Phase Transformations and Heat Treatments of Steels

Bankim Chandra Ray, Rajesh Kumar Prusty, Deepak Nayak

• Editorial: CRC Press
• Año: 2020
• ISBN:
  9780429671593
• Edición: 1
• Categoría: Technologie et ingénierie
• Categoría: Ingénierie minière

The perpetual flow of understanding between phase transformation that controls grain/microstructures and heat treatment which decides the size of grains/microstructures of steels is not well articulated in the perspective of undergraduate students. In Phase Transformations and Heat Treatments of Steels, theories of phase transformation have been used to obtain a desirable phase or combination of phases by performing appropriate heat treatment operations, leading to unification of both the concepts. Further, it includes special and critical heat treatment practices, case studies, local and in-service heat treatments, curative and preventive measures of heat treatment defects for several common and high-performance applications.

Features:

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• Presents fundamentals of phase transformation in steels

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• Analyzes basics of phase transformation due to heat treatment of steel under various environmental conditions

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• Explains application of heat treatment for different structural components

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• Discusses heat treatment defects and detection
• Emphasizes heat treatment of special steels and in-situ heat treatment practices

1

An Introduction to Metals

1.1 Elements, Atoms, and Isotopes

Chemical elements are the fundamental matters of all the materials. These elements are chemically distinct and exhibit unique physical and mechanical properties. The basic representative block of any element is again the atoms, which is comprised of electrons (negatively charged), protons (positively charged), and neutrons (neutral). Both proton and neutron are of almost similar weight, which is around 1.67 × 10⁻²⁷ kg, whereas electron is much lighter in weight around 9.11 × 10⁻³¹ kg. The weight of an atom is almost the same as that of the nucleus, which contains the neutrons and protons. However, the diameter of an atom (~10⁻¹⁰ m or 1 Å) is quite larger than that of the nucleus (~10⁻¹⁴ m). The magnitude of charge of an electron and a proton being equal, the atom contains exactly the same number of both the entities in order to maintain electrical neutrality. The atomic number of an element indicates the number of protons it possesses in a single atom which is also the same as the number of electrons. However, the difference between atomic weight and atomic number usually indicates the average number of neutrons in the atom. The periodic table designed by the Russian scientist Mendeleev is the ideal tool to find out these numbers for any specific element. Almost all the empty cells in the original Mendeleev’s periodic table have been filled due to discovery of new elements in subsequent time. Some elements may have a higher or lower number of neutrons than that of the electrons/protons. As these elements have the same number of electrons/protons, the atomic number is not changed. However, due to different neutrons, the atomic weight becomes different. These are called isotopes. The most common example of isotopes is hydrogen. A high fraction of hydrogen atoms comprises only one proton without any neutron, thus having an atomic weight of 1. A small fraction of hydrogen atoms contains one proton and one neutron giving rise to an atomic weight of 2, which are most commonly known as deuterium. Another small fraction of hydrogen atoms known as tritium (atomic weight 3) possess one proton and two neutrons. In all these cases, the number of protons is 1, and all these elements have the same atomic number, but with different atomic weight. Most of the elements in nature are mixture of such multiple isotopes, and thus, the atomic mass is not always a whole number and is the weighted average of the atomic weights of these isotopes. Continuing with the example of hydrogen, in commonly available hydrogen, the isotopes are mixed in such a proportion that the average atomic weight is 1.008. Taking the examples of iron (Fe), it is naturally available in the form of four stable isotopes. The most abundantly available form of iron is ⁵⁶Fe (~91.754%) followed by ⁵⁴Fe (~5.845%), ⁵⁷Fe (2.119%), and ⁵⁸Fe (0.282%), giving rise to an average atomic mass of 55.85.

1.1.1 Types of Elements

Based on the broad physical and mechanical properties of elements, they are categorized into three groups, i.e., (i) metals, (ii) metalloids, and (iii) nonmetals. Typically, metals are of shiny lustrous appearance, when prepared fresh or fractured. They are good conductors of heat and electricity. Metals can be plastically deformed and are normally malleable (can be made to thin sheets) and ductile (can be drawn into wires). Except mercury, all other metals remain in their solid state at normal room temperature and exhibit crystalline arrangement of atoms. Around 91 out of 118 elements in the periodic table are metal. However, the exact number is not available, as the boundaries between metals, nonmetals, and metalloids fluctuate due to lack of globally accepted basis of categorization. Metals constitute around 25% of the earth’s crust and are inseparable from the present era of civilization. To a large extent, the development of civilization is driven by development in the field of metals and associated products. In the same line, a nonmetal is defined as an element that lacks in the metallic properties. Low density, boiling temperature, and melting temperature are some key physical properties of nonmetals. Most of the nonmetals are gases at room temperature and usually poor conductors of heat and electricity. Some nonmetals are brittle solids at room temperature but good conductors of electricity and heat, e.g., carbon. Metalloids exhibit properties that are in between metals and nonmetals or a mixture of metals and nonmetals. Boron, silicon, germanium, arsenic, antimony, and tellurium are the well-accepted metalloids. ...