alloys of Iron and Carbon at each of the eight corners and another in the center (fig. 2, a). Each of the many individual The properties of iron are affected very mark- grains (crystals) of which the solid metal is com- edly by additions of carbon. It should be realized posed is built up of a very large number of these that in discussing iron-carbon alloys, we actually unit cells, all oriented alike in the same grain. are dealing with plain carbon steels and cast irons.