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Roop Lal Engineering Materials & Metallurgy (Asst. Prof.) Delhi Technological University Soft Copy Available at: www.RoopLalRana.com UNIT III Solidification: Phases in metal system, lever rule, solidification of metal and alloys, solid solution, eutectic, eutectoid and inter – metallic compounds, Iron carbon equilibrium diagram, TTT – diagram. Heat Treatment: Principles and purpose of treatment of plain carbon steels, annealing, normalizing, hardening, tempering, isothermal treatment, case hardening – carburizing, nitriding etc, precipitating hardening of aluminium alloys. INTRODUCTION The understanding of phase diagrams for alloy systems is extremely important because there is a strong correlation between microstructure and mechanical properties, and the development of microstructure of an alloy is related to the characteristics of its phase diagram. In addition, phase diagrams provide valuable information about melting, casting, crystallization, and other phenomena. Component: The term component is frequently referred to pure metals and/or compounds of which an alloy is composed. For example, in brass, the components are Cu and Zn. Solute and solvent, solute and solvent are terms that are commonly employed.“Solvent” represents the element or compound that is present in the greatest amount; on occasion, solvent atoms are also called host atoms. “Solute” is used to denote an element or compound present in a minor concentration. System : The term system has two meanings. First, “system” may refer to a specific body of material under consideration (e.g., a ladle of molten steel). Or it may relate to the series of possible alloys consisting of the same components, but without regard to alloy composition (e.g., the iron–carbon system). Solid Solution: In this case two metals are completely soluble in solid as well as in liquid state. They have the same type of lattice and similar atomic size. Copper and Nickel form an isomorphous system.The concept of a solid solution consists of atoms of at least two different types; the solute atoms occupy either substitutional or interstitial positions in the solvent lattice, and the crystal structure of the solvent is maintained. The addition of impurity atoms to a metal will result in the formation of a solid solution and/or a new second phase, depending on the kinds of impurity, their concentrations, and the temperature of the alloy. A solid solution forms when, as the solute atoms are added to the host material, the crystal structure is maintained, and no new structures are formed. Perhaps it is useful to draw an analogy with a liquid solution. If two liquids, soluble in each other (such as water and alcohol)

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Page 1: Roop Lal Engineering Materials & Metallurgy - …rooplalrana.com/wp-content/uploads/2016/02/Unit-III-EMM-Lect.pdf · Roop Lal Engineering Materials & Metallurgy ... UNIT III Solidification:

Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

UNIT III Solidification: Phases in metal system, lever rule, solidification of metal and alloys, solid solution, eutectic, eutectoid and inter – metallic compounds, Iron carbon equilibrium diagram, TTT – diagram. Heat Treatment: Principles and purpose of treatment of plain carbon steels, annealing, normalizing, hardening, tempering, isothermal treatment, case hardening – carburizing, nitriding etc, precipitating hardening of aluminium alloys.

INTRODUCTION The understanding of phase diagrams for alloy systems is extremely important because there is a strong correlation between microstructure and mechanical properties, and the development of microstructure of an alloy is related to the characteristics of its phase diagram. In addition, phase diagrams provide valuable information about melting, casting, crystallization, and other phenomena. Component: The term component is frequently referred to pure metals and/or compounds of which an alloy is composed. For example, in brass, the components are Cu and Zn. Solute and solvent, solute and solvent are terms that are commonly employed.“Solvent” represents the element or compound that is present in the greatest amount; on occasion, solvent atoms are also called host atoms. “Solute” is used to denote an element or compound present in a minor concentration. System : The term system has two meanings. First, “system” may refer to a specific body of material under consideration (e.g., a ladle of molten steel). Or it may relate to the series of possible alloys consisting of the same components, but without regard to alloy composition (e.g., the iron–carbon system). Solid Solution: In this case two metals are completely soluble in solid as well as in liquid state. They have the same type of lattice and similar atomic size. Copper and Nickel form an isomorphous system.The concept of a solid solution consists of atoms of at least two different types; the solute atoms occupy either substitutional or interstitial positions in the solvent lattice, and the crystal structure of the solvent is maintained. The addition of impurity atoms to a metal will result in the formation of a solid solution and/or a new second phase, depending on the kinds of impurity, their concentrations, and the temperature of the alloy. A solid solution forms when, as the solute atoms are added to the host material, the crystal structure is maintained, and no new structures are formed. Perhaps it is useful to draw an analogy with a liquid solution. If two liquids, soluble in each other (such as water and alcohol)

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

are combined, a liquid solution is produced as the molecules intermix, and its composition is homogeneous throughout. A solid solution is also compositionally homogeneous; the impurity atoms are randomly and uniformly dispersed within the solid. Eutectic Type: When two metals are completely soluble in the liquid state but partly or completely insoluble in the solid state, is termed as eutectic type. Fe-C, Al-Mn, Pb-Sn form an eutectic system. Peritectic Type: In this case liquid and solid combine to form a new solid. The melting points of two metals differ considerably. Ag and Pt form such a system. In the eutectic system, the crystals of β-solid solution precipitated at the beginning of solidification react with the liquid alloy having definite composition to form new crystals of α - solid solution. In the peritectic reaction, two phases are used to produce one different phase with reaction just the opposite of eutectic reaction. The peritectic reaction similar to other systems of solidification of different metals, but is comparatively less common. This reaction also occurs at constant temperature. One can write this reaction as

The temperature at which peritectic reaction occurs is denoted by Td. Below Td, the liquid phase disappears upon completion of the transformation. Pt-Ag is a good example of peritectic reaction. There is also peritectoid reaction. This is the reaction of two solids into a third solid. One can represent this reaction as

Monotectic Type: In this case the two liquid solutions are not soluble in each other over a certain composition range, i.e., there is a miscibility gap in liquid state between the two metals. In this type one liquid decomposes into another liquid solid. Cu and Pb form monotectic system. Eutectoid Type: In this one solid decomposes into two different solids. Obviously, solid to solid transformation takes place. Fe-C, Cu-Zn, Al-Cu, Cu-Sn, etc form eutectoid system. Eutectoid transformation takes place at a constant temperature and the products of transformation are present as intimate mixture having appearance under the microscope. The eutectoid reaction is of the form:

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

The eutectoid structure, frequently lamellar, is produced by precipitation from the solid solution. The crystal structure of new phase is known as the Widmanstatten structure. The eutectoid reaction occurs in Fe-C diagram in which austenite (a solid solution C in g - Fe) is decomposed into pearlite. The most important example of the use of this reaction is heat treatment of steel. Solubility Limit. For many alloy systems and at some specific temperature, there is a maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution; this is called a solubility limit. The addition of solute in excess of this solubility limit results in the formation of another solid solution or compound that has a distinctly different composition. To illustrate this concept, consider the sugar– water (C12H22O11) system. Initially, as sugar is added to water, a sugar–water solution or syrup forms. As more sugar is introduced, the solution becomes more concentrated, until the solubility limit is reached, or the solution becomes saturated with sugar. At this time the solution is not capable of dissolving any more sugar, and further additions simply settle to the bottom of the container. Thus, the system now consists of two separate substances: a sugar–water syrup liquid solution and solid crystals of un-dissolved sugar. This solubility limit of sugar in water depends on the temperature of the water and may be represented in graphical form on a plot of temperature along the ordinate and composition (in weight percent sugar) along the abscissa, as shown in Figure 1. Along the composition axis, increasing sugar concentration is from left to right, and percentage of water is read from right to left. Since only two components are involved (sugar and water), the sum of the concentrations at any composition will equal 100 wt%. The solubility limit is represented as the nearly vertical line in the figure. For compositions and temperatures to the left of the solubility line, only the syrup liquid solution exists; to the right of the line, syrup and solid sugar coexist. The solubility limit at some temperature is the composition that corresponds to the intersection of the given temperature coordinate and the solubility limit line. For example, at 20oC the maximum solubility of sugar in water is 65 wt%. As Figure 1 indicates, the solubility limit increases slightly with rising temperature

Figure 1 The solubility of sugar (C12H22O11) in a sugar–water syrup.

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

PHASES : Concept of a Phase : A phase may be defined as a homogeneous portion of a system that has uniform physical and chemical characteristics. Every pure material is considered to be a phase. For example, the sugar–water syrup solution discussed above is one phase, and solid sugar is another. Each has different physical properties (one is a liquid, the other is a solid); furthermore, each is different chemically (i.e., has a different chemical composition); one is virtually pure sugar, the other is a solution of H2O and C12H22O11. If more than one phase is present in a given system, each will have its own distinct properties, and a boundary separating the phases will exist across which there will be a discontinuous and abrupt change in physical and/or chemical characteristics. When two phases are present in a system, it is not necessary that there be a difference in both physical and chemical properties; a disparity in one or the other set of properties is sufficient. When water and ice are present in a container, two separate phases exist; they are physically dissimilar (one is a solid, the other is a liquid) but identical in chemical makeup. Sometimes, a single-phase system is termed “homogeneous.” Systems composed of two or more phases are termed “mixtures” or “heterogeneous systems.” Most metallic alloys and, for that matter, ceramic, polymeric, and composite systems are heterogeneous. Ordinarily, the phases interact in such a way that the property combination of the multiphase system is different from, and more attractive than, either of the individual phases. MICROSTRUCTURE Many times, the physical properties and the mechanical behavior of a material depend on the microstructure. In metal alloys, microstructure is characterized by the number of phases present, their proportions, and the manner in which they are distributed or arranged. The microstructure of an alloy depends on the alloying elements present, their concentrations, and the heat treatment of the alloy. After appropriate polishing and etching, the different phases may be distinguished by their appearance. For example, for a two-phase alloy, one phase may appear light and the other phase dark. When only a single phase or solid solution is present, the texture will be uniform, except for grain boundaries that may be revealed. PHASE EQUILIBRIA Equilibrium is another essential concept that is best described in terms of a thermodynamic quantity called the free energy. In brief, free energy is a function of the internal energy of a system, and also the randomness or disorder of the atoms or molecules. A system is at equilibrium if its free energy is at a minimum under some specified combination of temperature, pressure, and composition. In a macroscopic sense, this means that the characteristics of the system do not change with time but persist indefinitely; that is, the system is stable. A change in temperature, pressure, and/or composition for a system in equilibrium will result in an increase in the free energy and in a possible spontaneous change to another state whereby the free energy is lowered. The term phase equilibrium, refers to equilibrium as it applies to systems in which more than one phase may exist. Phase equilibrium is reflected by a constancy with time in the phase

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

characteristics of a system. For example suppose that a sugar–water syrup is contained in a closed vessel and the solution is in contact with solid sugar at 20oC. If the system is at equilibrium, the composition of the syrup is 65 wt% C12H12O11 – 35 wt% (Figure 1), and the amounts and compositions of the syrup and solid sugar will remain constant with time. If the temperature of the system is suddenly raised—say, to100oC — this equilibrium or balance is temporarily upset in that the solubility limit has been increased to 80 wt% C12H12O11 (Figure 1). Thus, some of the solid sugar will go into solution in the syrup. This will continue until the new equilibrium syrup concentration is established at the higher temperature. This sugar–syrup example illustrates the principle of phase equilibrium using a liquid–solid system. In many metallurgical and materials systems of interest, phase equilibrium involves just solid phases. Often in the case of solid systems, that a state of equilibrium is never completely achieved because the rate of approach to equilibrium is extremely slow; such a system is said to be in a nonequilibrium or metastable state. A metastable state may persist indefinitely, experiencing only extremely slight and almost imperceptible changes as time progresses. Often, metastable structures are of more practical significance than equilibrium ones. For example, some steel and aluminum alloys rely for their strength on the development of metastable microstructures during carefully designed heat treatments. Thus not only is an understanding of equilibrium states and structures important, but also the speed or rate at which they are established and the factors that affect the rate must be considered. ONE-COMPONENT (OR UNARY) PHASE DIAGRAMS Much of the information about the control of the phase structure of a particular system is conveniently and concisely displayed in what is called a phase diagram, also often termed an equilibrium diagram. Now, there are three externally controllable parameters that will affect phase structure—viz. temperature, pressure, and composition— and phase diagrams are constructed when various combinations of these parameters are plotted against one another. Perhaps the simplest and easiest type of phase diagram to understand is that for a one-component system, in which composition is held constant (i.e., the phase diagram is for a pure substance); this means that pressure and temperature are the variables. This one-component phase diagram (or unary phase diagram) [sometimes also called a pressure–temperature (or P –T) diagram] is represented as a two-dimensional plot of pressure (ordinate, or vertical axis) versus temperature (abscissa, or horizontal axis). Most often, the pressure axis is scaled logarithmically. We illustrate this type of phase diagram and demonstrate its interpretation using as an example the one for H2O, which is shown in Figure 2. Here it may be noted that regions for three different phases—solid, liquid, and vapor—are delineated on the plot. Each of

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

Figure 2 Pressure–temperature phase diagram for H2O Intersection of the dashed horizontal line at 1 atm pressure with the solid-liquid phase boundary (point 2) corresponds to the melting point at this pressure (T = 0OC ). Similarly, point 3, the intersection with the liquid-vapor boundary, represents the boiling point (T = 100oC). the phases will exist under equilibrium conditions over the temperature–pressure ranges of its corresponding area. Furthermore, the three curves shown on the plot (labeled aO, bO, and cO) are phase boundaries; at any point on one of these curves, the two phases on either side of the curve are in equilibrium (or coexist) with one another. That is, equilibrium between solid and vapor phases is along curve aO—likewise for the solid-liquid, curve bO, and the liquid vapor, curve cO. Also, upon crossing a boundary (as temperature and/or pressure is altered), one phase transforms to another. For example, at one atmosphere pressure, during heating the solid phase transforms to the liquid phase (i.e., melting occurs) at the point labeled 2 on Figure 2 (i.e., the intersection of the dashed horizontal line with the solid-liquid phase boundary); this point corresponds to a temperature of 0oC. Of course, the reverse transformation (liquid-to-solid, or solidification) takes place at the same point upon cooling. Similarly, at the intersection of the dashed line with the liquid-vapor phase boundary [point 3 (Figure 2), at 100oC] the liquid transforms to the vapor phase (or vaporizes) upon heating; 0oC condensation occurs for cooling. And, finally, solid ice sublimes or vaporizes upon crossing the curve labeled aO. As may also be noted from Figure 2, all three of the phase boundary curves intersect at a common point, which is labeled O (and for this H2O system, at a temperature of 273.16 K and a pressure of 6.04 X 10-3 atm). This means that at this point only, all of the solid, liquid, and vapor phases are simultaneously in equilibrium with one another. where three phases are in equilibrium, is called a triple point; sometimes it is also termed an invariant point inasmuch as its position is distinct, or fixed by definite values of pressure and temperature. Any deviation from this point by a change of temperature and/or pressure will cause at least one of the phases to disappear. Pressure-temperature phase diagrams for a number of substances have been determined experimentally, which also have solid, liquid, and vapor phase regions. In those instances when

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

multiple solid phases exist, there will appear a region on the diagram for each solid phase, and also other triple points. Binary Phase Diagrams Binary phase diagram is one in which temperature and composition are variable parameters, and pressure is held constant—normally 1 atm. Here we are concerned with binary alloys—those that contain two components. If more than two components are present, phase diagrams become extremely complicated and difficult to represent. An explanation of the principles governing and the interpretation of phase diagrams can be demonstrated using binary alloys even though most alloys contain more than two components. Binary phase diagrams represent the relationships between temperature and the compositions and quantities of phases at equilibrium, which influence the microstructure of an alloy. Many microstructures develop from phase transformations, the changes that occur when the temperature is altered (ordinarily upon cooling).This may involve the transition from one phase to another, or the appearance or disappearance of a phase. Binary phase diagrams are helpful in predicting phase transformations and the resulting microstructures, which may have equilibrium or non equilibrium character BINARY ISOMORPHOUS SYSTEMS Possibly the easiest type of binary phase diagram to understand and interpret is the type that is characterized by the copper–nickel system (Figure 3a). Temperature is plotted along the ordinate, and the abscissa represents the composition of the alloy, in weight percent (bottom) and atom percent (top) of nickel. The composition ranges from 0 wt% Ni (100 wt% Cu) on the left horizontal extremity to 100 wt% Ni (0 wt% Cu) on the right. Three different phase regions, or fields, appear on the diagram, an alpha ( α ) field, a liquid (L) field, and a two-phase (α + L) field. Each region is defined by the phase or phases that exist over the range of temperatures and compositions delimited by the phase boundary lines. The liquid L is a homogeneous liquid solution composed of both copper and nickel. The a phase is a substitutional solid solution consisting of both Cu and Ni atoms, and

Figure 3(a) : The copper–nickel phase diagram.

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

Figure 3(b) : A portion of the copper–nickel phase diagram for which

compositions and phase amounts are determined at point B.

having an FCC crystal structure. At temperatures below about 1080OC, copper and nickel are mutually soluble in each other in the solid state for all compositions. This complete solubility is explained by the fact that both Cu and Ni have the same crystal structure (FCC), nearly identical atomic radii and electro negativities. The copper–nickel system is termed isomorphous because of this complete liquid and solid solubility of the two components. A couple of comments are in order regarding nomenclature. First, for metallic alloys, solid solutions are commonly designated by lowercase Greek letters (α, β, γ etc.). Furthermore, with regard to phase boundaries, the line separating the L and α + L phase fields is termed the liquidus line, as indicated in Figure 3a; the liquid phase is present at all temperatures and compositions above this line. The solidus line is located between the α and α + L regions, below which only the solid phase exists. For Figure 3a, the solidus and liquidus lines intersect at the two composition extremities; these correspond to the melting temperatures of the pure components. For example, the melting temperatures of pure copper and nickel are 1085oC and 1453oC respectively. Heating pure copper corresponds to moving vertically up the left-hand temperature axis. Copper remains solid until its melting temperature is reached. The solid-to-liquid transformation takes place at the melting temperature, and no further heating is possible until this transformation has been completed. For any composition other than pure components, this melting phenomenon will occur over the range of temperatures between the solidus and liquidus lines; both solid and liquid phases will be in equilibrium within this temperature range. For example, upon heating an alloy of composition 50 wt% Ni–50 wt% Cu (Figure 3a), melting begins at approximately 1280oC (2340oF); the amount of liquid phase continuously increases with temperature until about 1320oC (2410oF), at which the alloy is completely liquid.

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

INTERPRETATION OF PHASE DIAGRAMS For a binary system of known composition and temperature that is at equilibrium, at least three kinds of information are available: (1) the phases that are present, (2) the compositions of these phases, and (3) the percentages or fractions of the phases. The procedures for making these determinations will be demonstrated using the copper–nickel system. Phases Present The establishment of what phases are present is relatively simple. One just locates the temperature–composition point on the diagram and notes the phase(s) with which the corresponding phase field is labeled. For example, an alloy of composition 60 wt% Ni–40 wt% Cu at 1100oC would be located at point A in Figure 3a; since this is within the α region, only the single α phase will be present. On the other hand, a 35 wt% Ni–65 wt% Cu alloy at 1250oC (point B) will consist of both α and liquid phases at equilibrium. Determination of Phase Compositions The first step in the determination of phase compositions is to locate the temperature –composition point on the phase diagram. Different methods are used for single - and two-phase regions. If only one phase is present, the procedure is trivial: the composition of this phase is simply the same as the overall composition of the alloy. For example, consider the 60 wt% Ni –40 wt% Cu alloy at 1100oC (point A, Figure 3a). At this composition and temperature, only the phase α is present, having a composition of 60 wt% Ni – 40 wt% Cu. For an alloy having composition and temperature located in a two-phase region, the situation is more complicated. In all two-phase regions (and in two-phase regions only), one may imagine a series of horizontal

lines, one at every temperature; each of these is known as a tie line, or sometimes as an

isotherm. These tie lines extend across the two-phase region and terminate at the phase boundary lines on either side. To compute the equilibrium concentrations of the two phases, the following procedure is used:

1. A tie line is constructed across the two-phase region at the temperature of the alloy.

2. The intersections of the tie line and the phase boundaries on either side are noted.

3. Perpendiculars are dropped from these intersections to the horizontal composition axis, from which the composition of each of the respective phases is read.

Determination of Phase Amounts The relative amounts (as fraction or as percentage) of the phases present at equilibrium may also be computed with the aid of phase diagrams. Again, the single - and two - phase situations must be treated separately. The solution is obvious in the single phase region: Since only one phase is present, the alloy is composed entirely of that phase; that is, the phase fraction is 1.0 or, alternatively, the percentage is 100%. From the previous example for the 60 wt% Ni – 40 wt% Cu alloy at (point A in Figure 3a), only the phase α is present; hence, the alloy is completely or 100% If the composition and temperature position is located within a two-phase region,

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

things are more complex. The tie line must be utilized in conjunction with a procedure that is

often called the lever rule (or the inverse lever rule), which is applied as follows:

1. The tie line is constructed across the two - phase region at the temperature of the alloy.

2. The overall alloy composition is located on the tie line. 3. The fraction of one phase is computed by taking the length of tie line from the

overall alloy composition to the phase boundary for the other phase, and dividing by the total tie line length.

4. The fraction of the other phase is determined in the same manner. 5. If phase percentages are desired, each phase fraction is multiplied by 100. When

the composition axis is scaled in weight percent, the phase fractions computed using the lever rule are mass fractions—the mass (or weight) of a specific phase divided by the total alloy mass (or weight). The mass of each phase is computed from the product of each phase fraction and the total alloy mass.

In the employment of the lever rule, tie line segment lengths may be determined either by direct measurement from the phase diagram using a linear scale, preferably graduated in millimeters, or by subtracting compositions as taken from the composition axis.

Consider again the example shown in Figure 3b, in which at 1150oC both α and liquid phases are present for a 35 wt% Ni–65 wt% Cu alloy. The problem is to compute the fraction of each of the α and liquid phases. The tie line has been constructed that was used for the determination of α and L phase compositions. Let the overall alloy composition be located along the tie line and denoted as Co and mass fractions be represented by WL and Wα for the respective phases. From the lever rule, may be computed according to

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

Of course, identical answers are obtained if compositions are expressed in weight percent copper instead of nickel. Thus, the lever rule may be employed to determine the relative amounts or fractions of phases in any two-phase region for a binary alloy if the temperature and composition are known and if equilibrium has been established. Its derivation is presented as an example problem. It is easy to confuse the foregoing procedures for the determination of phase compositions and fractional phase amounts; thus, a brief summary is warranted. Compositions of phases are expressed in terms of weight percents of the components (e.g., wt% Cu, wt% Ni). For any alloy consisting of a single phase, the composition of that phase is the same as the total alloy composition. If two phases are present, the tie line must be employed, the extremities of which determine the compositions of the respective phases. With regard to fractional phase amounts (e.g., mass fraction of the α or liquid phase), when a single phase exists, the alloy is completely that phase. For a two-phase alloy, on the other hand, the lever rule is utilized, in which a ratio of tie line segment lengths is taken. BINARY EUTECTIC SYSTEMS Another type of common and relatively simple phase diagram found for binary alloys is shown in Figure 7 for the copper–silver system; this is known as a binary eutectic phase diagram. A number of features of this phase diagram are important and worth noting. First, three single-phase regions are found on the diagram: α, β and liquid. The α phase is a solid solution rich in copper; it has silver as the solute component and an FCC crystal structure. The β -phase solid solution also has an FCC structure, but copper is the solute. Pure copper and pure silver are also considered to be α and β phases, respectively. Thus, the solubility in each of these solid phases is limited, in that at any temperature below line BEG only a limited concentration of silver will dissolve in copper (for the α phase), and similarly for copper in silver (for the β phase). The solubility limit for the α phase corresponds to the boundary line, labeled CBA, between the α/( α + β) and α/( α + L) phase regions; it increases with temperature to

Figure 7 The copper–silver phase diagram.

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

a maximum [8.0 wt% Ag at 779oC (1434oF ) at point B, and decreases back to zero at the melting temperature of pure copper, point A [1085 oC (1985 oF )]. At temperatures below 779oC (1434oF ), the solid solubility limit line separating the α and α + β phase regions is termed a solvus line; the boundary AB between the α and α + L fields is the solidus line, as indicated in Figure 7. For the phase, both solvus and solidus lines also exist, HG and GF, respectively, as shown. The maximum solubility of copper in the phase, point G (8.8 wt% Cu), also occurs at 779oC (1434oF ). This horizontal line BEG, which is parallel to the composition axis and extends between these maximum solubility positions, may also be considered a solidus line; it represents the lowest temperature at which a liquid phase may exist for any copper–silver alloy that is at equilibrium. There are also three two-phase regions found for the copper–silver system (Figure 7): α + L, β + L and α + β . The α and β phase solid solutions coexist for all compositions and temperatures within the α + β phase field; the α + and β+ phases also coexist in their respective phase regions. Furthermore, compositions and relative amounts for the phases may be determined using tie lines and the lever rule as outlined previously. As silver is added to copper, the temperature at which the alloys become totally liquid decreases along the liquidus line, line AE; thus, the melting temperature of copper is lowered by silver additions. The same may be said for silver: The introduction of copper reduces the temperature of complete melting along the other liquidus line, FE. These liquidus lines meet at the point E on the phase diagram, through which also passes the horizontal isotherm line BEG. Point E is called an invariant point, which is designated by the composition CE and temperature TE for the copper–silver system, the values of CE and TE are 71.9 wt% Ag and 779oC (1434oF ) , respectively. THE GIBBS PHASE RULE The construction of phase diagrams as well as some of the principles governing the conditions for phase equilibrium are dictated by laws of thermodynamics. One of these is the Gibbs phase rule. This rule represents a criterion for the number of phases that will coexist within a system at equilibrium, and is expressed by the simple equation

P + F = C + N (16)

where P is the number of phases present. The parameter F is termed the number of degrees of freedom or the number of externally controlled variables (e.g., temperature, pressure, composition) which must be specified to completely define the state of the system. Expressed another way, F is the number of these variables that can be changed independently without altering the number of phases that coexist at equilibrium.The parameter C in Equation 16 represents the number of components in the system. Components are normally elements or stable compounds and, in the case of phase diagrams, are the materials at the two extremities of the horizontal compositional axis (e.g. H2O, and C12H22O11 and Cu and Ni for the phase diagrams shown in Figures1 and 3a, respectively). Finally, N in Equation 16 is the number of non compositional variables (e.g., temperature and pressure). Let us demonstrate the phase rule by applying it to binary temperature – composition phase diagrams, specifically the copper–silver

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system, Figure 7. Since pressure is constant (1 atm), the parameter N is 1—temperature is the only non compositional variable. Equation 16 now takes the form

P + F = C + 1 (17) Furthermore, the number of components C is 2 (viz. Cu and Ag), and

P + F = 2 + 1 Or

F = 3 – P Consider the case of single-phase fields on the phase diagram (e.g., α, β and liquid regions). Since only one phase is present, P = 1 and F = 3 – P F = 3 – 1 = 2 This means that to completely describe the characteristics of any alloy that exists within one of these phase fields, we must specify two parameters; these are composition and temperature, which locate, respectively, the horizontal and vertical positions of the alloy on the phase diagram. For the situation wherein two phases coexist, for example, and phase regions, Figure 7, the phase rule stipulates that we have but one degree of freedom since

F = 3 – P F = 3 – 2 = 1 Thus, it is necessary to specify either temperature or the composition of one of the phases to completely define the system.

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THE IRON–IRON CARBIDE (Fe–Fe3C) PHASE DIAGRAM Pure iron, upon heating, experiences two changes in crystal structure before it melts. At room temperature the stable form, called ferrite, or α iron, has a BCC crystal structure. Ferrite experiences a polymorphic transformation to FCC austenite, or γ iron, at 912oC (1674oF ). This austenite persists to 1394 oC ( 2541 oF), at which temperature the FCC austenite reverts back to a BCC phase known as δ ferrite, which finally melts at 1538oC ( 2800 oF). All these changes are apparent along the left vertical axis of the phase diagram.

Figure 24 The iron–iron carbide phase diagram.

The composition axis in Figure 24 extends only to 6.70 wt% C; at this concentration the intermediate compound iron carbide, or cementite (Fe3C), is formed, which is represented by a vertical line on the phase diagram. Thus, the iron–carbon system may be divided into two parts: an iron-rich portion, as in Figure 24, and the other (not shown) for compositions between 6.70 and 100 wt% C (pure graphite). In practice, all steels and cast irons have carbon contents less than 6.70 wt% C; therefore, we consider only the iron–iron carbide system. Figure 24 would be more appropriately labeled the Fe–Fe3C phase diagram, since Fe3C is now considered to be a

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component, Convention and convenience dictate that composition still be expressed in “wt% C” rather than “wt% Fe3C”; 6.70 wt% C corresponds to 100 wt% Fe3C. Carbon is an interstitial impurity in iron and forms a solid solution with each of α and δ ferrites, and also with austenite, as indicated by the α, δ, and γ single phase fields in Figure 24. In the BCC α ferrite, only small concentrations of carbon are soluble; the maximum solubility is 0.022 wt% at 727oC(1341oF). The limited solubility is explained by the shape and size of the BCC interstitial positions, which make it difficult to accommodate the carbon atoms. Even though present in relatively low concentrations, carbon significantly influences the mechanical properties of ferrite. This particular iron–carbon phase is relatively soft, may be made magnetic at temperatures below768oC(1414oF )., and has a density of 7.88 g/cm3. The austenite, or γ phase of iron, when alloyed with carbon alone, is not stable below727oC ( 1341oF), as indicated in Figure 24. The maximum solubility of carbon in austenite, 2.14 wt%, occurs at 1147 oC(2097 oF). This solubility is approximately 100 times greater than the maximum for BCC ferrite, since the FCC interstitial positions are larger, and, therefore, the strains imposed on the surrounding iron atoms are much lower. As the discussions that follow demonstrate, phase transformations involving austenite are very important in the heat treating of steels. In passing, it should be mentioned that austenite is nonmagnetic. The ferrite is virtually the same as ferrite, except for the range of temperatures over which each exists. Since the ferrite is stable only at relatively high temperatures, it is of no technological importance and is not discussed further. Cementite (Fe3C) forms when the solubility limit of carbon in α ferrite is exceeded below 727oC and (1341oF) (for compositions within the α + Fe3C phase region). As indicated in Figure 24, Fe3C will also coexist with the phase between 727oC and (1341oF and 2097oF). Mechanically, cementite is very hard and brittle; the strength of some steels is greatly enhanced by its presence. Strictly speaking, cementite is only metastable; that is, it will remain as a compound indefinitely at room temperature. However, if heated to between 650 and 700oC (1200 and 1300oF) for several years, it will gradually change or transform into α iron and carbon, in the form of graphite, which will remain upon subsequent cooling to room temperature. Thus, the phase diagram in Figure 24 is not a true equilibrium one because cementite is not an equilibrium compound. However, inasmuch as the decomposition rate of cementite is extremely sluggish, virtually all the carbon in steel will be as Fe3C instead of graphite, and the iron–iron carbide phase diagram is, for all practical purposes, valid. The two-phase regions are labeled in Figure 24. It may be noted that one eutectic exists for the iron–iron carbide system, at 4.30 wt% C and 1147oC ( 2097oF ); for this eutectic reaction,

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or, upon cooling, the solid γ phase is transformed into α iron and cementite. The eutectoid phase changes described by above Equation are very important, being fundamental to the heat treatment of steels, as explained in subsequent discussions. Ferrous alloys are those in which iron is the prime component, but carbon as well as other alloying elements may be present. In the classification scheme of ferrous alloys based on carbon content, there are three types: iron, steel, and cast iron. Commercially pure iron contains less than 0.008 wt% C and, from the phase diagram, is composed almost exclusively of the ferrite phase at room temperature. The iron–carbon alloys that contain between 0.008 and 2.14 wt% C are classified as steels. In most steels the microstructure consists of both α and Fe3C phases. Upon cooling to room temperature, an alloy within this composition range must pass through at least a portion of the - phase field; distinctive microstructures are subsequently produced. Although a steel alloy may contain as much as 2.14 wt% C, in practice, carbon concentrations rarely exceed 1.0 wt%. Cast irons are classified as ferrous alloys that contain between 2.14 and 6.70 wt% C. However, commercial cast irons normally contain less than 4.5 wt% C.

Figure 26 Schematic representations of the microstructures for an

iron–carbon alloy of eutectoid composition (0.76 wt% C) above and below the eutectoid temperature.

The microstructure for this eutectoid steel that is slowly cooled through the eutectoid temperature consists of alternating layers or lamellae of the two phases (α and Fe3C) that form simultaneously during the transformation. This microstructure, represented schematically in

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Figure 26, point b, is called pearlite because it has the appearance of mother of pearl when viewed under the microscope at low magnifications. The thick light layers are the ferrite phase, and the cementite phase appears as thin lamellae most of which appear dark. Many cementite layers are so thin that adjacent phase boundaries are so close together that they are indistinguishable at this magnification, and, therefore, appear dark. Mechanically, pearlite has properties intermediate between the soft, ductile ferrite and the hard, brittle cementite. MODIFIED IRON-CARBON PHASE DIAGRAM

Fig. 27 Modified iron-carbon equilibrium or phase diagram. MICRO-CONSTITUENTS OF Fe-C SYSTEM We can observe the various constituents of Fe and Steel produced due to decomposition of austenite under a microscope. The following are the important micro-constituents of Fe-C system:

(i) Austenite: This is the solid interstitial solution of carbon in gamma iron (Feγ). It has a FCC lattice in which the interstices are larger than in the BCC lattice, because of

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which the solubility of carbon in Feγ is much higher and attains 2.14%. Austenite is ductile and has a higher strength than ferrite (HB 160-200) at a temperature 20-25°C. On cooling below 723°C it starts transforming into pearlite and ferrite. We may note that the austenite in a eutectoid steel is unstable at all temperatures.

(ii) Ferrite: This is a soft and ductile phase. Basically, this is a BCC iron phase with very limited solubility of carbon. Like austenite, ferrite may have other alloying elements in substitutional solid solution. Ferrite is the name given to pure iron crystals. The solubility of carbon in ferrite is 0.025 at 723°C. Ferrite has the following mechanical properties: st = 300 MPa, δ = 40%, γ = 70%, KCU = 2.5 MJ/m2, and HB = 80 -100. Below the critical temperature, the slow cooling of low carbon steel produces ferrite structure. Ferrite is very soft and highly magnetic and does not harden when cooled rapidly.

(iii) Cementite (Cem): This is essentially an iron carbide Fe3C (of almost constant

composition). It contains 6.69% C and has a complex rhombic lattice and under normal conditions it is extremely hard in nature and brittle. It is weakly ferromagnetic, but loses this property on heating to 210°C. The melting temperature of cementite is difficult to determine, since cementite decomposes on heating. In experiments with laser-beam heating, its melting point has been measured to be 1260°C. The brittleness and hardness of cast iron is mainly controlled by the presence of cementite in it.

(iv) Pearlite: Pearlite is a mechanical mixture of about 87% ferrite and 13% cementite having a two-phase microstructure and found in some steels and cast irons. Pearlite results from the transformation of austenite of eutectoid composition and consists of alternating layers (or lamellae) of a-ferrite and cementite. A steel with 0.8% carbon is wholly pearlite, with less than 0.8% carbon is hypoeutectoid and with more than 0.8% carbon is hypereutectoid steel. The former contains ferrite and pearlite and is soft while the latter contains pearlite and cementite which are hard and brittle.

(v) Bainite: This is a ferrite cementite aggregate, i.e., an austenitic transformation

product found in some steels and cast irons. This is formed by the growth of a ferrite nucleus. It forms at temperatures between those at which pearlite and martensite transformations occur. It is the product of isothermal decomposition of austenite. The microstructure consists of a-ferrite and a fine dispersion of cementite. Bainite is present in two forms: the feathery bainite obtained in the upper part of the temperature range and needle-like or accicular bainite produced by lower reaction temperature.

(vi) Martensite: This is a body-centered, tetragonal metastable iron phase produced by entrapping carbon on decomposition of austenite when cooled rapidly. Obviously, this is supersaturated in carbon that is the

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product of a diffusionless (athermal) transformation from austenite. It is essentially a supersatured interstitial solid solution of carbon in Fea. Martensite crystals have a lamellar shape and grow at an enormous rate equal to the speed of sound in steel (roughly 500 m/s). The growth of martensite crystals can be hampered by boundaries of austenitic grains or martensite lamellae that have formed earliers. Martensite is magnetic and is made of a needle-like fibrous mass and contains carbon upto 2% and extremely hard and brittle. We may note that the decomposition of austenite below 320°C starts the formation of martensite.

(vi) Troostite: This is obtained by quenching tempering martensite and composed of cementite phase in a ferrite matrix that can be resolved by a light microscope. In comparison to martensite, this is less hard and brittle and also weaker than martensite. It is also produced by cooling the metal slowly until transformation begins and then rapidly to prevent its completion at about 580-550°C. At the transformation temperature, the interlamellar spacing decreases to 1 × 10–7 – 2 ×10–7 m. It has dark appearance on etching.

(vii) Sorbite: This is also produced by the transformation of tempered martensite at a temperature of 640 - 590°C. It is produced when steel is heated at a fairly rapid rate from the temperature of the solid solution to normal room temperature. Its properties are intermediate between those of pearlite and troostite and has good strength. It is practically pearlite. We may note that troostite and sorbite division of pearlitic structures is quite conventional, since the mixture dispersity increases monotonously with decreasing temperature of transformation.

Formation and Decomposition of Austenite Pure iron, upon heating, experiences two changes in crystal structure before it melts. At room temperature the stable form, called ferrite (or α - iron), has a BCC structure. Ferrite experiences a polymorphic transformation to FCC austenite (or γ - iron), at 812°C. This austenite persists to 1394°C, at which temperature the FCC austenite reverts back to a BCC phase known as δ -ferrite, which finally melts at 1538°C. Formation of austenite from pearlite is the transformation that takes place on heating steel slightly above the equilibrium temperature (Ac1), the free energy of pearlite is more than that of austenite. When an austenite with 0.8% carbon (called eutectoid composition) is cooled below the eutectoid temperature (723°C), it decomposes into a mixture of ferrite and cementite (or iron cabide). This eutectoid mixture is called pearlite. Its microstructure contains alternate layers of ferrite and cementite. TYPES AND PROPERTIES OF CARBON-STEELS: The properties of carbon-steels depend upon the structure, i.e., iron-carbon system. Ferrite is relatively a soft and ductile material, whereas pearlite is harder and less ductile. Obviously, carbon steels having these substances will have therefore different properties. The hardness and tensile strength increases with an increase in carbon content. However, toughness,

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machinability and ductility decreases with an increase in carbon content (Fig. 28). However, we note that toughness, machinability, and ductility decreases with an increase in carbon content.

Fig. 28 Effect of carbon percentage (% C) on the structure of carbon-steel

Classification of carbon steels : Carbon Steels are generally classified into following three groups:

(i) Mild steel or Low Carbon Steel: It contains carbon upto 0.25%. It is tough, soft, ductile and low tensile strength steel. It is a general purpose steel and can be easily worked and welded. It is used where hardness and tensile strength are not the most important requirements. Typical applications of mild steel are body work for cars and ships, screws, wires, nails, structural steels, etc.

(ii) Medium Carbon Steel: Carbon content in this steel is 0.3 to 0.55%. Strength and hardness are improved while ductility is reduced. This type of steel can be forged, rolled and machined. It finds uses for agricultural tools, fasteners, dynamo and motor shafts, crank shaft, connecting rods, gears, etc.

(iii) Tool Steel or High Carbon Steel: It is hardest of plain carbon steels and contains

0.6 to 1.5% carbon. It possess good tensile strength. Tool steel is for withstanding wear, where hardness is more necessary requirement than ductility. Tool steel is generally used for machine tools, saws, hammers, cold chisels, punches, axes, dies, taps, drills, razors. These are always used in hardened and tempered condition.

ISOTHERMAL TRANSFORMATIONS-TTT DIAGRAM Time-Temperature-Transformation (TTT) diagram or S-curve refers to only one steel of a particular composition at a time, which is applicable to all carbon steels. This diagram is also called as C-curve isothermal (decomposition of austenite) diagram and Bain’s curve. The effect of time-temperature on the microstructure changes of a steel can be shown by TTT diagram. These diagrams are extensively used in the assessment of the decomposition of austenite in

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heat-treatable steels. We have seen that the iron-carbon phase diagram does not show time as a variable and hence the effects of different cooling rates on the structures of steels are not revealed. Moreover, equilibrium conditions are not maintained in heat treatment. Although, the iron-carbon equilibrium diagram reveals on the phases and corresponding microstructures under equilibrium conditions but several useful properties of the steels can be obtained under non-equilibrium conditions, e.g. variable rates of cooling as produced during quenching and better transformation of austenite into pearlite and martensite. The steels with different percentage of carbon, give different TTT diagrams. The diagram shows the rate at which austenite is transformed, at a given temperature, from all austenite to coarse pearlite; to fine pearlite, to upper bainite to lower bainite, martensite plus residual austenite, depending upon the carbon content. The transformation of the austenite takes place at a constant temperature, i.e., the liquid bath temperature in which the component is cooled for the required time. Since the changes takes place at a constant temperature, it is known as Isothermal Transformation. One can determine the amount of microstructural changes by the microscopic examination of the sample. In order to construct a TTT diagram, a number of small specimens of steel are heated to a temperature at which austenite are stable and then rapidly cooled to temperatures, e.g. 650°C, 600°C, 500°C, 250°C, etc. The specimens are held isothermally at these temperatures for different periods of time until the austenite is completely decomposed. Experimentally, it is observed that at the start of the cooling shown by the points B1, B2, B3 and B4, there is no decomposition of austenite. This time period is referred as incubation period. After this, austenite starts to decompose into the ferrite-cementite mixtures. After the lapse of a certain period of time, the process of decomposition of austenite is stopped, as shown by points E1, E2, E3 and E4 (Fig. 29). Experimentally, it is observed that the rate of decomposition of austenite is not constant and initially it is rapid and slows down gradually.

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Figure 30 : Demonstration of how an isothermal transformation diagram is generated from percentage transformation-versus - logarithm of time measurements

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Figure 30 : Isothermal transformation diagram for an iron–carbon alloy of eutectoid composition and the isothermal heat treatments (a), (b), and (c)

TRANSFORMATION OF AUSTENITE UPON CONTINUOUS COOLING Let us consider a number of specimens of eutectoid steel heated to a temperature t, above the critical points (Fig. 31). At this temperature, the steel is present in the form of stable austenite. Let the specimen of steel is cooled continuously below the lower critical point, i.e., 723°C at various cooling rates. Let the inclined curves V1, V2, V3, . . . on temperature-time graph (Fig. 31) represent these cooling processes. The slowest cooling rate is represented by the curve V1. Slightly higher cooling rate is represented by the curve V2. Still more rapid cooling rates are represented by the V3, V4 and V5. We may note that these curves are straight lines.

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Now, we superimpose these cooling curves (V1, V2, V3, . . .) on the time-temperature transformation diagrams as shown in Fig. 31. From Fig. 31, we note that the curve V1 crosses transformation curves 1 and 2 at points a1 and b1 respectively. Obviously, on slow cooling, the austenite completely transforms into a ferrite cementite mixture. Since the transformation takes place at the highest temperature therefore the ferrite cementite mixture is pearlite. The curve V2 also intersects both the transformation curves at points a2 and b2 respectively. Austenite, at this cooling rate completely transforms into ferrite-cementite mixture. The transformation occurs at lower temperature (as compared to V1), the resulting ferrite-cementite is sorbite. Similarly, the curve V3 also intersects both the curves at points a3 and b3 respectively. The resulting ferrite-cementite mixture is troostite. We note that the curve V4 does not cross both the transformation curves. It intersects only the curve 1 at point a4, and does not reach the stage of completion. Obviously, a part of austentic grains transform into ferrite-cementite mixture, while the other does not transform due to insufficient time. It is observed that the remaining part of austenite which has not been transformed, undergoes transformation into martensite on reaching the temperature Ms. It is shown by the intersection of the curve V4 and temperature Ms at the point M4. Obviously, the structure of steel, cooled at the rate of V4, consists partly of troostite and partly of martensite. This type of structure is common to all steels, which are cooled at a rate faster than those represented by V3 and slower than by V5. This cooling rate for carbon steels is achieved by quenching in oil. We may note that at any cooling rate, higher than V5, e.g. curve V6, of austenite does not transform into ferrite-cementite mixture. However, the austenite is transformed into martensite. Point M5 and M6 in Fig.31 represent this transformation. This cooling rate for carbon steel corresponds to the quenching in water. We may note that the austenite is never transformed into martensite. This untransformed austenite is known as retained austenite. The minimum cooling rate, at which all the austenite is rapidly cooled to temperature Ms and is transformed into martensite is called as critical cooling rate. It is represented by straight line V5, i.e. the tangent line drawn to the curve 1. We may see that curves V2 and V3 and others between them have more slope also intersect the line Ms.

Fig. 31 Transformation of austenite upon continuous cooling

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Obviously, the martensite is formed at the end of transformation. However, it has reported that martensite is never formed at such cooling rates. Perhaps, this may be due to the fact that the curves V2 and V3 and others intersect both the transformation curves. Thus the complete transformation of the austenite takes place at points b2 and b3 respectively. No austenite is left in the steel beyond these points. This means that nothing is to be transformed into martensite. This is why that point M3 in Fig. 31 has no physical sense.

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Heat Treatment: Introduction : Heat treatment refers to the heating and cooling operations required to alter the properties of metals and alloys. Changes in material’s properties result from changes made in microstructure of the material. Heat treatment can be applied to ingots, castings, semi-finished products, welded joints and various elements of machines. During heat-treatment of a metal piece, when it is heated to a definite temperature followed by cooling at a suitable rate, there occur changes in the micro-constituents of the metal. These changes in the micro constituents of the metal may be in their nature, form, size and distribution in the metal piece. Obviously, temperature of heating and rate of cooling are the main controlling factors of changes in micro-constituents. These changes in micro-constituents then control the changes in physical and mechanical properties of heat treated metal specimen. For various fabrication and manufacturing operations, heat treatment is a very important process.

The principle of heat treatment: The principle of the theory of heat treatment is that when an alloy has been heated above a certain temperature, it undergoes a structural adjustment or stabilization when cooled to room temperature. The cooling rate plays an important role in this operation. The structural modification is mainly based on the cooling rate. The heat treatments are normally applied to hypo-eutectoid carbon steels. For steel the eutectoid reaction in the iron-carbon diagram involves the transformation and decomposition of austenite into pearlite, cementite or martensite. Figure 32. shows the temperature ranges and heat treatment processes. Common microstructure of steel obtained during heat treatment is shown in Fig. 33. Figure shows the iron-iron carbide phase diagram in the vicinity of the eutectoid. The horizontal line at the eutectoid temperature, conventionally labelled A1, is termed the lower critical temperature, below which, under equilibrium conditions, all austenite will have transformed into ferrite and cementite phases. The phase boundaries denoted as A3 and Acm represent the upper critical temperature lines, for hypoeutectoid and hyper-eutectoid steels respectively. For temperatures and compositions above these boundaries, only the austenite phase will prevail. Other alloying elements will shift the eutectoid and the positions of these phase boundary lines.

Fig. 32.Heat treatment range for carbon steels Fig. 33. Microstructures of steel

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

The purpose of heat treatment The purpose of heat treatment is to achieve any one or more objectives cited as follows:

(i) To remove strain hardening of a cold worked metal and to improve its ductility. (ii) To relieve internal stresses set up during cold-working, casting, welding and hot-

working treatments. (iii) To remove gases from castings, to soften a metal to improve its machinability, and

to increase the resistance to wear, heat and corrosion. (iv) To improve the cutting ability, i.e., hardness of a steel tool, to improve grain

structure after hot working a metal and to remove effects of previously performed heat-treatment operations.

(v) To improve magnetization property, especially of steels, for producing permanent magnets.

(vi) To refine grain structure after hot working a metal. (vii) To soften and toughen a high carbon steel piece. (viii) To produce a single phase alloy in stainless steel, and to produce a hard, wear

resistant case on a tough core of a steel part. (ix) To harden non-ferrous metals and alloys, especially aluminium alloys and to

produce a single phase alloy in stainless steel. (x) To produce a hard, wear resistant case on a tough core of a steel part and to

toughen a hardened steel piece at the cost of its hardness.

Annealing : 1 Full Annealing This operation removes all structural imperfections by complete recrystallization. This operation is often utilized in low and medium carbon steels that will be machined or will experience extensive plastic deformation during a forming operation. This operation consist of:

(i) Heating the hypoeutectoid steel to about 50-70°C above the upper critical temperature (for hypoeutectoid steels) and by the same temperature above the lower critical temperature for hypereutectoid steels until equilibrium is achieved. This ensures that the metal is heated thoroughly and phase transformation has taken place throughout the whole volume.

(ii) The alloy is then furnace cooled; i.e., the heat-treating furnace is turned off and both furnace and steel cool to room temperature at the same rate, which takes several hours. The microstructural product of full anneal is coarse pearlite (in addition to any proeutectoid phase) that is relatively soft and ductile. The full-annealing cooling procedure is time consuming; however, a microstructure having small grains and a uniform grain structure results.

2 Process Annealing or Partial Annealing This is a heat treatment that is used to negotiate the effects of cold work, i.e. to soften and increase the ductility of a previously strain hardened metal. Process annealing is commonly utilized during fabrication procedures that require extensive plastic deformation, to allow a

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

continuation of deformation without fracture or excessive energy consumption. It is the recrystallization of cold work, i.e., recovery and recrystallization processes are allowed to occur. Ordinarily a fine grained microstructure is desired, and therefore, the heat treatment is terminated before appreciable grain growth has occurred. In other words, the exact temperature depends upon the extent of cold working, grain size, composition and time held at heat. Surface oxidation or scaling may be prevented or minimized by annealing at relatively low temperature (but above the recrystallization temperature) or in a non oxidizing atmosphere. Process annealing is very useful in mild steels and low carbon steels (Fig. 10.3).

Figure 34 : Process annealing Process annealing or sub-critical annealing which is done on coldworked low carbon steel sheet, wire or tubing to relieve internal stresses and to soften the material. The process is as follows:

(i) The steel is heated to 550-650°C, which is just below the lower critical temperature on iron-carbon diagram for steel.

(ii) Stresses throughout the metal are relieved and recrystallization causes new grains to form and grow.

Heating period is followed by slow cooling. Prolonged annealing causes the cementite in the pearlite to “ball up” or spheroidise. Ferrite grain growth also occurs. Obviously, annealing time and temperature control is very essential for proper process annealing.

Patening: It is mainly applied to medium to high carbon steels prior to drawing of wire or

between drafts. This mainly increases ductility for wire drawing. The process is as follows: (a) Heating to a temperature above the transformation range and (b) then cooling to a temperature below that range in air or in a bath of molten lead

or salt maintained at a temperature appropriate to the carbon content of the steel and the properties required of the finished product.

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

Stress-Relief Annealing Internal residual stresses may develop in metal pieces due to:

(i) Plastic deformation processes such as matching and grinding; (ii) Non uniform cooling of a piece that was processed or fabricated at an elevated

temperature, such as a weld or a casting; and (iii) A phase transformation that is induced upon cooling where in parent and product

phases have different densities. Distortion and warpage may result in case if residual stresses are not removed. They can be eliminated by this process-in which the piece is heated to the recommended temperature, held there long enough to attain a uniform temperature, and finally cooled to room temperature in air. We may note that the annealing temperature is ordinarily a relatively low one suchthat effects resulting from cold working and other heat treatments are not affected, castings, forgings, weldment and other work pieces may have residual stresses. In this process, the work pieces are first heated to about their recrystallization temperature and then these are cooled slowly. Like process annealing this stress-relief may also be performed on any metal.

Double Annealing: It is quite useful for steel castings. It removes the strains. It coalesces the

sulphide films (which embrittle the steel) in the ferrite and produces homogeneity by rapid diffusion. The steel is ultimately obtained with refined grains and in soft condition. In this process, steel is heated to a temperature considerably over the A3 point, i.e. above the critical range for a time period. Then rapid cooling is performed to a temperature below the lower critical temperature. Now, immediately reheating is done to a point just over the upper critical temperature for a time period. Finally, slow cooling allowed to room temperature.

Normalizing: This is used as a finishing treatment for carbon steels giving higher strength than annealing. There is no serious loss of ductility too. Heating and soaking in this process is same as in the full annealing but part is allowed to cool in air so that cooling rate is much faster. An annealing heat treatment called normalizing is used to refine the grains (i.e., to decrease the average grain size) and produce a more uniform and desirable size distribution. Fine grained pearlite steels are tougher than coarse-grained ones. The fine grain structure increases the yield and ultimate strengths, hardness and impact strength. Normalizing is accomplished by heating at approximately 55 to 85°C above the upper critical temperature, which is, of course, dependent on composition. Normalizing often applied to castings and forgings is stress relieving process. To some extent, it increases strength of medium carbon steel. It improves machinability, when applied to low carbon steel. Alloy steels in which the austenite a procedure termed austenizing is very stable can be normalized to produce hard martensitic structure. Cooling in air produces high rate of cooling which can decompose the austenitic structures in such steels and martensite is produced. This increases the hardness to great extent. The advantages of this method are: (i) In comparison to fully annealed material, normalizing produces stronger material.

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

Normalizing refines the grains. (ii) Normalizing produces homogenised structure. (iii) Normalizing is used to improve properties of steel castings instead of hardening and

tempering. (vi) Strength and hardness are increased. (vii) Better surface finish is obtained in machining. (vii) Resistance to brittle fracture is increased in hot-rolled steel. (viii) Crack propagation is checked.

Hardening It is a kind of heat treatment which forms a non-equilibrium structure in an alloy. To form a non-equilibrium structure in an alloy, it is heated above the temperature of the phase transformation in the solid state and then cooled (chilled) quickly; fast cooling is essential for preventing the equilibrium transformation during cooling. Structural and tool-making alloys are hardened in order to increase their strength. Their strength increases either due to the martensitic phase change or due to a reduction of the temperature of eutectoid reaction; in both cases there forms a fine grained eutectoid mixture. If the hardening procedure has resulted in that the metal at room temperature (20-25°C) has the fixed state of high temperature solid solution, the strengthening effect immediately after hardening is insignificant; it will be pronounced mainly on a repeated low-temperature heating or after holding at 20-25°C. In alloys possessing special properties, hardening makes it possible to change the structure-sensitive physical and chemical properties, for instance increases the electric resistance or corrosion resistance. Hardness is the resistance to indentation. Hardening capacity is important characteristics of steels. Hardening capacity is defined as the surface hardness of a hardened article and depends mainly on the carbon content of the steel. ‘Hardenability’ is a term that is used to describe the ability of an alloy to be hardened by the formation of martensite as a result of a given heat treatment. Note that the influence of alloy composition on the ability of a steel alloy to transform to martensite for a particular quenching treatment is related to a parameter hardenability. Moreover, for every different steel alloy there is a specific relationship between the mechanical properties and the cooling rate. Hardenability is a qualitative measure of the rate at which hardness drops off with distance into the interior of a specimen as a result of diminished martensite content. There are two testing methods to measure the hardenability of steels:

(i) cylinder series test and (ii) Jominy end-quench test.

The first method gives a single value of hardenability. The value is stated in terms of percentage of martensite at the centre when quenched in a certain manner. Severity of quench is an index that quantitatively defines the quenching condition. This index is denoted by H and defined as

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

When H →∝, it represents the severest condition of quench, i.e., surface of steel immediately reaches the temperature of the quenching medium. The Jominy End-Quench Test This is most convenient and widely used laboratory test for hardenability. With this test, except for alloy composition, all factors which may influence the depth to which a piece hardens (i.e., specimen size and shape, and quenching treatment) are maintained constant. A standard specimen (25.4 mm in diameter and 100 mm long) is austenitized at a prescribed temperature for a prescribed time. After removal from the furnace, it is quickly mounted in a fixture (Fig. 35a). The lower end of the specimen is then quenched by a standard jet of water, resulting into a varying rate of cooling. The cooling rate at the jet end is about 300°C/s while that at the other end is about 3°C/s. After the piece has cooled to room temperature, shallow flats 0.4 mm deep are ground along the specimen length and Rockwell hardness measurements are made for the first 50 mm along each flat (Fig. 35 b); for the first 12.8 mm, hardness observations are noted at 1.6 mm intervals, and for the remaining 38.4 mm, every 3.2 mm. This varying cooling rate produces a wide range of hardness along the length of Jominy specimen. A hardenability curve is obtained when hardness is plotted as a function of position from the quenched end.

Fig. 35 Jominy end-quench specimen (a) mounted during quenching, and

(b) hardness is measured along the length of the specimen

Influence of Quenching Medium, Specimen, Size, and Geometry We have discussed the influence of both alloy composition and cooling or quenching rate on the hardness. The cooling rate of a specimen depends on the rate of heat energy extraction, which is a function of the characteristics of the quenching medium in contact with the specimen surface and also on the size and geometry of the specimen. “Severity of quench” is widely used to express the rate of cooling. Interestingly, the more rapid the quench, the more severe the quench. Water, oil and air are the three most common quenching media. Out of

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

these three, water produces the most severe quench followed by oil, which is most effective than air. The degree of agitation of each medium also influences the rate of heat removal. The quenching effectiveness can be enhanced by increasing the velocity of the quenching medium across the specimen. Oil quenches are found suitable for the heat treating of many alloy steels. A water quench is too severe for higher. Carbon steels, because cracking and wraping may be produced. Air cooling of austenitized plain carbon steels ordinarily produces an almost totally pearlitic structure. During the quenching of a steel specimen, heat energy must be transported to the surface before it can be dissipated into the quenching medium. As a consequence, the cooling rate within and throughout the interior of a steel structure varies with position and depends on the geometry and size. The heat energy is dissipated to the quenching medium at the specimen surface and the rate of cooling for a particular quenching treatment depends on the ratio of surface to the mass of the specimen. The larger this ratio, the more rapid will be the cooling rate and, consequently, the deeper the hardening effect. Interestingly, irregular shapes with edges and corners have larger surface-to-mass ratios than regular and rounded shapes (e.g. spheres and cylinders) and are thus more amenable to hardening by quenching. We may note that there are a multitude of steels that are responsive to martensitic heat treatment, and one of most important criteria in the selection process is hardenability. Hardening may cause few defects due to prolonged or uneven heating, temperature higher than required, improper cooling. Few of these defects are:

(i) hard and soft spots due to uneven or prolonged heating, (ii) oxidation and decarburisation due to high temperature, (iii) mechanical properties achieved different from anticipated, (iv) wrappage due to high temperature, (v) change in dimension due to heating or cooling, and (vi) cracks either of circular or vertical nature due to uneven heating.

Tempering

The term ‘tempering’ is usually applied to steels and other alloys undergoing a polymorphic transformation in hardening (two-phase aluminium, bronzes, some titanium alloys) and the term ‘ageing’ to alloys which undergo no polymorphic transformations in hardening (aluminium alloys, austenitic steels, nickel alloys, etc.). Tempering improves the ductility and toughness of quenched steel while decreases hardness. Tempering releases the stresses and reduces the brittleness. Tempering causes the transformation of the martensite into less brittle structure, i.e., a fine pearlistic structure termed as troostite. Troostitie is much tougher, although somewhat softer than martensite. Most c.s. cutting tools have this type of structure. Once the tempering temperature has been reached, it is normal to quench the steel. All structures resulting from tempering are termed tempered martensite. The changes taking place during various temperature ranges are as follows: (a) 100°C-220°C Below 200°C tempering temperature only relieves the hardening stresses and very little change occurs in the micro-structure. However, the stress relieving treatment is given

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

when maximum hardness is desirable and brittleness is a problem. The strain is relieved due to removal of carbon atoms from their trapped positions. (b) 240°C-400°C Above 220°C the martensite starts to change into a fine pearlistic structure termed as troostite. In the range of temperature 240°-400°C, martensite decomposes rapidly into emulsified form of pearlite called as secondary troostite. This type of material is very fine in nature and hence provides good shock resistance. The fine edge tools are usually tempered within the range 270°C-300°C. (c) 400°C-550°C Tempering above 400°C causes the cementite particles to ball up giving a coarse structure called sorbite, which is more ductile and tougher than troostite. Within this range, the precipitate troostite begins to coalesce forming a coarser form of globular pearlite called as sorbite. We may note that both troostite and sorbite are now preferably called tempered martensite. This treated is usually preferred in such components as beams, springs and axles. (d) 600°C-700°C Within this range, heating hardened steel causes spheroidisation, the structure being known as spherodite. This structure is formed due to further coalescence of the carbide within the alloy. Spheroidised steels exhibit fairly good machinability as the hard carbide particles are embedded in the soft ferrite matrix and consequently do not have to be cut by the cutting tool. When the shperoidized steel is heated to just above its lower critical temperature the pearlite present will alter to austenite and cooling to room temperature will yield a structure of lamellar pearlite plus pro-eutectoid ferrite or cementite depending upon carbon content. Usually the temperature of tempering is judged by colour appearance on shop floors. However, for accuracy, the exact temperature measurement are to be made.

isothermal treatment Special Treatments

In case of large sections where the water quenching may produce cracks, special treatments are preferred for hardening. The cracks on the surface are caused due to fast skin cooling and changes into martensite while the inner core cools slowly and transforms later accompanied by dilation. This dilation causes outer skin to crack. In order to avoid this type of cracking special treatments have been developed. Prior to their discussion, it would be proper to have some idea about isothermal transformation. (i) Isothermal Transformation: Usually austenite is not converted into martensite instantaneously but the process continues for some time. It is observed that different steels take different time for full transformation and the time depends upon the temperature from where cooling begun. Conditions of constant temperature are termed isothermal. The related phase diagrams are referred to as isothermal transformation diagrams, or sometimes as time-temperature transformation (or T-T-T) diagrams. The selected specimen is austenized and then quenched in liquid bath held at temperature to be investigated. The specimen is held for a different length of time in the bath and then quenched in the water.

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

One can study the resulting structure under microscope or any other associated property, e.g. hardness may be studied. It is observed that definite times are required for the initiation and completion of the transformation and these times vary with the temperature. The progress of transformation, say for 10%, 50%, or 90% may also be determined. Figure 35, illustrates complete transformation diagram for eutectoid steel. The transformation temperature is lowered from A1 temperature to about 550°C, and the nucleation and completion time decreases and pearlite lamellae becomes finer. The nucleation and completion time increases from about 550°C to 250°C. The transformation product is bainite (composed of two equilibrium phases that are ferrite and cementite). Minimum nucleation time is identified as the nose or knee. The transformation product is martensite below about 250°C. Although, this forms almost instaneously, but the amount formed depends upon the temperature. The upper and lower limits of martensite transformation temperature are represented by Ms and MF respectively. These diagrams are also referred to as S-curves because of their shape.

Fig. 36 Isothermal transformation diagram for a eutectoid steel. (ii) Austempering or Isothermal Quenching: The component or specimen to be hardened is first austenized and then quenched into a lead or salt bath held at just above the martensite transformation temperature. The component is kept in the bath until the bainite transformation is completed. Now, the component is removed from the bath and cooled in air till the room temperature is reached. The bainite so produced is somewhat softer than martensite of the same carbon content and distortion is also minimum. Moreover, the austempered steel has improved shock resistance and low notch sensitivity. Figure 37, shows the process of austempering.

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

Fig. 37. Austempering on the TTT curve

Molten salts, 45% NaNO3 and 55% KNO3 or molten alkalies 20% NaOH and 80% KOH are used as quenching medium. Their temperature is maintained between 150-450°C. Interestingly, austempering is a heat treatment and no reheating is required as in tempering. Austempering is often limited to section thickness of 20 mm. This is the only limitation in austempering, i.e. only small sections are suitable for austempering as big sections cannot be cooled rapidly to avoid the formation of pearlite. Austempering is applicable to a few plain carbon steels and requires facility of molten salt bath. One may regard this as a disadvantage over quenching and tempering. (iii) Martempering or Steeped Quenching: The sample or the specimen to be hardened is fully austenized and then quenched into a lead or salt bath held at a temperature just above that at which martensite starts to form. It is maintained at this temperature until its temperature becomes uniform throughout, i.e. there remains no difference in outside and inside temperatures. Then it is water quenched to form complete martensitic structure and bainite formation is prevented. This process separates successfully the cooling contraction from the austenite - martensite expansions and thus prevents quenching in large size articles. The steel can be tempered to low temperature to further refine the structure. Effect of such treatment is to minimise cracking, distortion, and the thermal shock of the quenching. The hardness and ductility are usually similar to those obtained by direct quenching to the martensitic state followed by tempering. However, the impact toughness may be better.

Surface Hardening Surface hardening of articles, like most methods of surface strengthening (chemical heat treatment, strain hardening, knurling, etc.) offers an additional advantage that large compressive stresses appear in the surface layers of hardened articles. A number of components require only a hard surface to resist wear and tear and a tough core to resist shock loads instead of complete component being made hard. These two properties do not exist in one steel. For toughness, one finds that the core should not exceed 0.3% carbon content, also sometimes it is not desirable to harden complete component. For example, it is undesirable to have case harden screw threads. The threads would be brittle and distortion during hardening would need expensive thread grinding operations to correct the distortions. Surface hardening is classified into two types:

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

1. Selective heating of the surface layers: In this case, the time available to change the micro-structure of steel is short. Obviously, hardened and tempered steels respond well but annealed steels do not. Moreover, composition of the steel must be such that the quenching will produce martensite and so harden the steel. Clearly, carbon content to be 0.4% or more. (i) Flame hardening can be one of the method of selective heating. The surface

of a component is heated to 850°C with an oxyacetylene flame and quenched with cold water immediately. Flame heating transforms the structure of the surface layers to austenite and quenching changes the austenite surface layers to martensite resulting into a hard surface. Depth of hardening depends on the heat supplied per unit time. Flame hardening is mostly used in case of carbon and alloy steels having carbon content 0.4% to 0.6, e.g. shafts, gears, cylinder linears, crank shafts bearing journals, etc.

(ii) induction hardening is another method of selective heating. Steel components are placed within a coil through which a high frequency is allowed to pass. Surface layers of the component are heated between 850-1000°C. Subsequently cooling transforms the austenite to martensite. The heating coil is often made of tube perforated with fine spray holes so that it can be used both for heating and quenching. We may note that the depth of heating produced by this method is related to the frequency of the alternating current. Higher the frequency of a.c. current, less will be the hardened depth, but there will be more rapid rise in temperature.

(iii) Case Hardening: In this method C and/or N2 are introduced in the surface

layer. In carburizing the part is surrounded by material or atmosphere rich in carbon and on heating this carbon is first released and then absorbed in steel. More recently case carburizing is more effectively performed by heating steel part in the atmosphere of natural gas, coke even gas, butane or propane or the volatized form of liquid hydrocarbons like terpenes and benzene or the volatilized form of liquid hydrocarbons like terpenes and benzene. Volatilized form of alcohol and glycols or ketones have also been used. In these cases the thickness of hardened layer is proportional to root of the time of treatment in hour. The depth of hardness depends upon the number of times the process is repeated. The component is finally raised to a red heat and quenched in water. However, one will have to take the precaution that the part to be hardened should be covered with the compound.

2. Carburizing, This is another method of surface hardening. The composition of surface layers are changed. This process is usually carried out on a steel containing less than 0.2% carbon. Carburizing is usually employed for treating certain types of machine elements which have to have a wear-resistance working surface and tough core, gear wheels, shafts, pins,

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(Asst. Prof.) Delhi Technological University

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camshafts, cams, worms, etc. The initial medium for carburizing (carbon saturation) is usually called a carburizer. Two methods of carburizing are in use: in a solid and in a gaseous carburizer. In both cases, however, the carburizing process passes through a gaseous phase. The most popular solid carburizer consists of charcoal with an addition of 20-25% barium carbonate to intensify the process and of 3-5% of CaCO3 to prevent the carburizer particles from caking. Articles to be carburized and the carburizer are placed into a container (steel box) and heated in a furnace to 910-930°C. During heating, charcoal reacts with the oxygen of remaining air

2C + O2 → 2CO BaCO3 + C → BaO + 2CO

The leading reaction of carburizing that takes place in the surface of metal is the disproportioning reaction:

2CO ... ... CO2 + C The active carbon that forms by the reaction is absorbed in the surface of metal, which is in the austenitic state, while CO2 reacts with charcoal to form new portions of CO. Thus, the carbon formed in the reversible disproportioning reaction is transferred into the surface being saturated.

In gas carburizing, the carburizer is diluted natural gas (consisting almost fully of methane), a controlled atmosphere produced in special gas producer and a liquid hydrocarbon (kerosene, benzene, etc.) supplied dropwise into an airtight furnace where it forms an active gaseous medium. The main leading reaction for methane is

CH4 → 2H2 + C In some cases when the gas mixture contains CO (an endothermic-base controlled atmosphere), the reaction 2CO → CO2 + C is also possible. Depending on the composition of gaseous mixture and the concentration of carbon in steel, the atmosphere in the furnace may be carburizing, decarburizing or neutral. Note that carburizing will take place when the carbon concentration in the surface of steel is smaller than the carbon potential of gaseous mixture at the given temperature. Normally, this process is carried out on a steel containing less than 0.2% carbon. To achieve adequate hardness the surface of the component should have a carbon content of 1% approximately. In this process the carbon is usually added to the surface layers upto 0.7 to 0.8% by a carefully regulated depth, which is followed by a quenching process to convert the surface layers into martensite. The structure obtained on carburizing is coarse-grained, since the metal is held for an appreciably long time at the carburizing temperature. The time of isothermal holding in carburizing depends on the grade of steel and the desired depth of layer. The process of gas carburizing occurs more quickly than that of solid carburizing, since no time is lost for heating the carburizer box. Besides, gas carburizing can be more easily controlled and automatized. By carburizing process hardness can be increased from 200 HB (0.8 carbon content in the core) to 750 HB (0.7% carbon content in the surface).

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(Asst. Prof.) Delhi Technological University

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Other methods used for carburising are: Pack Carburizing: The component to be carburised is heated above A3 temperature between 850°C to 1060°C for about five hours in a sealed metal box containing charcoal and barium carbonate as mentioned earlier. Oxygen present in the box reacts with the carbon to produce carbon monoxide. Carbon present in the atmosphere gets diffused into the surface austenitic layers of the component. When carburising is complete, metal boxes are allowed to cool down so that they can opened and unpacked. The components are then cleaned and made available for subsequent heat treatment. This method is mainly used for large components or where a thick surface layer has to be hardened. Salt Bath Carburizing: This method involves the component in a bath of suitable carbon rich salts between 750-850°C. Usually, sodium cyanide is mainly used as a medium. Carbon from the molten salt diffuses into the component. The advantages and disadvantages of cyanide salts are: (i) Cyanide salt is poisonous in nature. Obviously, one will have to take utmost care while working with such a salt. A few grains of this salt under the fingernails could prove fatal if transformed to mouth via smoking or food. (ii) After the treatment, removal of the salt from the hardened surface is more problematic in case of blind holes or threaded parts. (iii) In this process heating and carburisation are more uniform with less chance of distortion. (iv) The components can be hardened by quenching straight out of cyanide salt without need for further heat treatment. Nitriding: This is the process of diffusion saturation of steel surface with nitrogen. The component is heated in a mixture of ammonia and hydrogen so that nitrogen diffuses into the surface layers and hard nitride compounds are formed. The process is employed for increasing the wear resistance and endurance limit of machine elements (crankshafts, cylinder sleeves, worm gears, shafts, etc.). Prior to nitriding, steel articles are subjected to hardening and high temperature tempering (improving heat treatment) and finishing. After nitriding, they are ground and polished. In a nitriding process, the component is heated in a mixture of ammonia and hard nitride compounds are formed. Common nitriding is carried out at a temperature of 500-600°C in a muffle or container through which dissociating ammonia is being passed. This process is used with those alloys of steel which contain elements that form stable nitrides, e.g. chromium, molybdenum, tungsten, vanadium, aluminium, etc. It is probable that the reaction of ammonia dissociation takes place at the steel surface; nitrogen ions are absorbed by the surface and then diffuse into the depth of the metal. The time taken for nitrogen to react with the steel surface is about 100 hours. If ammonia is heated in an isolated volume, only the reaction 2NH3 → N2 + 3H2 is possible and the molecular nitrogen formed by this reaction cannot diffuse into the metal without being ionized. The depth to which the nitrides are formed depend on the temperature and time allowed for the reaction, and in normal conditions, it is unlikely to exceed 0.7 mm. The components are cooled slowly in the ammonia hydrogen atmosphere. The process of nitriding is usually completed with the formation in the metal surface of the e-phase with an HCP lattice and ordered arrangements of atoms in a wide range of concentrations.

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

When nitriding carbon steels, the rate of nitrogen diffusion decreases with an increase of carbon content and the formation of carbonitride phases becomes possible. A thin white layer of nitrides is formed on the surface of the component, which should be removed by mechanical or chemical means as this layer adversely affects the mechanical properties. Nitriding is a rather time-consuming process. Nitrided layers of alloyed steels have higher values of hardness and wear resistance compared with carburized layers. Since no quenching treatment is required with nitriding, cracking and distortions are less likely to occur. With this process very high surface hardness can be obtained with special alloys. The hardness is retained upto about 500°C compared to carburising in which case the surface becomes softer at above 200°C. The nitriding process is however used less frequently than carburizing, mainly in view of its longer duration and smaller depth of the strengthened layer, which limits the allowable contact loads onto the metal surface. In recent time, sources of highly concentrated energy (electron and laser beams) have come into use for heat treatment of some articles. Pulses of electron and laser beams can be used efficiently for local heating of the surface of articles which makes it possible to carry out surface hardening of working of cutting tools and certain areas of the articles to be subjected to wear. In some cases, a thin surface layer is heated to melting and then cooled quickly so that a fine-grained or amorphous structure forms in it. In surface hardening by means of sources of highly concentrated energy, water quenching becomes needless, since a thin locally heated surface layer is cooled quite rapidly by the underlying metal. Electron accelerators and continuous wave gas lasers and pulsed lasers can be employed for the purpose. Age Hardening (Precipitation Hardening): One can enhance the strength and hardness of some metal alloys by the formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix. However, this must be accomplished by phase transformations that are induced by appropriate heat treatments. The process is termed as precipitation hardening, as the small particles of the new phase are termed “precipitates”. This process is also called as ‘age hardening’, as the strength develops with time, or as the alloy ages. The alloys that are hardened by this process are aluminium-copper, copper, berryllium,copper-tin, and magnesium-aluminium. Few ferrous alloys are also precipitation hardenable. Age hardening is a characteristic of alloys with a falling solid solubility, e.g. the solubility of copper in aluminium to form the solid solution is only 9.5% at room temperature. It increases with rise in temperature and reaching a maximum of 5.7% at 548°C. Obviously, the solubility of copper in aluminium depends on the temperature. When an alloy containing 4% copper is cooled slowly from the alpha region, coarse particles of CuAl2 get precipitated. In this condition, the alloy is comparatively weak and brittle. When carried out the age hardening treatment as follows, this improves mechanical properties:

(i) The alloy is heated to a prescribed temperature of 500°C to dissolve and other alloying element.

(ii) The alloy is quenched from the above temperature to super-saturate the alpha solid solution at room temperature keeping precipitation rate very low. A super-saturated solid solution of 4% copper in aluminium is obtained. Precipitation of copper as CuCl2 is prevented by rapid cooling.

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Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

Soft Copy Available at: www.RoopLalRana.com

(iii) Finally the alloy is kept at room temperature and hardening (age) occurs spontaneously.

Note that when the solute atoms form a solid solution with solvent atoms it produces an alloy which is harder and also has more strength than the pure solvent metal. This is termed as solute hardening. Interestingly, precipitation hardening and the treating of steel to form tempered martensite are totally different phenomena, even though the heat treatment procedures are similar. The principal difference between the two lies in the mechanisms by which hardening and strengthening are achieved.