Phase Rule PDF

ENGINEERING CHEMISTRY PHASE RULE Unit 3: Phase Rule Phase rule: Introduction: Phase rule is an important tool used for quantitative treatment of systems in equilibrium. It enables to predict the conditions that must be specified for a system to exhibit equ...

ENGINEERING CHEMISTRY PHASE RULE Unit 3: Phase Rule Phase rule: Introduction: Phase rule is an important tool used for quantitative treatment of systems in equilibrium. It enables to predict the conditions that must be specified for a system to exhibit equilibrium. Phase diagrams are of considerable significance both industrially and commercially, particularly for steels, alloys ceramics and semiconductors. These are the basis of separation procedures in petroleum industry, food formulations and in the preparation of cosmetics. Phase diagrams are an important tool to deal with the behavior of heterogeneous systems. Phase rule helps in predicting the effect of temperature, pressure and concentration on heterogeneous system in equilibrium Phase Rule: The phase rule is a generalization put forward by Willard Gibbs in 1847 which explains the equilibrium existing in heterogeneous system. The phase rule is defined as if an equilibrium is dependent only on temperature, pressure and concentration variations and is not influenced by gravity, surface forces, electrical or magnetic forces then the number of degrees of freedom(F) of the system is related to the number of components(C) and phase(P) by the phase rule equation. F = C- P + 2 Where, F = number of degree of freedom or variables C = no. of components of the system P = no. of phases of the system Terms Involved in Phase Rule Phase: Phase can be defined as a homogenous, physically distinct and mechanically separable portion from other parts of the system by definite boundary surfaces. Examples: 1) At freezing point water exists in three phases Ice ↔ water ↔ water vapour (solid) (liquid) (vapour) 2) If two liquids are completely miscible it is said to exist in one phase only. Ex: water- alcohol – homogenous system. 3) If two liquids are immiscible they are said to exist in two phases. Ex: Benzene – water – heterogonous system. 4) A solution of a substance in a solvent is said to exist in a single phase. Ex: sugar dissolved in water 5) A gaseous mixture is said to exist in a single phase only Ex: mixture of nitrogen and hydrogen 6) Different solid components constitute separate phase while different gaseous constituents are considered as single phase Ex: CaCO3(s) ↔ CaO(s) + CO2(g) → 3 phases  161 DEPARTMENT OF HUMANITIES & SCIENCES ENGINEERING CHEMISTRY PHASE RULE AND SURFACE CHEMISTRY Component(C): The number of components in a system is the minimum no. of independently variable chemical species by means of which the composition of each phase can be expressed. It is an element or compound present in a system. Eg: 1. In water system all three phases ice, water and vapour has the same chemical composition H2O. The composition of each phase can be expressed in terms of single constituent represented byH2O. Hence it is a one- component system. Eg: 2. If we take Na2SO4-water system to describe the composition of different phases two independently variable chemical species Na2SO4 and H2O are required Hence it is a two – component system. Na2SO4(s) + 7H2O ↔ Na2SO4.7H2O(s) Phase Component Na2SO4.7H2O = Na2SO4+7H2O Na2SO4 = Na2SO4+OH2O Solution = xNa2SO4+yH2O 3) Dissociation of ammonium chloride, which is in equilibrium with its products ammonia and hydrochloric acid NH4Cl ↔ NH3(g) + HCl(g) The composition of all the phases can be stated in terms of NH4Cl only. Hence C=1 4) Decomposition of CaCO3: There are 3 chemical species CaCO3, CaO and CO2. But the composition of each phase can be expressed in terms of any two of the species. PHASE COMPONENT CaCO3 → CaO + CO2 CO2 → CaCO3 – CaO CaO → CaCO3 – CO2 Components (C): In a chemically reactive system involving reactions between various species, the number of components of a system may be defined as the number of chemical constituents of the system minus the number of equations relating to these constituents in a equilibrium state. In a chemically reactive system, the number of components is given by C=N–E Where N: No. of chemical species E: No. of independent equations relating the conc. of N species. Examples: (1). Dissociations of NH4Cl in a closed vessels NH4Cl(s)↔NH3(g) + HCl(g) DEPARTMENT OF HUMANITIES & SCIENCES 162 ENGINEERING CHEMISTRY PHASE RULE AND SURFACE CHEMISTRY The proportions of NH3 and HCl are equivalent [NH3] = [HCl] ∴ N = 3 (NH4Cl, NH3 and HCl) E=2 ∴C=N–E = 3-2 = 1, i.e. it is a one component system. (2). When NH4Cl is heated in a closed vessel along with NH3 or HCl. NH4Cl(s)↔ xNH3(g) + y HCl(g) [NH3] ≠ [HCl] ∴ Only one equations (i) relates the concentration of constituents ∴C=3–1=2 ∴ The system has two components. (3). KCl – NaCl – H2O system N = 3 (i,e. KCl, NaCl, H2O) Since there is no independent equation relating their concentrations. ∴E=O ∴C=3–0=3 ∴ System is a 3- component system For KCl – NaBr – H2O system N = 5 (i,e. KCl, NaBr, NaCl, KBr, H2O) E= 1 (i,e. KCl + NaBr↔KBr + KCl ∴ C =N – E = 5 – 1 = 4 This system is regarded as a 4 component system. Degree of freedom: It is defined as, “the minimum number of independently variables such as pressure temperature and composition that must be specified in order to define the state of the system completely.  If degree of freedom F=0, the system is known as non-variant or invariant  If F=1, then the system is invariant  If F=2, then system is bivariant Examples: 1) In the water system Ice (s) ↔ Water (l) ↔ Vapour (g) Three phases will be in equilibrium only at particular temperature and pressure. The system is, zero variant, or non-variant or invariant and has no degree of freedom. 2) For a system consisting of water in contact with its vapour. Water (l) ↔ Vapour (g) DEPARTMENT OF HUMANITIES & SCIENCES 163 ENGINEERING CHEMISTRY PHASE RULE We must state either temperature or pressure to define the system completely. Hence the degree of freedom is one (or) the system is univariant. 3) For a gaseous mixture of N2 and H2, we must state both the temperature and pressure. Hence, the system is bivariant (degree of freedom is two). Merits of phase rule: 1. It is applicable to both physical and chemical equilibria. 2. It is a convenient method of classifying the equilibrium states in terms of phases, components and degree of freedom. 3. It indicates different system with same degree of freedom behave similarly. 4. It helps us to predict the behaviors of a system, under different sets of variables. 5. It helps in deciding whether the number of substances remains in equilibrium or not. Limitations of phase rule: 1. It can be applied only for heterogeneous systems in equilibrium. 2. Only three variables like P, T, & C are considered, but not electrical, magnetic and gravitational forces. 3. It is applied only to a single equilibrium system. 4. It requires utmost care in deciding the number of phases existing in equilibrium. 5. Solid and liquid phases must not be in finely divided state, otherwise deviations occur. 6. Only three degrees of freedom are allowed to influence the equilibrium system. Applications of Phase rule: 1. Useful in understanding the properties of materials such as metals, alloys and composites 2. Used in several metallurgical processes, solvent extraction and steam distillation 3. In quality control of ultra pure materials like semiconductors, it is used to check impurity concentration. Phase Diagram:  A diagram which illustrates the conditions of equilibrium between various phases of a substance is called a phase diagram or equilibrium diagram.  A polyphasic equilibrium which can be studied with the help of any two intensive properties, temperature and pressure and pressure or temperature and composition or pressure and composition are represented by a two dimensional phase diagram  A diagram which illustrates the condition of equilibrium between various phases of a substance is called a phase diagram.  A phase diagram is a graphical representation of chemical equilibrium.  Hence a phase diagram is a plot showing the conditions of pressure and temperature under which two or more physical states can exist together in a state of dynamic equilibrium. The diagram consists of a) the regions or areas b) the lines or curves and c) the point One-component system: DEPARTMENT OF HUMANITIES & SCIENCES 164 ENGINEERING CHEMISTRY PHASE RULE In any system, the minimum number of phases is one. It is evident from the phase rule equation F = C- P + 2 =1-1+ 2=2 For one component system the maximum number of degree of freedom is two. The maximum number of phases in any system is three, from the phase rule equation F=C–P+2 =1-3+2=0 The minimum number of degree of freedom is zero for one component system. From the above calculations, it is clear that for any one-component system the maximum number of degree of freedom is two. Therefore, such a system can be presented completely by a two dimensional diagram. Hence, we may draw the phase diagram by taking pressure and temperature as the two axes. These are known as pressure-temperature (P-T) diagrams, concentration (or) composition remains constant. Water system: Water can exist in three possible phases, namely, solid, liquid and vapour. Water system is a typical example for one component system. It shows the following different equilibria 1. Liquid ↔ Vapour 2. Solid ↔ Vapour 3. Solid ↔ Liquid Phase diagram for water system: DEPARTMENT OF HUMANITIES & SCIENCES 165 ENGINEERING CHEMISTRY PHASE RULE Phase diagram consists of Curves Areas Point OA = Vapourisation curve AOB = vapour only O = Triple point OB = Sublimation curve BOC = Ice only OC= Fusion curve AOC = Liquid only OA1= Meta stable curve Each equilibrium involves two phases. It shows the relation between the solid, liquid and gaseous states of a substance as a function of temp and pressure Vapourisation curve OA:  It is called the vaporization curve of water  It represents the equilibrium between liquid water and vapour.  At any point on the curve the following equilibrium will exist. Liquid water ↔ vapour Water at any given temperature is associated with vapour pressure. With increase in temperature, there is an increase in vapour pressure. The rate of increase is higher at higher temperature than at lower temperature. This increasing trend continues till the system reaches a critical temperature of 374oC and a critical vapour pressure of 218 atm. Beyond point A, the two phase liq water and vapour merge into each other and only vapour phase exists. It is not possible to liquefy water vapour beyond 374oC by the application of any amount of pressure. The degree of freedom of the system is one, i.e., invariant. This is predicted by the phase rule. F=1-2+2; F=1 Sublimation Curve OB:  It is called the sublimation curve or vapour pressure curve of ice.  It represents the equilibrium between ice and vapour.  At any point on the curve the following equilibrium will exist. Ice ↔ Vapour. With decrease in temperature initially the vapour pressure drops steeply followed by slow decrease. Therefore this curve has positive slope. The curve starts at ‘O’ and ends at ‘B’ i.e.is at absolute zero (-273oC). At this point no vapour can exist and therefore only ice is present. The degree of freedom of the system is one i.e. univariant. This is predicted by the phase rule. ∴ F = C-P+2; F = 1-2+2; F = 1 Fusion Curve OC:  It is called the fusion curve or melting point curve of ice  It represents the equilibrium between ice and water.  At any point, on the curve the following equilibrium will exist. Ice ↔ water DEPARTMENT OF HUMANITIES & SCIENCES 166 ENGINEERING CHEMISTRY PHASE RULE It indicates the effect of pressure on the melting point of ice. As ice occupies more volume than liquid water, according to Le-Chatelier’s principle any rise in pressure shifts the equilibrium in favour of liquid water. This is seen by the sloping of the curve OC towards the vapour pressure axis indicating melting point of ice is decreased by increasing pressure. Therefore the fusion curve has a negative slope. ∴ F = C – P+2 = 1 -2 +2 = 1 ∴ It has one degree of freedom and the system is univariant Metastable curve OA1: Water always does not freeze at 0oC. Some times, water can be cooled below its freezing temperature, ie water can be super cooled by carefully eliminating solid particles. The curve OA represents vapour pressure curve of super cooled water. Along the OA1, the super cooled liquid is in metastable equilibrium with vapour. As soon as small particle of ice is kept in contact with super cooled liquid, it changes into ice and the curve merges in OB. At any point on the curve the following equilibrium will exist. Super-cool water ↔ vapour The degree of freedom of the system is one i.e., univariant. Triple point:  The curves OA, OB, OC and OA1 meet at point O, and this point of intersection is called the Triple point where all three phases, ice, water and vapour are in equilibrium.  The Temperature and pressure at the triple point of water are 0.0098oC and 4.58 mm respectively.  According to phase rule, the degree of freedom is zero i.e., invariant. ∴ F = C-P+2; F=1-3+2; F = 0 Thus the triple point represents an invariant system, i.e. any variation in even one of the variable (either pressure or temperature) breaks the three phase equilibria and transforms into a two phase equilibria represented by the curves Areas: AOB, AOC and BOC contain vapour, liquid and ice phases respectively, within these single phase areas, the system is bivariant. This is predicted by the phase rule. ∴ F=C-P+2; F=1-1+2; F=2 To locate any point in an area, temperature and pressure needs to be known. Two – Component system: In a two-component system, when P=1 Then, F= C- P+2 DEPARTMENT OF HUMANITIES & SCIENCES 167 ENGINEERING CHEMISTRY PHASE RULE =2–1+2= 3 This means that three variables must be specified in order to describe the state of the phase. For graphical representation of these variables on a phase diagram requires a 3- dimensional solid model phase diagram of a binary system should be represented by a three dimensional diagram of temperatures, pressure and composition, which cannot be drawn on paper. In order to simplify the system, the phase diagrams are represented in a two- dimensional way- considering only two variables, the third one being a constant. If pressure is kept constant, the phase diagram is called Isobaric If temperature is kept constant, it is called isothermal and If composition is kept constant, the phase diagram is called isoplethal. Condensed System: For two- component solid/liquid system, the gas phase is usually absent and the effect of pressure on the equilibrium is very small. Then it is necessary to consider the other two variables, i,e. temperature & concentration. Such a solid/liquid system with the gas phase absent is called a condensed system. The degree of freedom in such system is reduced to one and the corresponding phase rule equation is called reduced or condensed phase rule and it is written as F1 = C – P + 1 Types of two-component system: TYPE A: When the components are completely miscible with one another in liquid state and they do not form any compound and on solidification they give rise to intimate mixture known as eutectic. Eg: Pb – Ag, Pb – Sn, KI – water system TYPE B: System in which the two components form a stable compound. Eg: FeCl3 – water system, Zn – Mg system NaCl - water system SILVER – LEAD SYSTEM: Silver – lead system is an example for two – component system. The four possible phases of silver – lead system are (i) solid silver (ii) solid lead (iii) solution of silver & lead in molten state (iv) vapour. The boiling points of silver and lead being very high, vapour phase is practically absent. Thus Ag/Pb system can be considered as condensed system with three phases. In such condensed system, pressure changes have negligible impact on the system. Hence phase diagram is plotted between temperature and composition at constant pressure of 1 atm. So for silver – lead system ‘P’ is negligible. ∴ Isobaric phase diagram are studied and condensed phase rule equation is applied. DEPARTMENT OF HUMANITIES & SCIENCES 168 ENGINEERING CHEMISTRY PHASE RULE Phase diagram-Ag/Pb system The phase diagram consists of (1) Two curves OA, & OB (2) Five areas: AOB, BOD, AOC, COE, & DOE (3) Eutectic point ‘O’ Curve OA: It is the melting point curve or freezing point curve of Ag. The point A represents the melting point of pure silver (M.P = 961oC). The melting point of silver falls gradually by the addition of lead till the silver melt gets saturated with lead as represented by the curve OA. Any further addition of lead beyond O, it simply separates out as solid Pb. This saturation limit is represented by ‘O’ is called the eutectic point. Along ‘OA’ solid silver & melt of silver & lead are in equilibrium. So the system is two phase, two component system. ∴ F1 = C – P + 1 =2–2+1=1 Hence the curve ‘OA’ represent a univariant system, where fixing any one of the variable, automatically fixes the other variable and even a slight variation of the variable leads to the total disappearance of one of the phases. Curve OB: This is freezing point or melting point curve of Pb. The point B represent the melting point of pure lead (M.P = 327oC). By the gradual addition of silver, melting point of lead falls, till the lead melt gets saturated with silver. Any further addition of silver beyond O, simply separate out as solid Ag. Along curve OB, solid lead and melt of silver & Pb are in equilibrium. ∴ F1 = C – P + 1 =2–2+1=1 ∴ The system is univariant. EUTECTIC POINT O:  The curves OA & OB meet at eutectic point ‘O’. At ‘O’ the silver solution becomes saturated with lead and lead solution becomes saturated with silver. On further addition of DEPARTMENT OF HUMANITIES & SCIENCES 169 ENGINEERING CHEMISTRY PHASE RULE silver or lead, solid will get separated out. This saturation limit is represented by ‘O’ called eutectic point.  At this point all 3 phases i.e; solid Pb, solid Ag and liquid solution are in equilibrium  On applying reduced phase rule equation F1 = C – P + 1 =2–3+1=0  Hence point ‘O’ represents an invariant system, where any minor variation in one of the variables disturb the three phase equilibrium resulting in a atom phase equilibrium.  The temperature and composition of the eutectic mixture corresponding to eutectic point O at 1atm Pressure are 303oC and 2.4% Ag + 97.6% Pb respectively.  The eutectic point refers to lowest freezing point of liquid mixture of two metals.  If the temperature is raised above eutectic temperature, the solid phases Ag & Pb disappear and if the system is cooled below the eutectic point, only solid Ag/Pb exists where solution phase is non-existent. Area AOB: This area represents a homogenous solution of Ag & Pb. ∴ F1 = C – P + 1 =2–1+1=2 The system is bivariant Area AOC: The area AOC has solid Ag & liquid melt in equilibrium. ∴ F1 = C – P + 1 =2–2+1=1 System is univariant Area BOD: The area BOD has solid lead & liquid melt in equilibrium. ∴ F1 = C – P + 1 = 2 – 2+ 1 =1 System is univariant Area COE: The area COE has solid eutectic and solid Ag in equilibrium and the system is univariant (F1 = 2 – 2 + 1 = 1) Area DOE: The area has solid lead & solid eutectic in equilibrium. This is also a univariant system like area COE EUTECTIC SYSTEM: 1. Eutectic means easy to melt (Greek: eutectos = easy melting) 2. A system in which two components are miscible in all proportions in the liquid state, but do not react chemically and each component reduces the melting point of other is called eutectic system. 3. A solid solution of two- component system having the lowest freeing point is called ‘Eutectic mixture’. 4. The lowest freezing point that can be obtained corresponding to eutectic mixture is known as ‘Eutectic point’. At the eutectic point three phases co-exist. ∴ F1 = C – P + 1 = 2 – 3 + 1 = 0 (a non-variant or invariant point) Characteristics of Eutectic Point: 1. Eutectic point refers to lowest freezing point of a liquid mixture of any two metals at a given pressure. DEPARTMENT OF HUMANITIES & SCIENCES 170 ENGINEERING CHEMISTRY PHASE RULE 2. Below the eutectic temperature, no liquid phase of any composition can exist in the system. 3. The system is invariant at the eutectic point and it has definite values of temperature and composition. 4. Similar to molten metal, eutectic mixture freezes at a constant temperature. The solid eutectic so formed has a fixed composition and is called an alloy. 5. If the liquid melt of the two components is cooled just below the eutectic point, both the components solidify in the form of small crystals without any change in composition. 6. Because of their crystal characteristics, the eutectic mixtures of alloys have greater strength than their components. 7. Inspite of having a definite composition and fixed freezing point eutectic mixture is never considered as a compound as the crystals of components are clearly under microscope. Applications of Eutectic system: 1. Eutectic mixture composed of alloys are used as safety devices in boilers, as plugs in automobiles. 2. Because of their low melting points eutectics are also used in preparing solders which are used in joining two metal pieces together. Eg: lead – tin solders. 3. They are also used in safety fuses used in building to protect them against fire hazards. Eg: Wood metal, which is an alloy of 50%, Bi + 25%, Pb + 12.5%, Sn + 12.5% Cd. Application of Ag-Pb System in Desilverisation of Pb (or) Pattinson’s Process:  The principle of (Ag-Pb) system is utilized in the pattinson’s process of desilverisation of lead.  During extraction of Pb from its ore PbS (Galena), a very small amount of Ag remains associated with Pb.  Ag is soluble in Pb to some extent. Thus obtained is known as Argentoferrous lead.  The process of removing this traces of Ag from Argentoferrous lead is known as desilverisation of Pb. If a sample of argentoferrous lead containing less than 0.5% Ag, is allowed to cool gradually, lead will separate out and the solution will become progressively richer in Ag, till the percentage 2.6% of Ag is reached and on further cooling the whole mass will solidify as such, on the other hand, if lead –silver alloy containing Ag greater than 2.6% is allowed to cool, then pure silver separated along curve BC, till the eutectic composition O is reached Iron–Carbon System: Iron-carbon system represents an interstitial solid solutions, which are formed when the alloying elements differ widely in their atomic sizes. Pure iron is not suitable for fabrication of structural components because of its weak mechanical properties. Carbon, though a non-metallic element forms alloys with iron to give various types of steel and improves the mechanical properties of the base metal to a large extent. The size of carbon atom is small compared to that of iron atoms and hence occupies interstitial position in crystal structure of iron, which in turn depends on the temperature. Iron has 3 allotropic forms: DEPARTMENT OF HUMANITIES & SCIENCES 171 ENGINEERING CHEMISTRY PHASE RULE α – iron BCC exists up to 9100C 𝛾 - iron FCC exists from 910 – 14000C ð - iron BCC exists from 14000C – 15390C Cooling curve of Iron: Depending on temperature, iron may exist in BCC (or) FCC structures. At room temperature iron is in BCC and at 9100C it changes to FCC and then at 14000C back to BCC. 7700C is called the curie point. At this temperature, the magnetic properties of iron disappear, when the temp drops below the curie point then the magnetic properties reappear. Iron – carbon phase diagram: The Fe-C phase diagram consists of various important microconstituents, they are: 𝑎 - Ferrite: 1. It is the solid solution of carbon in ∝ - iron 2. It contains carbon up to 0.25% at 7230C 3. Possess BCC structure. 4. Exist at room temperature. 5. Soft, ductile and weak. Austenite: 1. It is the solid solution of carbon in 𝛾- iron 2. It has FCC crystal structure. 3. Possess higher solubility carbon up to 2% at 11300C 4. Does not exists below 723oC 5. Maximum concentration of carbon is 0.83% at 723oC. 6. Very soft and ductile at temp at which it exists. ( i.e: 723oC to 1400oC). - Ferrite: 1. It is solid solution of carbon in ð- iron. DEPARTMENT OF HUMANITIES & SCIENCES 172 ENGINEERING CHEMISTRY PHASE RULE 2. Maximum concentration of carbon in ferrite is 0.99% at 1493oC 3. It has BCC structure 4. It has soft structure. Cementite: 1. It is an iron- carbide compound with composition of Fe3C 2. It has complex orthorhombic structure 3. It exist at all temperatures studied from 0 to 18000C 4. It is hard & brittle 5. Effects the properties of steel & cast iron 6. Its presence in steel along with ferrite makes them tough, hard & strong. Pearlite: 1. It consists of alternate layers of ferrite and cementite 2. It is the product of decomposition of austenite by an eutectoid reaction. 3. It contains 0.8% carbon 4. Properties are in between those of ferrite and cementite. Softer and more ductile than cementite but harder and stronger than ferrite. Martensite: 1. It is transformed form of austenite. 2. It is supersaturated interstitial solid solution of carbon in ∝- ferrite with a distorted BCC or tetragonal structure. 3. Hard & brittle and has very little ductility The melting point of pure iron is 1539oC. At this temp, pure iron is called ð- ferrite having BCC structure. DEPARTMENT OF HUMANITIES & SCIENCES 173  ENGINEERING CHEMISTRY PHASE RULE The phase diagram extends form pure iron to the compound iron carbide (Fe3C) 1. The region above the curve ABC represents the liquid state. ∴ The line ABC is known as liquidus line above which only one liquid phase exists. 2. The region below the curve ADBE represents only one phase i.e. solid state of various composition of iron and carbon. ∴ The curve ADBE is known as solidus line 3. The region between the curves ABC & ADBE i.e. liquidus & solidus line represents a two phase system since it is a mixture of solid and liquid. 4. Pure iron melts at 15390C. By progressive addition of carbon to iron, the melting point of iron is decreased up to point ‘B’, where the carbon content is 4.3% at 1134 0C. On further increase in carbon content, melting point again rises, i.e. up to point ’C’ where carbon content is 6.67% and it is known as cementite (Fe3C). 5. The region ADBE represent the existence of austenite and liquid and the area BCE consists of cementite + liquid. 6. The region ADGEA represents the condition in which a solid solution of carbon in 𝛾- iron known as austenite exists. 7. The maximum amount of carbon that can be held in austenite is 2% at 1130 0C indicated by point D, which marks the limit (2%) of carbon content in steel. 8. FG represent the beginning of the change of γ –iron to ∝- iron which completes along HG line. 9. The beginning of separation of cementite from austenite is represented by the line DG, and its completion is represented by GI. 10. The transformation temperature of 9100C for γ – to ∝- change is lowered down to 7230C as the carbon content goes up to 0.8%, i.e. at point ‘G’ which is in solid state. The point G DEPARTMENT OF HUMANITIES & SCIENCES 174 ENGINEERING CHEMISTRY PHASE RULE is similar to B, but it represents changes taking place in solid state. Hence it is known as “eutectoid point”. 11. At this point ‘G’ austenite is transformed into pearlite. This is called eutectoid transformation. At this point three phases austenite, ferrite & cementite co-exist in equilibrium. 12. The eutectic mixture austenite and cementite is called ledeburite. The eutectic liquid freezes to ledeburite at 11300C with composition of 4.3% carbon at point ‘B’. 13. Further cooling transforms austenite to cementite, then to ferrite gradually and again at the eutectoid point ‘G’ the remaining austenite is transformed into pearlite at 7230C. Features of Fe – C phase diagram: * Upto 0.088% C are regarded as pure iron. * 0.088% to 2% C represents steel. * Above 2% C represents cast iron. Steel are further divided into two parts (i) Hypo- eutectoid steel (0.08 to 0.8% C ) (ii) Hyper – eutectoid steel(0.8 to 2% C) Eutectoid steel – steel containing 0.8% carbon. Heat Treatment of Steel: Heat treatment is combined operations of heating and cooling of metal (or) an alloy in solid state in order to get desired properties. During heat treatment, the size and shape of grain or the composition of the phase undergoes change with respect to the micro constituents and internal stress will be relieved. Heat treatment aims at (i) Increasing strength, toughness, hardeness, ductility to steel. (ii) Relieving internal stress and strain. (iii) Normalizing steel which has been subjected to mechanical or heat treatment. The heat treatment methods include: 1) Transformation hardening 2) Tempering 3) Annealing 4) Normalizing 5) Case hardening Transformation Hardening (Hardening)  When plain carbon steel is heated to a temperature above 7230C for long time, dissolution of more carbon takes place with the formation of austenite phase in FCC structure.  This on slow cooling FCC changes to BCC and excess carbon forms cementite.  If the steel is quenched by plunging into water at 2040C, the carbon atoms do not have sufficient time to form cementite, but remain trapped in the BCC lattice.  Excess carbon gets precipitated in the hot metal and, prevents slipping of planes.  Thus quenched steel is quite hard and strong and has lower ductility.  This transformation of austenite to martensite which is hard steel, by heat treatment is known as “Transformation Hardening”. DEPARTMENT OF HUMANITIES & SCIENCES 175 ENGINEERING CHEMISTRY PHASE RULE Tempering:  If simple carbon steel, which is in austenite state is quenched by plunging into water (or) brine, the unstable 𝛾- iron in FCC and held carbon in solid solution changes to stable ∝- iron at that temperature.  Quenched steel is not useful for construction purpose because of its brittleness. Therefore quenching is always followed by another heat treatment process called tempering.  The quenched steel is tempered by reheating to below ∝ - to 𝛾 - iron transition temperature.  At elevated temp, there is greater atomic mobility. The stresses and strains are relieved and excess carbon is rejected as Fe2.4C.  The sample is then cooled in air, water, oil etc to get resistance to abrasion and the steel is capable of withstanding shock loads. Annealing:  Annealing involves heating and holding the steel at the critical temperature for some time to facilitate the dissolution of carbon in 𝛾-iron followed by a slow cooling in a controlled manner in a furnace.  By annealing the steel becomes soft, ductile and machineable.  However annealing decreases the hardness and strength of steel.  Annealed hyper eutectoid steels contain cementite. They are not soft but can be machined easily. In contrast annealed hypo eutectoid steels contain ferrite and are relatively soft and malleable. Annealing consists of 3 stages: (i) Heating to the right temperature (ii) Keeping at that temperature for some time (iii) Slow cooling in a furnace. Advantages: 1) Annealing removes internal stresses 2) It changes ductility, toughness, electrical and magnetic properties 3) It removes gases entrapped within the mass 4) It improves machinability. Normalising:  Normalising means bringing back the structure and properties that are normal for a sample.  It is another heat treatment involving cooling of the hot steel more rapidly as compared to annealing but less rapidly when compared to quenching.  It produces steel with high tensile strength, yield strength & impact resistance.  Normalising is the corrective treatment for grain refinement in steel after it has been subjected to rolling, forging etc. Case Hardening: This technique is used for surface modification. In case hardening carbon is introduced into a solid ferrous alloy by heating the metal in contact with a carbonaceous solid, liquid or gas. DEPARTMENT OF HUMANITIES & SCIENCES 176 ENGINEERING CHEMISTRY PHASE RULE In case hardening, the material is placed in carburizing cases of about 1cm thick layer of placing material. It gives its carbon to steel. Charcoal, charred leather and barium carbonate are used. The box is sealed and heated to 900-9500C where α- iron is converted to γ – iron which absorbs carbon at its surface. This process is called pack carburizing. N KUNDANA, Assistant Professor , BVRITN DEPARTMENT OF HUMANITIES & SCIENCES 177