ENGINEERING MATERIALS

ENGINEERING MATERIALS

IRON-IRON CARBIDE PHASE DIAGRAM

COMPONENTS AND PHASES • Components: The elements or compounds which are mixed initially (e.g., Al and Cu) • Phases: The physically and chemically distinct material regions that result (e.g., a and b). Aluminum- Copper Alloy Adapted from Fig. 9.0, Callister 3e. 3 3

PHASE DIAGRAM A material scientist and engineer must have adequate knowledge of the understanding of phase diagrams for alloy systems because there is a strong correlation between microstructure and mechanical properties. The development of microstructure of an alloy is related to the characteristics of its phase diagram and furthermore phase diagrams provide valuable information about melting, casting, crystallization, and other phenomena. Components are pure metals and/or compounds of which an alloy is composed. For example in brass copper and zinc are the components.

PHASE DIAGRAM In material system may be defined as a specific body of material under consideration or it may be series of possible alloys consisting of the same components, but without regard to alloy compositions such as a system of iron and carbon, iron-carbon system. Equilibrium of the system may be defined in terms of a thermodynamic parameter known as free energy. It is the internal energy with respect to disorder of the atoms or molecules. A system is said to be in equilibrium if its free energy is a minimum under some specified combination of temperature, pressure and composition.

PHASE DIAGRAM PHASE A phase is a region in material that differs in its microstructure and/or composition from another region. For example water can exist in solid, liquid and vapor phases. PHASE DIAGRAIM The graphical representations of what phases are present in a materials system at various temperature, pressure, and composition is known as Phase Diagram. Most phase diagrams are constructed by using equilibrium conditions and are widely used by engineers and scientists to understand and predict many aspects of the behavior of materials.

MORE ABOUT PHASES PHASE EQUILIBRIUM The term phase equilibrium is usually refers to equilibrium as it applies to system in which more than one phase may exist. An familiar example of two phases of a pure substance in equilibrium is a glass of water containing ice cubes. Similarly during boiling of water liquid water and water vapor are two phases in equilibrium. The simplest and easiest type of phase diagram is in which composition is held constant and temperature and pressure are variables.

TYPES OF PHASE DIAGRAMS The one component phase diagram or unary phase diagram is called a pressure-temperature (P-T) diagram. This diagram is represented as a two-dimensional plot of pressure (vertical axis) versus temperature (horizontal axis). In the pressure-temperature phase diagram of water there exists a triple point at low pressure & temperature where solid, liquid and vapor phases of water coexist. Liquid and vapor phases exist along the vaporization line and liquid and solid along the freezing line. These lines are two-phase equilibrium lines. Pressure-temperature phase diagrams have been determined experimentally for a number of substances, which have also solid, liquid, and vapor phase regions.

In the same way P-T phase diagram can also be constructed for other pure substances. For example the equilibrium P-T phase diagram for pure iron has three separate and distinct solid phases of iron, alpha, gamma, and delta. Alpha and delta phases of iron have BCC crystal structures whereas gamma phase has an FCC structure. The triple point in the iron P-T diagram where three different phases coexist: (1) liquid, vapor, and delta ferrous, (2) vapor, delta ferrous, and gamma ferrous, and (3) vapor, gamma ferrous, and alpha ferrous. Under equilibrium condition, alpha and gamma iron can exist at a temperature of 910 ºC and 1 atm pressure. Above 910 ºC only single-phase gamma exists, and below 910 ºC only single-phase alpha exists. Also there are three triple points in the iron PT diagram where three different phases coexist.

In a phase diagram for a typical substance at a fixed volume, vertical axis is Pressure, horizontal axis is Temperature. At higher T, a higher P is necessary to maintain the substance in liquid phase. At the triple point the three phases; liquid, gas and solid; can coexist. Above the critical point there is no detectable difference between the phases.

Another type of extremely common phase diagram, binary phase diagram, is one in which temperature and composition are variable parameters, and pressure is held constant (1 atm). Binary phase diagrams are maps that represent the relationship between temperature and the compositions and quantities of phases at equilibrium, which influence the microstructure of an alloy. A mixture of two metals is called a binary alloy and constitutes a two component system since each metallic element in an alloy is considered a separate component. For example pure copper is a one-component system, whereas an alloy of copper and nickel is a two-component system. BINARY PHASE DIAGRAMS

Sometimes a compound in an alloy is also considered a separate component. For example, plain-carbon steels containing mainly iron and iron carbide are considered two-component systems. This type of phase diagram would be discussed in detail. In some binary metallic systems the two elements are completely soluble in each other in both the liquid and solid states. In such systems only a single type of crystal structure exists for all compositions, and these are called Isomorphous systems. Binary phase diagrams are helpful in predicting phase transformations and the resulting microstructures, which may have equilibrium or nonequilibrium character.

Probably the easiest type of binary phase diagram to understand and interpret is the type that is characterized by copper-nickel (two components) system. In this system temperature is plotted on the y-axis and composition (weight %) is plotted on the x-axis. The composition ranges from 0% to 100%. Three different phases or fields can be identified such as (1) alpha region, (2) liquid region, and (3) a two-phase alpha+liquid region. Each region is defined by the phase / phases that exist over the range of temperatures and compositions separated by the phase boundary lines. This system is also isomorphous as copper and nickel are mutually soluble in each other for all compositions because their crystal structures are same.

TENARY PHASE DIAGRAM In ternary phase diagrams there exist three components as compared to two and one component. Compositions on ternary phase diagrams are usually constructed by using an equilateral triangle as a base. Compositions of ternary systems are represented on the base of this triangle with the pure component at each end of the triangle. The ternary phase diagram of iron, chromium, and nickel is important since the commercially most important stainless steel has a composition of 74% iron, 18% chromium, and 8% nickel. Ternary phase diagrams also are important for the study of some ceramic materials. However such diagrams are very complicated to construct.

INTERPRETATION OF PHASE DIAGRAMS Following three types of information may be available from a binary system of known composition and temperature if the system is in equilibrium: Type of phases present in the system Compositions of these phases The percentages or fractions of the phases In order to do it first of all temperature-composition point on the phase diagram is located. In case only one phase exists, the composition of the phase is simply the same as the overall composition of the alloy.

INTERPRETATION OF PHASE DIAGRAMS In case of two phases situation is a bit complicated. In such case a series of horizontal lines at different temperatures may be considered. These are known as tie-line or isotherm. In order to do it a tie-line is constructed across the two-phase region at the temperature of the alloy. The intersections of the tie-line and the phase boundaries one either side are noted. Perpendiculars are dropped from these intersections to the horizontal composition axis from which the composition of each of the respective phase can be determined. Similarly the relative percentage of the phases present at equilibrium may also be computed with the aid of phase diagram.

INTERPRETATION OF PHASE DIAGRAMS FOLLOWINGS ARE SOME OF THE SIMPLE RULES TO BE FOLLOWED FOR THE INTERPRETATION OF BINARY PHASE DIAGRAMS: A PHASE DIAGRAM CONSISTS OF LINES THAT DIVIDE IT INTO A NUMBER OF AREAS, OR FIELDS. THESE FIELDS MAY BE SINGLE PHASE, OR TWO PHASE. SINGLE-PHASE AREAS ARE ALWAYS SEPARATED BY A TWO-PHASE ZONE, AND THREE PHASES CAN ONLY CO-EAXIST AT A POINT , KNOWN AS EUTECTIC POINT. WHEN A VERTICAL LINE REPRESENTING THE COMPOSITION OF SOME ALLOY IN THE SYSTEM CROSSES A LINE IN THE PHASE DIAGRIM, IT INDICATES THAT SOME CHANGE IS TAKING PLACE IN THE ALLOY.

INTERPRETATION OF PHASE DIAGRAMS FOR ANY POINT IN A TWO-PHASE REGION THE COMPOSITION OF THE TWO PHASES IN EQUILIBRIUM WITH ONE ANOTHER CAN EASIBLY BE DETERMINED. IF A HORIZONTAL TIE-LINE IS DRAWN THROUGH THE POINT OF THE INTERSECTIONS OF THIS LINE WITH THE PHASE BOUNDARY LINES DENOTE PHASE COMPOSITIONS. THE RELATIVE PROPORTIONS OF THE PHASE CAN BE DETERMIENED BY USING THE LEVEL RULE. FOR EXAMPLE, BY USING THE LEVER RULE THE WEIGHT % LIQUID AND WEIGHT % SOLID FOR ANY PARTICULAR TEMPERATURE CAN BE DETERMINED IN THE TWO-PHASE OF THE BINARY COMPOUND.

IRON-IRON CARBIDE PHASE DIAGRAM Possibly the most important of all binary alloy systems is for Iron (Fe) and Carbon (C). In each and every field of engineering where discussion is about the integrity and reliability of structures and machines, this binary alloy system is talked about. Both steels and cast irons are essentially iron-carbon alloys. Iron-carbon alloys containing from a very small amount, 0.03%, to about 1.2% carbon, 0.25 to 1.00 % manganese, and minor amounts of other elements are known as plain-carbon steels. The phases present in very slowly cooled iron-carbon alloys at various temperatures and compositions of iron with up to 6.67% of carbon are shown in the Iron-Iron Carbide phase diagram.

IRON-CARBON (Fe-C) PHASE DIAGRAM (Adapted from Fig. 9.24, Callister 6e. (Fig. 9.24 from Metals Handbook, 9th ed., Vol. 9, Metallography and Microstructures, American Society for Metals, Materials Park, OH, 1985.) Adapted from Fig. 9.21,Callister 6e. (Fig. 9.21 adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.) 21

IRON-IRON CARBIDE PHASE DIAGRAM Iron-Iron Carbide phase diagram is in fact not a true equilibrium diagram since the compound Iron Carbide that is formed is not a true equilibrium phase. Under certain conditions Iron Carbide, also known as Cementite, can decompose into the more stable phases of iron and carbon. However, for most practical conditions Iron Carbide is very stable and, therefore, is treated as an equilibrium phase.

PHASES OF IRON-IRON CARBIDE DIAGRAM The Iron-Iron Carbide phase diagram has following solid phases: Alpha Ferrite: This phase is an interstitial solid solution of carbon in the BCC iron crystal lattice. As indicated in the phase diagram, carbon is only slightly soluble in alpha ferrite, reaching a maximum of 0.218% at 723 degrees C. The solubility of carbon decreases to 0.005% at 0 degrees C. Austenite (Gamma): The interstitial solid solution of carbon in gamma iron is called austenite. Austenite has an FCC crystal structure and a much higher solid solubility for carbon than alpha ferrite. The solid solubility of carbon in austenitic is a maximum of 2.11% at 1148 degrees C and decreases to 0.77% at 723 degrees C.

PHASES OF IRON-IRON CARBIDE DIAGRAM Cementite: The intermetallic compound iron carbide is called cementite. It has negligible solubility limits and a composition of 6.67% carbon and 93.3% iron. Cementite is a hard and brittle compound. Delta Ferrite: The interstitial solid solution of carbon in delta iron is called delta ferrite. It has a BCC crystal structure like alpha ferrite but with a greater lattice constant. The maximum solid solubility of carbon in theta ferrite is 0.09 at 1465 degrees C.

It must be mentioned here that carbon is an interstitial impurity in iron and forms a solid solution with each of alpha, austenitic and delta ferrite. Although carbon is present in relatively low concentrations, it significantly influences the mechanical properties of ferrite. Ferrous alloys are those in which iron is the prime component, but carbon and other alloying elements may be present.

There are three types of ferrous compounds as per the classification scheme of ferrous alloys based on carbon content, Iron, Steel and Cast Iron. Pure iron contains less than 0.008 wt% C; iron-carbon alloys that contain between 0.008 and 2.14 wt% C are classified as Steels. Cast Irons are those ferrous alloys that contain between 2.14 and 6.70 wt% . Commercial cast irons normally contain less than 4.5 wt% C.

BINARY EUTECTIC ALLOY SYSTEM Many binary alloy systems have components that have limited solid solubility in each other such as the Lead-Tin system. In such systems there are two regions of restricted solid solubility known as alpha and beta phases. Alpha and beta phases are solid solutions which can dissolve in solid solution to a maximum composition and temperature. In simple binary eutectic systems there is a specific alloy composition known as the eutectic composition that freezes at a lower temperature than all other compositions. This reaction is shown as Liquid ----------- α solid solution + β solid solution EUTECTIC, PERITECTIC AND MONOTECH ALLOY SYSTEMS

BINARY PERITECTIC ALLOY SYSTEM Another type of reaction that often occurs in binary equilibrium phase diagram is the peritectic reaction. As the melting points of the two components are quite different, therefore, this reaction is a part of a more complicated binary equilibrium diagram. In the peritectic reaction a liquid phase reacts with a solid phase to form a new and different solid phase. In the general form this reaction can be written as Liquid + α------> β During the equilibrium of an alloy of peritictic composition through the peritectic temperature, all the solid-phase alpha reacts with all the liquid to produce a new solid-phase beta. EUTECTIC, PERITECTIC AND MONOTECH ALLOY SYSTEMS

BINARY MONOTECCTIC ALLOY SYSTEM Another type of reaction that occurs in some of the binary phase diagrams is the monotectic reaction. In this type of reaction a liquid phase transforms into a solid phase and another liquid phase as shown. Liquid (1) ------------ α + Liquid (2) Over a certain range of compositions the two liquids are immiscible like oil in water and therefore constitute individual phases. This type of reaction occurs in the copper-lead system. Practically speaking lead is added in small amounts in many alloys to make the machining easier by reducing ductility. Such alloys are also used for bearings where small amount of lead smear out at wear surfaces between the bearing and shaft and thus reduce friction. EUTECTIC, PERITECTIC AND MONOTECH ALLOY SYSTEMS

So far three types of invariant reactions have been discussed which commonly occur in binary phase diagrams. Two other important invariant reactions occurring in binary systems are the Eutectoid and peritectoid types. Eutectic and eutectoid reactions are similar in that two solid phases are formed from one phase on cooling. However, in the eutectoid reaction the decomposition phase is solid whereas it is liquid in eutectic reaction. Similarly in the peritectoid reaction two solid phases react to form a new solid phase, whereas in the peritectic reaction a solid phase reacts with a liquid phase to produce a new solid phase. EUTECTIC, PERITECTIC AND MONOTECH ALLOY SYSTEMS

Eutectic Reation- A phase transformation in which all the liquid phase transforms on cooling into two solid phases isothermally. Eutectic Composition – The composition of the liquid phase that reacts to form two new solid phases at the eutectic temperature. Eutectic Point – The point determined by the eutectic composition and temperature. Hypoeutectic Composition – One that is to the left of the eutectic point. Hypereutectic Composition – One that is to the right of the eutectic point SOME IMPORTANT DEFINITIONS

SOME MORE DEFINITIONS Proeutectic Phase – A phase that forms at a temperature above the eutectic point. PeritecticReation - A phase transformation in which, upon cooling, a liquid phase combines with a solid phase to produce a new second solid phase. MonotecticReation - A phase transformation in which upon cooling, a liquid phase transforms into a solid phase and new liquid phase. Hypoeutectic Alloy - An alloy composition between that of the left-hand-side end of the tie line defining the eutectic reaction and the eutectic composition.

SOME MORE DEFINITIONS Hypereutectic Alloy - An alloy composition between that of the right-hand-side end of the tie line defining the eutectic reaction and the eutectic composition.

MECHANICAL FAILURE OF STRUCTURES BRITTLE FAILURE DUCTILE FAILURE RUPTURE FATIGUE CREEP FATIGUE + CREEP STEADY LOADING CYCLIC LOADING THERMAL LOADING THERMAL CYCLIC LOADING

MECHANICAL FAILURE OF STRUCTURES WHILE SELECTING MATERIAL TO AVOID ANY PREMATURE FAILURE OF STRUCTURES AND MACHINES, ONE MUST CONSIDER FOLLOWING BASIC PARAMETERS: DESIGN MATERIAL MANUFACTUREING AVAILABILITY AND COST OF MATERIALS QUALITY RECYCLING OF MATERIALS RUPTURE FATIGUE CREEP STEADY LOADING CYCLIC LOADING TEMERATURE

PARAMETERS TO BE CONSIDERED WHILE DESINING: MECHANICAL PROPERTIES ELASTIC MODULI AND STIFFNESS YIELD AND MAXIMUM STRENTHTS FATIGUE STRENGTH CREEP STRENGTH FACTURE TOUGHNESS HARDNESS DUCTILITY ABRASION RESISTANCE PHYSICAL PROPERTIES DENSITY ELECTRICAL CONDUCTIVITY MAGNETIC PROPERTIES THERMAL CONDUCTIVITY THERMAL EXPANSION THERMAL STABILITY

PARAMETERS TO BE CONSIDERED WHILE DESINING: CHEMICAL PROPERTIES RESISTANCE TO CHEMICALS CORROSION RESISTANCE RESISTANCE TO CHEMICALS CORROSION RESISTANCE OXIDATION RESISTANCE WEATHERING RESISTANCE MANUFACURING CHARACTERISTICS CASTABILITY FORMABILITY MACHINABILITY

COST AND AVAILABILITY MATERIAL COST MANUFACTURING COST AVAILABILITY PRICE STABILITY QUALITY FULFILMENT OF CUSTOMER REQUIREMENTS FIT TO USE EASILY AND ALWAYS AVAILABLE CUSTOMER SATISFACTION QUALITY MANAGEMENT SYSTEM (ISO-9001) ENVIRONMENTAL MANAGEMENT SYSTEM (ISO-14001) OCCUPATIONAL HEALTH & SAFETY ASSESSMENT SYSTEM OHSAS 18001

QUESTIONS AND QUERIES IF ANY! IF NOT THEN GOOD BYE AND HAVE A NICE WEEK END IRON AND STEELS – THEIR PRODUCTION

ASSIGNMENT N0. 02 Q. NO. 1 DEFINE PHASE DIAGRAM AND ITS TYPES IN YOUR OWN WORDS. Q. NO. 2 WHAT ARE TIE-LINES? HOW WOULD YOU DETERMINE COMPOSITIONS OF DIFFERENT PHASES IN AN ALLOY? Q. NO. 3 HOW DOES FAILURE OF STRUCTURES AND MACHINES OCCUR? EXPLAIN DIFFERENT TYPES OF FAILURES WITH ONE PRACTICAL EXAMPLE. Q. NO. 2 WHAT PARAMETERS A DESIGNER MUST TAKE INTO ACCOUNT WHILE SELECTING AN APPORPRIATE MATERIAL FOR HIS / HER PRODUCT?

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