Unit-III Phase diagram & Iron Carbon Diagram.pptx

Iron Carbon Diagram

1. Unit –III Phase Diagrams and Iron-Carbon Diagram

2. INTRODUCTION One of the important objectives of engineering metallurgy is to determine the properties of material. Th e propertie s of material i s a function of the microstructure which is further dependent on the overall compositio n and variables suc h as temp., pressure and composition. Equilibriu m diagram or phase diagram is a graphical representatio n of variou s phases present i n th e material system at various temp. and compositions.

3. Solid Solutions In solid state the alloy may be present in one or more of the following forms Solid Solution An intermediate phase. Mechanical mixture of metals. Mechanical mixture of Solid solutions Mechanical Mixture of Chemical compound of Metals Alloys are more widely used than pure metals Properties of alloys are better than pure metals .

4. A solid solution is formed when two metals are completely soluble in liquid state and also completely soluble in solid state. In other words, when homogeneous mixtures of two or more kinds of atoms (of metals) occur in the solid state, they are known as solid solutions. Solvent and Solute The more abundant atomic form is referred as solvent The less abundant atomic form is referred as solute .

5. Types of Solid Solutions Substitutional Solid Types Interstitial Solid Solutions

6. Substitutional Solid Types If the atoms of the solvent or parent metal are replaced in the crystal lattice by atoms of the solute metal then the solid solution is known as substitutional solid solution. □ For example, copper atoms may substitute for nickel atoms without disturbing the F.C.C. structure of nickel In the substitutional solid solutions, the substitution can be either disordered or ordered. Figure 1.1 shows disordered substitutional solid solution. Here the solute atoms have substituted disorderly for the solvent atoms on their lattice site. Fig.1.1 shows an ordered substitutional solid solution. Here the solute atoms have substituted in an orderly manner for the solvent atoms on their lattice site

7. USEFUL TERMINOLOGY

8. A pure substance, under equilibrium conditions , as eithe r of a phase namel y vapor, may exist or solid, l d iq ep u e id ndingupon the conditions of temperature and pressure. A phase can be defined as a homogeneous portion of a system that has uniform physical and chemical characteristics i.e. it is a physically distinct from other phases, chemically homogeneous and mechanically separable portion of a system. Other words, a phase is a structurally homogeneous portion of matter.

9. It is a region that differs in it’s microstructure and composition from another region. Forthe same composition, different crystal structures represent different phases. A solid solution has atoms mixed at atomic level thus it represents a single phase. A single-phase system is termed as homogeneous, and systems composed of two or more phases are termed as mixtures or heterogeneous. Most of the alloy systems and composites are heterogeneous

10. 1. System A system is a portion of interacting or interdependent component parts forming a complex whole is called system This refers to any portion of objective space within specified boundaries subject to specified variables. An alloy more syste m i s a combination elements forming of alloys considered within a specified two or which are temp. , pressure and concentration. range of

11. Systems are classified according to number of system it constitutes Types of systems:- Binary system: 2 components Ternary system: 3 components Quaternary System: 4 components

12. 2 . Phase : Phase is defined as homogeneous , physically distict portion of the system. Each portion is physically and chemically homogeneous composition throughout and regardless of sample taken in any portion. But each phase have same composition but different properties . In an equilibrium diagram, liquid is one phase and solid

13. Many phases exists in some alloys. Example: A liquid may exist with solid solution or two solid solutions of different combinations may exists. Many phases exists in same alloy Grey Cast Iron for example is made up of following phases in order to decreasing amount 1. Solid solution phase of iron containing some carbon , silicon, phosphorus 2. A soft metalloid , graphite (carbon) 3. A hard brittle compound, Fe 3 C (Cementite). 4. A compound of the impurity Sulphur (MnS) 5. A compound of impurity , Phosphorus (Fe3P) 6. A very small amount of non metallic compound.

14. 3. Variable : A particular phase exists under various conditions of temp., pressure and concentrations. These parameter are known as the variables of the phase. These variables specify the combination of phases present in system

15. Componen t – These are th e substances, chemical el t e h m e e r nt s or chemica l compound s whos e presence is necessary and sufficient to make a system. Pure metal is a one-component system whereas an alloy of two metal is a two-component system. Fe – 1 component Cu- Ni (Copper Nickel alloy)– 2 components Elements Cu and Ni In Ice water steam system, the component is H 2 O

16. 4. Alloy : It i s a mixture of two or more elements having metallic properties. Inthe mixture, metal is in the large proportion and the others can be metals or non-metals. Th e elemen t i n th e larges t amount i s calle d as base metal(Parent metal) or solvent and the other elements are called as alloying elements or solute . The components of alloy are usually completely soluble in liquid state

17. Phase equilibrium – it refers to the set of conditions where more thanone phase may exist. It can be reflected by constancy with time in the phase characteristics of a system. In most metallurgical and materials systems, phase equilibrium involves just solid phases.

18. PHASE DIAGRAM OF WATER

19. HUME-ROTHERY’S RULES FOR SOLID SOLUTION Solid solution is an alloy of two or more elements wherein the atomic crystal structure of the alloying element (solute) is same as that of the base metal matrix (solvent). The solubility limit of the solute in the solvent is govern by certain factors . These governing factors are known as Hume- Rothery’s rules for solid solubility.

20. 1. Atomic size or Relative Size Factor: - Greater the difference between the atom size of two metals involved , smaller is the solubility. -If atomic size of solute and solvent differ more than 15% ,then solid solubility is extremely smaller. -If atomic size differ exceeds less than 15% solid solubility is more.

21. 2. Chemical-affinity Factor :- The ions of all metals are electropositive but some are more electro positive than others. Greater the difference in electro positivity , greater will be the chemical affinity of one atom for another atom so that they will tend to form compound than solid solution. The greater the chemical affinity more restricted is their solid solubility Greater affinity greater the tendency of formation of compound. If Electropositive are similar then they will probably for solid solution Compound contains two or more elements that are chemically bound together whereas a solid solution has few substances that

22. 3.Relative Valence (Valency) Factor :- Valency is the number of atoms of a particular element that is combined with one atom of another element to form a molecule. Valency is also known as molecular weight. Valency is a measure of the combining power of an atom. The valency of an element is determined by the number of electrons in its outermost shell. The valency of an element can be increased either by gaining or losing electron. A metal of Higher valency can dissolve a small amount of lower valency metal. While the lower valency metal can have good solubility for higher valency metal.

23. 4.Crystal Structure Factor- Metals having same crystal structure will have greater solubility. Since atoms tend to assume relatively fixed positions. This gives rise to the formation of crystals in solid state. Difference in crystal structure limits the solid solubility.

24. GIBB’S PHASE RULE Dr.Gibbs studied the relationship between the number of phases and the effect of variables such as pressure, temperature and composition. The Gibb’s phase rule states that under equilibrium condition, the following condition must be satisfied P + F = C + 2 P = Number of phase in system F = Degree of Freedom i.e Number of variables that can be changed independently without affecting the number of phase. C = Number of Components (i.e. element) 2 = It represents any two variable amongst three

25. In general, all equilibrium diagrams are studied c a o t nstan t pressur e , hence rul e i s modifie d to. P + F = C + 1 Th e phase rul e helps determine maximu m phase p n r u e m s e b n e t r i n o a f n alloy syste m under equilibriu m condition s at any point in phase diagram.

26. COOLING CURVE FOR PURE METAL used to determine phase transition temperature record T of material vs time, as it cools from its molten state through solidification and finally to RT (at a constant pressure!!!) B1 Latent heat

27. NUCLEATION Nucleation is the beginning of a phase transformation. Nucleation is the process by which atoms or molecules come together to form a new phase or structure. This process is crucial in the formation of crystals, as well as in the formation of bubbles in a liquid or gas. It is marked by the appearance in the molten metal of tiny regions called nuclei of new phase which grow to solid crystals until the transformation is complete. 1. Homogeneous or self Nucleation. 2. Heterogeneous Nucleation.

28. Homogeneous or self Nucleation. Heterogeneous Nucleation. Interiors of a uniform substance Slower process It occurs with much more difficulties. Nuclei are formed from atoms of solidifying metals. Lower temperature. It requires supercoiling to form first nuclei. Start at the nucleation sites on the surface contacting liquid or vapor. Faster process Occurs easily. At impurity atoms or container surface acts as a nucleating agent. Higher temperature. It requires little or no supercoiling.

29. COOLING CURVE FOR PURE METAL Region AB, P + F = C + 1 1 + F = 1 + 1 F=1(T without changing liquid phase) Region BC, P + F = C + 1 2 + F = 1 + 1 F=0 (No variable) Region CD, P + F = C + 1 1 + F = 1 + 1 F=1 (T without changing a solid phase)

30. COOLING CURVE FOR BINARY SOLID SOLUTION Region AB, P + F = C + 1 1 + F = 2 + 1 F=2 (T and C without changing liquid phase) Region BC, P + F = C + 1 2 + F = 2 + 1 F=1 (T without changing liquid-solid phase ) Region CD, P + F = C + 1 1 + F = 2 + 1 F = 2 (T and C without changing solid phase)

31. COOLING CURVE FOR BINARY EUTECTIC ALLOY

32. Region AB, P + F = C + 1 1 + F = 2 + 1 F=2 (Bi-varient) Region BC, P + F = C + 1 3 + F = 2 + 1 F=0 ( Non Varient) Region CD P + F = C + 1

33. Binary Eutectic is homogeneous mixture of two solids which forms at constant temperature during cooling and melts at constant temperature during heating. Binary eutectic transformation reaction can be as follows Constant Temperature L S1 + S2 Where S1- one solid S2- another solid The temperature at which this transformation reaction is obtained is called eutectic temperature

34. COOLING CURVE FOR OFF-EUTECTIC BINARY ALLOY

35. Region AB, P + F = C + 1 1 + F = 2 + 1 F = 2 ( T & C withou t changin g Liqui d Phase) Region BC, P + F = 2 + 1 2 + F = 2 + 1 F= 1 ( Univarient) Region CD P + F = C+ 1 3 + F = 2 + 1 F=0 ( Non-varient )

36. Region CD P + F = C + 1 2 + F = 2 + 1 F=1 ( Univarient)

37. PLOTTING OF EQUILIBRIUM OR PHASE DIAGRAM Sample 1 2 3 4 5 6 7 8 9 10 11 % Cu 100 90 80 70 60 50 40 30 20 10 % Ni 10 20 30 40 50 60 70 80 90 100

38. LEVER RULE It is the method used to find out the exact amount of a particular phase existing in a binary system for a given alloy at any temperature under consideration. Let us consider an alloy A and B. Z be the composition of alloy under consideration and T be the temperature at which phase content is to be found.

39. According to lever rule, % of liquid = Intercept distance between Z% B alloy under consideration and the solidus line / Distance between solidus and liquidus line So, % of liquid Similarly, % of Solid = L(FD ) * 100 L(CD) = Intercept distance between Z% B alloy under consideration and the liquidus line / Distance between solidus and liquidus line So, % of Solid = L(CF) X 100 L(CD)

40. NOW, Amoun t of soli d = Amount of liquid L(CF) / L (CD) = L (FD)/ L (CD) L(CF) L(FD) Therefor, amount of solid x L(FD) = amount of liquid x L(CF) It means, the line CD acts as a lever arm and the point F acts as a fulcrum point as shown in fig, hence it is called as lever rule OR lever arm principle.

41. Tie line – connects the phases in equilibrium with each other - essentially an isotherm THE LEVER RULE How much of each phase? Think of it as a lever (teeter-totter) M L M α M ⋅ S = α M 20 T B 1200 1300 T(°C) L (liquid) α (solid) L + α L α liquidus + s o lidus B tie line 30 40 C L C o C α wt% Ni 50 R S R S ⋅ L R =Must balance

42. INTRODUCTION TO IRON CARBON DIAGRAM

43. 0.08 to 2 % carbon in Iron – Steel Iron- Ductile, Less strength & Hardness Increase the strength- by adding alloying elements Carbon alloying element in Iron (Steel) Plain carbon steel 1. Low carbon steel 2.Medium carbon steel 3.High carbon steel

44. ALLOTROPY OF IRON In actual practice it is very difficult to trace the cooling of iron from 1600°C to ambient temperature because particular cooling rate is not known. Particular curve can be traced from temperature, time and transformation (TTT) curve. However allotropic changes observed during cooling of pure iron are depicted in Fig.

45. Molten-Fe (Liquid state of iron) Delta-Fe (Body centered) austenite structure

46. TRANSFORMATION DURING HEATING AND COOLING OF STEEL

47. Principal phases of steel and their Characteristics Phase Crystal structure BCC Characteristics Ferrite Soft, ductile, magnetic Austenite FCC Soft, moderate strength, non- magnetic Cementite Compound of Iron & Carbon Fe 3 C Hard &brittle

48. 01/03/19 PROF.MAYU R S MOD I 81

49. 1. Austenite Austenite is a solid solution of free carbon (ferrite) and iron in gamma iron. On heating the steel, after upper critical temperature, the formation of structure completes into austenite which is ductile and non-magnetic. It is formed when steel contains carbon up to 1.8% at 1130°C. On cooling below 727°C, it starts transforming into pearlite and ferrite. Structures in Fe-C-diagram

50. Ferrite Ferrite contains very little or no carbon in iron. It is the name given to pure iron crystals which are soft and ductile. The slow cooling of low carbon steel below the critical temperature produces ferrite structure. Ferrite does not harden when cooled rapidly . Should be cold worked to be hardened

51. Cementite Cementite is a chemical compound of carbon with iron and is known as iron carbide (Fe 3 C). Cast iron having 6.67% carbon is possessing complete structure of cementite. It is extremely hard. It is magnetic below 200°C.

52. Pearlite Pearlite is a eutectoid alloy of ferrite and cementite. As the carbon content increases beyond 0.2% in the temperature at which the ferrite is first rejected from austenite drop until, at or above 0.8% carbon, no free ferrite is rejected from the austenite. This steel is called eutectoid steel , and it is the pearlite structure in composition.

53. 01/03/19 PROF.MAYU R S MOD I 88 SYMBO TEMP . ⁰ SIGNIFICANC A (curie 210 temp.) Above temp. Cementite losses its magnetism A 1 (LCT) 727 Above temp. perlite gets transformed in austenite A 2 (curie 768 temp.) Above temp . Ferrite losses its magnetism A 3 (critical hypo- eutectoid steeeel) 727-910 Above temp. Free ferrite gets dissolved to 100 % ferrite. A CM(critic al hyper- eutectoid steeeel) 727-1147 Above temp. Free cementite gets dissolved to 100 % austenite. A 4 (UCT) 1400- 1492 Above temp. Austenite gets transformed into δ – ferrite.

54. Peritectic reaction 0.55 % C 0.55 % C 149 3 ⁰C 140 ⁰C 153 9 ⁰C

55. The Austenite to ferrite / cementite transformation in relation to Fe-C diagram

56. Peritectic Reaction

57. Peritectic Reaction

58. Eutectoid Reaction

59. Eutectic Reactions

60. Eutectic Reaction

61. Eutectic Reaction