MEE 313 Phase Diagram PDF

3.0. PHASE DIAGRAM. A Phase Diagram is a chart showing The thermodynamic conditions of a substance at different Pressures & Temperatures. The regions around the lines show the phase of the substance of the lines show where the phases are in equilibrium. Common components of a phase diagram are line...

3.0. PHASE DIAGRAM. A Phase Diagram is a chart showing The thermodynamic conditions of a substance at different Pressures & Temperatures. The regions around the lines show the phase of the substance of the lines show where the phases are in equilibrium. Common components of a phase diagram are lines of equilibrium or phase boundaries which refers to lines that mark conditions under which multiple phases can coexist at equilibrium. Phase transition can occur along lines of equilibrium. Triple points are points on phase diagrams where lines of equilibrium intercept. They mark conditions at which three different phases can coexist. Eg water phase diagram has a triple point corresponding to the single temperature and pressure at which solid, liquid of gaseous water can coexist in a stable equilibrium (273.16k and a partial vapour pressure of 611-657 Pa). Solidus line is the temperature below which the substance is stable in a solid State. Liquidus line is the temperature above which the substance is stable in liquid state. There may be a gap between solidus & liquidus. Within the gap, the substance coexists as a mixture of crystals & liquid.  3.1. Types of Phase Diagrams. Two Dimensional Phase Diagram: The simplest phase diagram are Pressure-Temperature diagram of a single simple substance such as water. The axis correspond to pressure and temperature. The phase diagram shows in Pressure-Temperature space, the lines of equilibrium or phase boundaries between the three phases of solid, liquid, and gas. Three Dimensional Phase Diagram: It is possible to have 3D graphs showing thermodynamics quantities. For example for a single component, a 3D Cartesian coordinate type graph can show temperature on one axis, pressure on the second axis & specific volume on third axis. Such a 3-D graph is called PV-T diagram. The equilibrium condition are shown on a curved surface in 3D with areas for solid, liquid & vapour phase and areas where solid and liquid and vapour or liquid vapour coexist in equilibrium. The line on the diagram called triple point is where solid, liquid & vapour all coexist in equilibrium. 3.2. Binary Mixtures. Other much more complex types of phase diagrams can be constructed, particularly when more than one pure component is present. In that case, concentration becomes an important variable. Phase diagrams with more than two dimensions can be constructed that shows the effect of more than two variables on the phase of the substances. Phase diagrams can use other variables in addition to or in place of temperature, pressure & composition. For example, the strength of applied electrical or magnetic field and they can also involve substances that take on more than just 3 states of matter. One type of phase diagram plots temperature against relative concentration of two substances in a binary mixture called binary phase diagram. as shown below. Such a mixture can be either a solid solution, eutectic, peritectic etc. These two types of mixture result in a very different graph. Another type of binary diagram is a boiling point diagram for a mixture of two components, that is a chemical compound. In addition to the above mentioned type of phase diagram, there are thousands of other possible combinations. Some of the major features of phase diagram include Congruent points where a solid phase directly transforms into liquid. There's also a Peritectoid, a point where two solid phases combine into one solid phase during cooling. The inverse of this, when one solid phase transform into two solid phases is called Eutectoid. A complex phase diagram of great technological importance is that of Iron - Carbon system for less than 7% Carbon. 3.3. Lever Rule It's a tool used to determine weight percent of each phase of a binary equilibrium phase diagram. It is used to determine percent weight of liquid & solid phases for a given binary composition & temperature in the liquidus & solidus line. The % weight of element B at the liquidus is given by Wl and that of element B at the solidus is given by Ws. The % weight of solid & liquid can be calculated. Lever rule has to determine the compositions of the alloy at given temperatures. The ends of the T- line show the compositions of two phases that exist in equilibrium with each other at the given temperature From the diagram we see that x & L will exist. The x phase is Y% B and the L phase is X% B at this temperature, T. Remember though that the overall composition of the sample is unchanged- we are only having the composition of the constituent phases within the sample. Consider another alloy of Composition, Co. at temperature Tx °C, we can know the phases present - B&L. To get the composition of these phases, we draw T-line from the point to the nearest phase diagram boundaries. It is clear that L is X% B and B is Y% B. If the temperature of the alloy is reduced from Tx to Ty, the composition of the two phases will vary. The composition of L & B phases have both decreased in weight % B to L is X'%B and B is Y'% B. This implies that both the L&B phases are getting richer in A as the sample is cooled. Now that we know the composition of the two phases, we need to find out how much of each phase exist at the given temperature. The ratio of the two phases. 3.4. Gibb's Phase Rule. Gibb's Phase Rule provide theoretical foundation based on thermodynamics for characterizing the chemical state of a system and predicting the equilibrium relation of phases present as a function of physical condition such as pressure & temperature. It describes the possible number of degree of freedom in a closed system at equilibrium in terms of the maximum number of stable phases, 'P' and the number of system components, 'C' F=C-P+2 F= Degree of freedom, P = number of phases, C = number of system components.. In other words, the number of degree of freedom for a system at equilibrium is the number of intensive variables (often taken as pressure, temperature and composition fractions) that may be arbitrarily specified without changing the number of phases. In a region with 'P' stable phases, the values of C - P + 2 state variables can be changed independently and preserving the same set of stable phases. If F = 0, then an invariant equilibrium is defined (i.e. equilibrium can be attend only for a single set of values of all the state variables. For example, 3 phase equilibrium assemblage of binary system at constant pressure. If F = 1, then a uni-variant equilibrium is defined (i.e. the state of stable phases depends on the value of one stable variable only e.g. 2 phase equilibrium assemblage of binary system at constant pressure. 4.0. SOLD SOLUTION & ALLOY SYSTEMS. Solid solution is a homogeneous Crystalline structure in which one or more atoms or molecules may be partly substituted for the original atoms and molecules without changing the structure. The elements or compound in the solid solution share a common lattice. Examples of solid solutions are metal alloys. Much of the steel used in construction for example is actually a solid solution of iron & carbon. The Carbon atoms which fit neatly within the iron crystal lattice add strength to its structure. Naturally, the structure of metallic alloys is more complicated than that of pure metals and depend mainly on how their constituents can interact with one another. In some instances, the simple substances (constituents of an alloy) do not interact chemically with one another in the solid state. The structure of such an alloy is a mechanical mixture of particles or grains of the two (or more) constituents. In other cases, the constituents of an alloy can interact chemically with one another & form chemical compounds or can be mutually soluble & form solutions. Sometimes, intermediate phases can form in an alloy polls to A. Mechanical mixture: two constituents, A & B, will form a mechanical mixture in an alloy if they are mutually in soluble in each other in the solid state & can't form compounds with each other by chemical interaction. These conditions given, an alloy will contain pure crystals of A & B which may be readily seen in the microstructure, provided that they are large enough. The radiogram of the alley will reveal the presence of two lattices of A & B. If the properties of A crystal & B crystal in the alloy were tested separately, this result will be fully identical with the properties of pure metals A & B. The mechanical properties of such alloy depend on the relative contents of the constituents & also on the size of shape of grains and are usually somewhere between the characteristics of the pure constituents. B. Chemical Compounds: upon formation of chemical compounds: 1. The ratio of the number of atoms of the two elements exactly correspond to the Stoichiometric ratio, which may be expressed by a simple formula (in general case, a chemical compound of two elements may be designated as AnBm) 2. A typical crystal Lattice (different from the lattices of the constituents) will form with an ordered arrangement of atoms of the two components. A chemical compound may also be characterized by a definite melting point(or dissociation point) and an ability to change its properties abruptly with a change in composition. If a chemical compound is formed by metallic elements only, the points of its crystal lattice will be occupied by positive ions which are held in place by electron gas, thus the lattice will have what's called a metallic bond. C. Solid Solution Based on a Component of an Alloy: when molten, most metallic alloys employed in industries are homogeneous liquids i.e. liquid solutions. On passing to solid state, many of them retain their homogeneity & therefore solubility. The solid phase that forms on crystallization on such an alloy is called solid solution. The solid solution contains two or more elements, whereas metallographic analysis of such alloys dictates uniform grains in them as its pure metals Radiographic analysis reveals in solid solutions only one type of crystal lattice as in pure metals. Consequently, in contrast to metallic structure is a single phase system consisting of crystals of one type & having a definite type of crystal lattice. As distinct from chemical compounds, a solid solution can exist within a definite range of concentration rather than at a strictly definite ratio of the constituents. The structure of the single- component solid solution is such that the lattice of the base metal (solvent) contains atoms of the other substance (solute). Two principally different case possible here: 1.Substitutional Solid Solutions: The dissolution of constituent B in base metal A occurs through partial replacement (substitution) of the A atom by B atoms in the base metal lattice. This is usually by elements close to each other on the periodic table where their atomic sizes are about equal e.g. zine & Copper forming brass. 2. Interstitial Solid Solutions: atoms of solute C occupies the places between atoms of base metal A. The atoms of solute C are much smaller in size than the atoms of the base metal e.g. iron & Carbon where carbon atoms occupy places between iron atoms. With either the substitutional or interstitial solid solutions, atoms of solute are distributed at random of the lattice of the solvent. When the solid solution forms, the solvent element retains its original Crystal lattice. Atoms of the solute element distort the lattice of the solvent and change the average size of its elementary cell. Substitutional Solid Solutions may be either limited or unlimited. With unlimited solubility, any quantity of A atoms can be substituted by B atoms. Therefore, if the concentration of B atoms is increased, more & more B atoms will occupy lattice sides in place of A atoms, until all A atoms will be replaced. Thus, the composition of alloys can change gradually from 100% A to 100% B. This is passible however only when the two metals/elements have identical crystal structure i.e. both being isomorphic. If two metals with identical crystal lattices defer in atomic radii, the crystal lattice on the formation of a solid solution is severely distorted which brings about an accumulation of elastic energy. When the distortion reaches a definite limit, the crystal lattice becomes unstable and the solubility limit sets in. Thus, the second condition for the formation of unlimited solid solution is that the difference in the atomic radii should be sufficiently small. Finally, it has been noted that unlimited solubility is observed mainly in elements occupying close positions in the periodic table, i.e. similar in structure of valence shell and physical nature. If the metal being melted belongs to two distant groups, they are often liable to form chemical compounds rather than solid solution. If two metals do not conform to the conditions mentioned, they will be limitedly soluble in each other. it has been found that a solubility is lower with larger difference in the size of atoms and properties of two components which form the solution. Limited solubility usually diminishes with reducing temperature. D. Order Solid Solution: As noted, atoms of the solute are distributed in common solid solutions at random. Under particular circumstances, however, they occupy positions in the lattice sites, i.e. their disordered arrangement changes to an ordered one. The process is called ordering, and solutions with ordered arrangement of solute atoms, ordered solid solution. The process of ordering may be complete or incomplete. In complete, all atoms occupy predetermined positions in an ordered solid solution, while in the incomplete case, part of the atoms occupy predetermined positions & other part is arranged at random. Ordered solid solution can be regarded as intermediate phases between chemical compounds & solid solutions. With complete ordering, these phases resemble chemical compounds because: 1. They contain a definite number of atoms that can be expressed by a formula. 2. Atoms are arranged regularly in the lattice. These phases can be related to the solid solutions since they retain the original lattice of the solvent. 4.1. Correlation Between Alloy & Forms Of Constitutional Diagram Since the forms of Constitutional diagram depends on the phases that can be formed by two or more components & the properties of an alloy are also determined by the components or phases formed by the constituents of that alloy, then it is evident that a definite correlation should exist between the form of the Constitutional Diagram & the properties of the alloy. In the figure, there are 4 principal forms of Constitutional Diagram & the corresponding tendencies in variations of properties of alloys at varying concentrations. 1.With formation of mixtures (a), the properties of alloy varies by a linear law (additively). Therefore, the properties of alloys are intermediate between the properties of the pure components. 2. With formation of solid solution (b), the properties of alloy vary by a curvilinear law & some properties especially the electrical resistivity, may differ substantially from those in pure components. Therefore, electrical resistivity of a mechanical mixture may be only slightly greater than impure components, whereas the electrical resistivity of a solid solution may differ quite substantially. This is why the decomposition of the solid solution into two or more phases usually results in an increase of electric resistivity. 3. With the formation of limited solid solution(c), the properties in the range of concentrations corresponding to single phase solid solution vary as a curvilinear law & in the two-phase region of the diagram, the properties vary by a linear law, with the extreme points on the line representing the properties of pure phases of maximally saturated solution forming the mixture. 4. With the formation of a compound (d), the concentration of a chemical compound correspond to the maximum (minimum) of a curve (a bend of a straight line). This bend point is called the singular point 5.0. IRON & STEEL PRODUCTION. The production of steel at an integrated iron & steel plant is accomplished using several interelated processes. The major operations are 1. Coke Production* 2.Sinter Production* 3.Iron Production* 4. Iron Preparation 5. Steel Production. 6. Semi-finished Product Preparation 7. Finished Product Preparation 8. Heat & Electricity Supply 9. Handling & Transport of Raw, Intermediate of Waste materials. Coke is a fuel with few impurities and a high carbon content, usually made from coal. It is a solid carbonaceous material derived from destructive distillation of low-ash, low-sulphur, litiminous coal. Cokes made from coal are grey, hard and porous. Coke is one of the raw materials used in burning furnace for making iron. Others are the iron ore itself, sinter & limestone. Coke is made by heating coal until it becomes almost pure carbon. Coke gas, a by-product of industrial coke production from pit. Coal is created by high temperature dry distillation of coking coals in the absence of oxygen. The gas mainly consists of hydrogen (50-60%), methane (15-50%) and a small percentage of Carbon monoxide, Carbon & nitrogen. The sintering process converts fine-size raw materials including iron ore, coke breeze, limestone, mill scale, and fine dust into an agglomerated product, Sinter of suitable size for charging into the burning surface. The raw materials are sometimes mixed with water to provide a cohesive matrix, and then placed on a continuous, traveling grate called the sinter strand. A burner hood, at the beginning of a Sinter strand ignites the coke in the mixture, after which the combustion is self supporting and it provides sufficient heat, 1300-1480°C to cause surface melting & agglomeration of the mix. The fused Sinter is discharged at the end of the sinter strand where it is crushed & screened. Sinter product is cooled in open air or in a circular cooler with water sprays or mechanical fans. Iron is produced in burning furnaces by the reduction of iron bearing materials with a hot gas. The large, refractory line furnace is charged through its top with iron as ore, pellets and/or sinter; flux as limestone, dolomite, and siter; and coke for fuel. Iron oxides, coke and fluxes react with the black air to form molten reduced iron, Carbon monoxide (CO) & slag. The molten iron & slag collect in the hearth at the base of the burning furnace. The by-product gas is collected through offtakes located at the top of the burning surface and is recovered for use as fuel. The molten iron and slag are removed, or cast from the burning furnace periodically. The casting process