Seetharaman Fundamentals of Metallurgy

1. Auflage 2005
ISBN: 978-1-84569-094-6
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 576 Seiten

Reihe: Woodhead Publishing Series in Metals and Surface Engineering

As product specifications become more demanding, manufacturers require steel with ever more specific functional properties. As a result, there has been a wealth of research on how those properties emerge during steelmaking. Fundamentals of metallurgy summarises this research and its implications for manufacturers.The first part of the book reviews the effects of processing on the properties of metals with a range of chapters on such phenomena as phase transformations, types of kinetic reaction, transport and interfacial phenomena. Authors discuss how these processes and the resulting properties of metals can be modelled and predicted. Part two discusses the implications of this research for improving steelmaking and steel properties.With its distinguished editor and international team of contributors, Fundamentals of metallurgy is an invaluable reference for steelmakers and manufacturers requiring high-performance steels in such areas as automotive and aerospace engineering. It will also be useful for those dealing with non-ferrous metals and alloys, material designers for functional materials, environmentalists and above all, high technology industries designing processes towards materials with tailored properties. - Summarises key research and its implications for manufacturers - Essential reading for steelmakers and manufacturers - Written by leading experts from both industry and academia

Autoren/Hrsg.

Seetharaman, S.

Leseproben

2 Thermodynamic aspects of metals processing
R.E. Aune; S. Seetharaman 2.1 Introduction
According to Albert Einstein,1 thermodynamics is marked by its simplicity, the different kinds of things it relates to and the wide area of its applicability. Thermodynamics is by definition a subject that describes the link between heat and motion. With the development of physics and chemistry and the application of mathematical principles, the subject area today covers all forms of energy including, thermal, electrical, mechanical energies and the impact of the same in the change of the states of the systems we are interested in. The area of thermodynamics stretches from the atomistics to macro systems including huge metallurgical reactors. The applications of the concepts of thermodynamics to metallurgy reached significant advancements during the past four decades with stalwarts like Wagner from Germany, Darken, Chipman and Elliott from the US, Richardson in UK as well as Hillert from Sweden. The present chapter is intended to cover the areas of thermodynamics that are of relevance to metallurgy, particularly, the processing of metals and the properties of metals and alloys. For further reading, the readers are requested to resort to the classical textbooks in this area presented in the bibliography at the end of this chapter. The present chapter is also to an extent inspired by the course literature in thermodynamics at the Royal Institute of Technology by the present authors and their predecessors. Since thermodynamics is a subject that can be almost philosophical, it is imperative to have clear definitions of the various terms involved in order to apply its concepts in metallurgical applications. As mentioned earlier, we are concerned with the changes of state of a system due to energy impact. Thermodynamics does not provide any information as to the rate of this change. 2.2 Basic concepts in thermodynamics
2.2.1 State and state functions
System A system in thermodynamics is a limited but well-defined part of the universe focused on presently. The rest of the universe can be considered as the surrounding. The aim is to examine the interaction between the system and the universe in a simple but well-specified way. An open system can exchange with its surroundings both matter and energy. A closed system on the other hand can exchange only energy with the surroundings, but not matter. An isolated system can neither exchange matter nor energy with the surroundings. A homogeneous system is identical in physical and chemical properties in all parts of the system, as for example, liquid steel at 1600 °C. A system that has differences in physical and chemical properties within the system is referred to as a heterogeneous system, as for example, water and ice at 0 °C. A system is composed of different types of molecules. For example, air, as a system consists of nitrogen, oxygen and other minor gas molecules. These are referred to as components of a system. Considering a gas system consisting of three molecular types, H2, O2 and H2O, there is mutual reaction between these molecular types, namely: H2+O2=2H2O   (2.1) The number of these molecular types in the system can be altered by introducing some of these molecules from the surroundings or change, for example, the total pressure of the system. Thus, it is enough to define two of the three types of molecules, viz. the system has two components. A heterogeneous system that may have different physical properties with the same components throughout may have different phases. For example, water at its freezing point contains a solid (ice) and a liquid, and thus, two phases. Within each phase, the molar properties have the same value at every point. Generally, the three phases, solid, liquid and gas are considered in a heterogeneous system. In metallurgy, it is often necessary to consider different allotropic modifications in the solid state, as for example, a, ? or d iron with different crystal structures. Water at the triple point will have three phases, viz. ice, water and steam. State The macroscopic definition of ‘state’, which is relevant to metallurgy, is defined by its macroscopic properties like temperature, pressure, volume, vapour pressure, viscosity, surface tension, etc. With a focus on the chemical properties, surface energy is also related to the state of the system. For example, in defining 10 moles of nitrogen, it is important to define the temperature and pressure, as the other properties get defined implicitly. State properties The properties stated above along with others being defined in the following chapters that define the state of the system are the state properties. The properties that are additive are called the extensive properties, as for example mass, volume and energy and, in the case of a homogeneous system, are proportional to the total mass. On the other hand, properties, to which a value could be assigned at each point in the system are intensive properties. Some of the common intensive properties are temperature, pressure, density etc. Since the ratio of two extensive properties is independent of total mass, and may be assigned a value at a point, these ratios fall under intensive properties. Examples of such properties are molar properties like volume per mole. In order to differentiate these from intensive properties like temperature and pressure, the latter are often classified as potentials. It is useful to introduce, at this point, the concept of chemical potential, represented usually as µ, which is the potential corresponding to the chemical energy in the system. The equality of the potentials and the inequality in molar properties between phases is illustrated in Fig. 2.1. 2.1 Two phases in thermodynamic equilibrium. A system is in a state of mechanical equilibrium if the pressure at all points in the system is the same. If the system has no thermal gradients, it is supposed to be in thermal equilibrium. The system is in chemical equilibrium if the chemical potential is uniform throughout the system. The system is in complete thermodynamic equilibrium if it is having mechanical, thermal as well as chemical equilibria. The properties of the system can be varied by interaction between the system and the surrounding. Mass transfer could change the material content of the system while heat transfer could alter the energy content. In order to define the macroscopic state of the system unequivocally, all the properties of the system need be known. On the other hand, due to the interdependency of the properties, it is sufficient to define only a few. For example, in the case of a gas in a container, it is only necessary to define the temperature and pressure. The volume of the system, V in m3.mol- 1 gets implicitly defined by the gas law. Thus, in this case, we can define pressure, P, N.m- 2 and temperature, T, K are the independent variables and volume is the dependent variable. The gas law =NRT/P   (2.2) where N is the number of moles in a system is an equation of state. A schematic representation of the various states of a system, where the variables are T, P and V, is presented in Fig. 2.2. In some cases, especially in the case of phase transformations, it is sometimes advantageous to define a new term ‘inner state variable’3 as, for example, the degree of change due to conditions imposed on the system, as represented by the symbol ?. Since thermodynamics is concerned with the changes associated with the interactions between the system and the surroundings, the degree of change could be a useful parameter in following the path of a reaction. This is illustrated in Fig. 2.3. 2.2 The states of a defined amount of gas–schematic diagram.2 The letters 'a', 'b', 'c' and 'd' refer to different states of the system. P1 and P2 refer to two different pressures where P1 > P2. T1 and T2 are the two temperatures, T1 < T2. 2.3 Gradual change of an inner state variable caused by a quick change in temperature.3 2.2.2 The first law of thermodynamics
Energy change between the system and its surroundings is defined by the first law of thermodynamics. It is also variously considered as a definition of energy or a law of conservation of energy. U=dQ+dW   (2.3) where dU is the change in the internal energy of the system (without defining the microscopic state), dQ is the energy added to the system and dW is the work imposed on the system. (Please note that the symbol ‘d’ is used for infinitesimal, defined change in the state property of a system while, ‘d’ is used for an infinitesimal, undefined amount of energy or work coming in from the surroundings). It is to be noted that the term ‘energy’ includes all forms of energy including thermal, electrical, chemical and other known forms. Similarly, the term...