MODULE III

PHASE DIAGRAM

HEAT TREATMENT

STRENGTHENING OF MATERIALS

1

System

• A substance or group of substances isolated from its surroundings and subjected to changes in composition, temperature, pressure or volume.
• It is classified based on the number of components that constitute the system.
• An alloy system is a series of possible alloys consisting of the same elements or components.

2

Component

• A chemical constituent (element, ion or compound) of a system which may be used to specify its composition.
• A system having one component is called unary system and systems having two or three components are called binary or ternary systems.
• In steel, the components are Fe and C.
• They refer to the independent chemical species that comprise the system.

​

3

Phase

• A homogenous portion of a system, that has uniform physical and chemical characteristics.
• A phase is a physically distinct, chemically homogenous and mechanically separable region of a system.
• Every pure material is considered as a phase
• Water : solid , liquid & gas

4

Phase Rule

• The Gibb's Phase rule establishes the relationship between the number of components, the number of phases and the number of degrees of freedom in a system.
• In the simple form it can be expressed mathematically as
• F=C-P+2
• where Fis the number of degrees of freedom,
• C the number of components and
• P the number of phases in the system.
• The degrees of freedom refers to the independent variables (like temperature, pressure or concentration) which can be changed without affecting the other variables.

5

• Out of these independent variables, pressure is usually ignored in the case of liquids and solids.
• Now the phase rule can be written as
• F=C-P+1
• This modified form is applicable to all alloy systems involving metals

​

6

Phase diagram

• Phase diagram is a graphical representation of the physical states of a substance under different conditions of temperature and pressure.

7

8

Binary phase diagrams

• Determined at atmospheric pressure.
• Composition and temperature –independent variables
• X axis-% of B from zero at left extreme to 100% at right.

​

9

Construction

• No of samples of the binary alloy with different compositions is prepared
• Separately melted and allowed to solidify.
• Cooling curves are recorded and the points corresponding to solid- liquid transition are noted.
• Join the curves
• Liquidus: solidification start
• Solidus: solidification end

10

11

Classification of binary system

• Isomorphous system: two components are mixed, completely soluble in solid and liquid state.
• Eutectic system:
• Soluble in liquid state
• insoluble in solid state
• Partially soluble in solid state

​

12

Isomorphous system

• The two components are completely soluble in liquid and solid states.
• Cu-Ni, Ge-Si, Ag-Cu etc
• Cu-Ni phase diagram
• 3 phase regions:
• liquid (L)
• Alpha(α)
• Two phase(α+L)

13

+

14

Information available

• The phases that are present
• The compositions of these phases
• The relative percentages of the phases

15

• Phases that are present:
• X- 60% Ni, 40% Cu at 1150^oC : single phase represented by α
• Temp below 1085 ^oC, Cu and Ni completly soluble in solid and liquid states
• Y-35% Ni, 65% Cu at 1250 ^oC, (L+α)

16

17

• Determination of composition:
• X- 60% Ni, 40% Cu at 1150^oC : single phase represented by α
• When two phases are present
• A tie line is drawn horizontally across the two phase region corresponding to the selected temperature
• The inter section of the tie line with phase boundaries on both sides are noted
• Vertical lines are drawn downwards from the intersecting points to the composition axis, from which composition of the phases is obtained.

​

18

• Y: 1250 ^oC and 35%Ni and 65%Cu point C_o
• Liquidus curve: 31.5%Ni and 68.5%Cu -C_L
• Solidus curve :42.5%Ni and 57.5% Cu -C_a

19

31.5

35

42.5

• Determination of phase amount:
• Lever rule:
• W_L= S/(R+S) X 100%
• W_α= R/(R+S) X 100%
• Y: C_o =35% Ni
• Liquidus curve: C_L =31.5%Ni
• Solidus curve : C _α =42.5%Ni
• Fraction of liquid of composition 31.5% Ni=0.68
• Fraction of solid composition 42.5% Ni=0.32
• Overall composition= 0.68 x 31.5+ 0.32 x42.5=35%

​

20

Development of microstructure

• In Cu-Ni phase diagram , 36% Ni and 64% Cu cooled from 1300 ^oC
• A: liquid- No changes in microstructure
• B:meets liquidus curve (1265 ^oC ) , solidification begins and first crystal of α solid solution begins to form. α 47 Ni and L36 Ni
• C: α43 Ni, L 32 Ni
• D: Solidification ends
• E: α phase

21

22

• Coring: As an alloy of isomorphous type solidifies, the centre of a grain will be rich in the component having higher melting point.
• The concentration of the element having low melting point increases with position from centre to the grain boundary
• This is coring
• The phenomenon by which concentration gradients are established across the grains is called segregation.

23

Eutectic systems

• Completely insoluble in solid state
• Limited solubility in solid solutions
• In eutectic system there is always an alloy of a specific composition ( eutectic composition) which solidifies at a fixed lower temperature than the alloys of all other composition( eutectic temperature).
• The point on the phase diagram determined by the eutectic composition and eutectic temperature is called eutectic point.

24

Alloy system with limited solubility at solid state

• Solid solutions are formed near both ends of the phase diagram.
• These solid solutions are known as terminal solid solutions and the curves showing the solubility limit are known as solvus lines.
• Ag-Cu,
• Terminal solid solutions are denoted by α and β
• Three single phase region: α, β and liquid region

​

25

26

• α: Cu-solvent and Ag - solute
• Β: Cu- solute and Ag – solvent
• 7.9% max solubility of Ag in Cu
• 8.8% max solubility of Cu in Ag
• Lowest MP-both liquidus lines meet

​

27

Invariant reactions

• 3 separate phase
• Gibb’s phase rule
• F=C-P+1
• No of components:2
• No of phases:3

​

• DOF=0

​

• Reaction is called invariant

28

• Peritectic reacion
• Ag- Pt, Fe- Ni, Fe-C, Cu-Zn: MP of component vastly different

29

• Eutectoid reaction
• Solid state reaction and reverses on heating
• Fe-C, Cu-Zn and Al-Cu
• For Fe- C system

30

• Peritectoid reaction:
• Reverse of Eutectoid reaction
• Ag-Al alloy

31

• Monotectic reaction
• Two liquid phases involved are immiscible like water and oil

32

33

IRON CARBON DIAGRAM

• ALLOTROPIC FORMS OF IRON

34

IRON CARBON DIAGRAM

• Iron carbon alloys having carbon content upto 2.14%- steel
• Cast iron- carbon content more than 2.14%

​

• Phase diagram-iron rich portion and for compositions with C.
• Max C content: 6.7%
• Right extreme in diagram, 6.7% carbon and 100% Fe₃C

35

IRON CARBON PHASE DIAGRAM

36

37

38

Phases in Fe–C Phase Diagram

􀂾 α-ferrite - solid solution of C in BCC Fe

• Stable form of iron at room temperature.

• The maximum solubility of C is 0.022 wt%

• Transforms to FCC γ-austenite at 912 °C

􀂾 γ-austenite - solid solution of C in FCC Fe

• The maximum solubility of C is 2.14 wt %.

• Transforms to BCC δ-ferrite at 1395 °C

• Is not stable below the eutectic temperature

(727 ° C) unless cooled rapidly

39

􀂾 Fe₃C (iron carbide or cementite)

• This intermetallic compound is metastable, it

remains as a compound indefinitely at room T, but

decomposes (very slowly, within several years)

into α-Fe and C (graphite) at 650 - 700 °C

􀂾 Fe-C liquid solution

􀂾 δ-ferrite solid solution of C in BCC Fe

• The same structure as α-ferrite

• Stable only at high T, above 1394 °C

• Melts at 1538 °C

Invariant reactions

1. Peritectic reaction

• Liquid+ δ-ferrite Austenite

​

• Eutectoid reaction

Austenite α-ferrite + cementite

​

3. Eutectic reaction

Liquid Austenite + Cementite

40

Isothermal Transformation Diagram

• The curve is plotted using a no. of specimens heated to austenitic temperature and then rapidly transferring one by one to a liquid salt bath of a particular temp.
• After keeping each specimen at this temp for various periods of time, it is quenched to room temp.
• Then the micro constituents of each specimen are to be observed to determine the amount of austenite transformed isothermally at that particular temperature
• Curve is plotted to show the amount of austenite transformed corresponding to various periods of time.

​

​

41

• Isothermal transformation curves corresponding to different temperatures can be constructed.

​

• T (Time) T(Temperature) T(Transformation) diagram

​

• T TT diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition.

​

• It is used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy.

​

42

43

44

• When austenite is cooled slowly to a temperature below LCT (Lower Critical Temperature), the structure that is formed is Pearlite.
• As the cooling rate increases, the pearlite transformation temperature gets lower.
• The microstructure of the material is significantly altered as the cooling rate increases.

• If these cooling curves are superimposed on the TTT diagram, the end product structure and the time required to complete the transformation may be found.

45

• The area on the left of the transformation curve represents the austenite region.
• Austenite is stable at temperatures above LCT but unstable below LCT.
• Left curve indicates the start of a transformation and right curve represents the finish of a transformation.
• The area between the two curves indicates the transformation of austenite to different types of crystal structures.
• (Austenite to pearlite, austenite to martensite, austenite to bainite transformation.)

46

• Figure represents the upper half of the TTT diagram.
• As indicated in Figure, when austenite is cooled to temperatures below LCT, it transforms to other crystal structures due to its unstable nature.
• A specific cooling rate may be chosen so that the transformation of austenite can be 50 %, 100 % etc.
• When slow cooling is applied, all the Austenite will transform to Pearlite.
• If the cooling curve passes through the middle of the transformation area, the end product is 50 % Austenite and 50 % Pearlite, which means that at certain cooling rates we can retain part of the Austenite, without transforming it into Pearlite.

​

47

• Figure indicates the types of transformation that can be found at higher cooling rates at low temperatures.
• If a cooling rate is very high, the cooling curve will remain on the left hand side of the Transformation Start curve.
• In this case all Austenite will transform to Martensite.
• If there is no interruption in cooling the end product will be martensite.

48

• In Figure the cooling rates A and B indicate two rapid cooling processes.
• In this case curve A will cause a higher distortion and a higher internal stresses than the cooling rate B.
• The end product of both cooling rates will be martensite.
• Cooling rate B is also known as the Critical Cooling Rate, which is represented by a cooling curve that is tangent to the nose of the TTT diagram.
• Critical Cooling Rate is defined as the lowest cooling rate which produces 100% Martensite while minimizing the internal stresses and distortions.

​

49

• At temperatures below 540^oC,austenite transforms into another microconstituent .
• The end product is Bainite, which is not as hard as Martensite.

50

540

ISOTHERMAL TRANSFORMATION DIAGRAM

• It was stated that temperature and cooling rate (time) play very important roles in the rate of austenite to pearlite transformation in steels.
• The curve is plotted using a number of specimens heated to austenitic temperature and then rapidly transferring one by one to a liquid salt bath of a particular temperature, say 650°C.
• After keeping each specimen at this temperature for various periods of time, it is quenched to room temperature.
• Then the microconstituents of each specimen are observed to determine the amount of austenite transformed isothermally at that particular temperature.
• Then a curve is plotted show the amount of austenite transformed corresponding to various periods of time.
• The resultant curve is known as isothermal transformation curve at 650 °C.

​

​

51

• In a similar manner, isothermal transformation curves corresponding to different temperatures like 680°C, 700°C etc. can be constructed.
• Information obtained from these curves are then compiled to obtain the isothermal transformation diagram or time temperature transformation diagram shown in figure.
• This diagram is also known as Time-Temperature Transformation diagram or TTT diagram.
• Along the horizontal axis time is represented in logarithmic scale, while the vertical axis, represents temperature.
• The solid curve on left side represents the time required at each temperature to begin the transformation to pearlite.
• This curve is named as P_S curve, indicating start of pearlite transformation. The other solid curve on the right side is named P_F curve, indicating the time needed to finish pearlite transformation (100% pearlite).
• In between, the dashed curve represents the time at which 50% transformation is completed.

​

52

• The eutectoid temperature is indicated by a horizontal line, above which only austenite phase exists.
• If the alloy is supercooled below this temperature, then only the transformation to pearlite occurs.
• The time needed to commence transformation depends on this temperature.
• Both the start and finish curves are nearly parallel and will never meet the eutectoid line.
• Towards the left side of P_S curve, only unstable austenite exists, while only pearlite exists to the right of P_F curve.
• In between both austenite and pearlite are present.
• When transformation takes place at temperatures just below the eutectoid, thicker layers of ferrite and cementite are observed in the microstructure and is called coarse pearlite.
• As the temperature is reduced, the rate of diffusion also reduces and thin layers of ferrite and cementite are resulted.
• The thin layered structure is generally called as fine pearlite and is observed near to 540°C.

​

53

• At temperatures below 540°C, austenite transforms into another microconstituent, called bainite.
• Bainite also contains ferrite and cementite.
• It forms as needles or plates and is composed of a ferrite matrix with needle like particles of cementite.
• The TTT curve can be extended from 540-215°C to represent transformation to bainite.
• All the three curves in this figure are C shaped and have a "nose" at point N (540°C) where the transformation takes place faster.
• The bainite formed just below the nose of TTT curve is called upper bainite and the microstructure is having a feathery appearance.
• At lower temperatures the cementite grains become too fine and an acicular or needle like pattern is observed.
• This is lower bainite which is also called acicular bainite.
• Bainite also exhibits considerable variation in properties depending on the temperature at which it is formed.

​

54

• On rapid cooling (quenching) to relatively low temperatures, the austenite transforms into another microconstituent called martensite.
• Martensite is a single phase structure that results from diffusion less transformation.
• This transformation occurs only when the cooling rate is rapid enough to prevent diffusion of carbon.
• It is an instantaneous transformation as the martensite grains nucleate and grow at a very rapid rate.
• Martensite does not appear on the iron-carbon diagram as it is a non equilibrium phase.
• But the austenite to martensite transformation is represented on the TTT diagram.
• Beginning of this transformation is indicated by the horizontal line marked M_S.
• Two other horizontal dashed lines named M₅₀ and M₉₀ indicate completion of 50% and 90% martensite transformation.
• This transformation is independent of time, but, it is a function of the temperature to which the alloy is rapidly cooled or quenched.
• This type of transformation is called athermal transformation.

​

55

56

Continuous cooling transformation diagram

57

58

59

Heat Treatment

Heat Treatment

• Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape.

​

​

• Heat Treatment can also be used to alter certain manufacturability objectives such as improve machining, improve formability, restore ductility after a cold working operation.

​

60

• Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the commercial use of steels exceeds that of any other material.
• Steels are heat treated for one of the following reasons:
• Softening:
• Hardening:
• Material Modification

​

​

​

61

Objectives of Heat Treatment

• to improve mechanical properties like tensile strength, ductility and impact resistance.
• to increase resistance to wear, heat and corrosion
• to improve surface hardness
• to improve machinability
• to relieve internal stresses set up in the material during cold working.
• to refine the grain structure after hot working.
• to improve magnetic and electrical properties.

62

63

Heat Treatment Processes

Softening: (Annealing)

• Softening is done to reduce strength or hardness, remove residual stresses, improve toughnesss, restore ductility, refine grain size or change the electromagnetic properties of the steel.

​

64

Process Annealing or Stress Relief Anneal �

• Process Annealing is used to treat work-hardened parts made out of low-Carbon steels (< 0.25% C).
• This allows the parts to be soft enough to undergo further cold working without fracturing.
• Process annealing is done by raising the temperature to just below the lower critical temperature, line A₁on the diagram.
• This temperature is about 550 - 650ºC.
• This is held long enough to allow recrystallization of the ferrite phase, and then cooled in still air.

​

​

​

65

Full Annealing

• Annealing is reheating steel followed by slow cooling.
• The steel is heated about 50-90^oC above the upper critical temperature, held for a set time and then cooled slowly in the furnace (10-20^oC/hour).

​

• It is completed �a) to remove internal stress or to soften or �b) to refine the crystalline structure (This involves heating to above the upper critical temperature ).�

​

​

66

• Full annealing is the process of slowly raising the temperature about 50 ºC above the Austenitic temperature line A₃ or line A_CM in the case of Hypoeutectoid steels (steels with < 0.77% Carbon) and 50 ºC into the Austenite-Cementite region in the case of Hypereutectoid steels (steels with > 0.77% Carbon).
• It is held at this temperature for sufficient time for all the material to transform into Austenite or Austenite-Cementite as the case may be. It is then slowly cooled at the rate of about 20ºC/hr in a furnace into the Ferrite-Cementite range.

​

​

67

Spheroidization

• Spheroidization is an annealing process used for high carbon steels (Carbon > 0.6%) that will be machined or cold formed subsequently. This is done by one of the following ways:

​

1. Heat the part to a temperature just below the Ferrite-Austenite line, line A₁ or below the Austenite-Cementite line, essentially below the 727ºC line. Hold the temperature for a prolonged time and follow by fairly slow cooling.

​

• Cycle multiple times between temperatures slightly above and slightly below the 727ºC line, say for example between 700 and 750ºC, and slow cool.

​

• For tool and alloy steels heat to 750 to 800ºC and hold for several hours followed by slow cooling.

​

​

68

• Prolonged time at the elevated temperature will completely break up the pearlitic structure and cementite network and will produce a spheroidal or globular form of a carbide in a ferritic matrix called spheroidite.

​

• This structure is desirable when minimum hardness, maximum ductility and maximum machinability are required.

​

• All these methods result in a structure in which all the Cementite is in the form of small globules (spheroids) dispersed throughout the ferrite matrix.

​

• This structure allows for improved machining in continuous cutting operations such as lathes and screw machines.

​

• Spheroidization also improves resistance to abrasion.

​

​

69

Normalizing

• Normalizing is the process of raising the temperature to over 50ºC, above line A₃ or line A_CM fully into the Austenite range. It is held at this temperature to fully convert the structure into Austenite, and then removed from the furnace and cooled at room temperature under natural convection.

​

70

Normalising is used to

• To refine the grain structure and to create a more homogeneous austenite when a steel is to be reheated for quench hardening or full annealing

​

• To encourage reduced grain segregation in castings and forgings and provide a more uniform structure

​

• To provide moderate hardening

​

71

Quench and Temper treatments

72

Hardening

• Hardening: Hardening of steels is done to increase the strength and wear properties.
• Hardening involves heating a steel to its normalising temperature and cooling (Quenching) rapidly in a suitable fluid e.g oil, water or air.
• If the steel is cooled quickly (quench) by immersing it in oil or water, the carbon atoms are trapped, and the result is a very hard, brittle steel.

​

​

​

73

• Hardening involves heating a steel to its normalising temperature and cooling (Quenching) rapidly in a suitable fluid e.g oil, water or air.
• If the steel is cooled quickly (quench) by immersing it in oil or water, the carbon atoms are trapped, and the result is a very hard, brittle steel.
• This steel crystal structure is now a body centered tetragonal (BCT) form called martensite.

​

74

Quenching Medium�There are a number of fluids used for quenching steels listed below in order of quenching severity

• Brine
• Water
• Oil
• Special liquids
• Air

​

75

Tempering

• Tempering is a process done subsequent to quench hardening.
• Quench-hardened parts are often too brittle.
• This brittleness is caused by a predominance of Martensite.
• This brittleness is removed by tempering.
• Tempering results in a desired combination of hardness, ductility, toughness, strength, and structural stability.

​

​

76

• Tempering process involves reheating the hardened steel to temperatures below the lower critical temperature followed by slow cooling.
• All hardened steel are usually tempered immediately after hardening
• Temperature varies from 150-680ºC
• Low temperature 150-250ºC
• internal stress removed.
• Toughness and ductility increased
• For cutting tools made of tool steels and low alloy steels
• 3000-450ºC
• Resulting microstructure- martensite+ fine pearlitic structure called troostite
• Decrease hardness and strength, with increase in ductility
• Springs, axles, hammers, chisels
• Above 450 ºC
• Martensite transforms to sorbite
• High ductility with adequate hardness and strength
• Connecting rods, shafts, gears

77

ISOTHERMAL HEAT TREATMENTS

78

Austempering

• The part is not quenched through the Martensite transformation.
• Instead the material is quenched above the temperature when Martensite forms M_S
• It is held at this temperature till the entire part reaches this temperature.
• As the part is held longer at this temperature, the Austenite transforms into Bainite.
• Bainite is tough enough so that further tempering is not necessary, and the tendency to crack is severely reduced.

​

​

79

• Austempering begins with heating steel above the austenitising temperature.
• It is then quenched in a molten salt bath maintained at a constant temperature within the lower side of bainitic range (200 - 400°C).
• After holding the steel in this temperature for sufficiently long time, it is then cooled in air.
• When the component is kept in the bainitic transformation range for sufficient time, the austenite gets transformed completely into lower bainite or acicular bainite.
• The transformation product has better mechanical properties than tempered martensite and hence austempered components rarely need tempering.

80

• Two important parameters which control the process are the cooling rate for first quench and holding time in the quenching bath.
• The cooling rate has to be faster than the critical cooling rate.
• As the bainite is formed at constant temperature, the properties of austempered steel are uniform throughout the section, but it is limited to fairly thin sections.
• As compared to conventional hardening and tempering, this process results in better ductility at high hardness levels, improved impact and fatigue strength and freedom from distortion

​

81

Martempering/ Marquenching

• Martempering is similar to Austempering except that the part is slowly cooled through the martensite transformation.
• The structure is martensite, which needs to tempered just as much as martensite that is formed through rapid quenching.
• The steel which is austenitised is cooled rapidly to a temperature range (180-250°C) just above the start of martensitic range (M).
• This is done with the help of molten salt bath maintained at constant temperature.
• After holding in the bath for sufficient time, the component is cooled in air to room temperature.
• The cooling rate at the first phase should be greater than critical cooling rate so that no austenite is transformed to pearlite.
• Holding at the constant temperature ensures uniform temperature throughout the steel component.

​

​

82

• The second phase of cooling at moderate rate prevents large differences in temperature between outside and core of the section.
• Formation of martensite occurs uniformly throughout the component, thereby avoiding excessive residual stresses.
• Martempering also minimise cracking and distortion, at the same time reduces the thermal shock of the quenching process.
• However, hardness and ductility are almost similar to those obtained by direct quenching to the martensite state followed by tempering.
• Martempering is often followed by tempering in order to increase ductility of the steel component.
• As the success of the process depends on the formation of martensite, alloy steels are best suited for the process.
• Any steel which can be hardened can also be martempered successfully.

​

83

84

Hardenability

• Hardenability of a steel should not be confused with the hardness of a steel.
• The Hardness of a steel refers to its ability to resist deformation when a load is applied, whereas hardenability refers to its ability to be hardened to a particular depth under a particular set of conditions.
• Information gained from this test is necessary in selecting the proper combination of alloy steel and heat treatment to minimize thermal stresses and distortion when manufacturing components of various sizes.

​

• The Jominy End Quench Test measures Hardenability of steels.

​

85

Jominy End Quench Test

• A standard specimen of 25.4mm diameter and 100mm length is heated above the austenitising temperature for a specified period of time.
• The specimen is removed from the furnace and quickly transferred to a fixture as shown in figure.
• The lower end of the specimen is quenched by a jet of water at a specific flow rate.
• This results in different rates of cooling along the length of specimen from the most severe water quench at bottom end to air cooling at the other end.

86

• Surface hardness is measured along the length of specimen and a curve is plotted between hardness (usually Rockwell C) and distance from the quenched end.
• The resulting curve is called Jominy curve or hardenability curve.
• The hardenability information from the Jominy test can be used in two ways.
• If the quench rate for a given part is known, the Jominy test can predict the hardness of that part.
• Conversely, measurements of hardness can identify the quench rates.
• A hardenability curve for a particular steel will give the hardness as a function of cooling rate.
• There are standard plots available in hand books that will give you the cooling rates experienced for any point in a steel specimen of a particular size, geometry and quench.
• Jominy curve could be used for comparing hardenability between various steels.
• After drawing the Jominy curve, a horizontal line is drawn at HRc 57 (which is equal to 50% martensitic structure) to meet the Jominy curve at a particular point.
• This distance, known as Jominy distance is also used as a measure of hardenability.
• Hardenability dependent mainly upon the amount of alloying elements present in the steel.
• Other factors like composition, homogeneity and grain size of austenite also affect hardenability of steel.

​

87

Hardness Tests

• Hardness of a material is a measure of its resistance to indentation or scratching.
• It is measured by pressing a hard sphere or pyramid into the surface and then measuring the penetration.
• It is also a measure of the resistance to plastic deformation

​

88

Rockwell Hardness

• Different scales are used with different types of indenters.
• The indenter is pressed against the object with 10 kg load and then the load is increased to either 60, 100 or 150 kg and the increment in penetration is noted.

​

89

90

Brinnel hardness Tests

• Indentor – steel/tungsten carbide sphere of diameter D
• Method: The diameter (d) of the indentation is measured under a given load P. The hardness number is calculated as

​

​

​

​

​

91

Vicker’s Hardness Test

• Indenter: Pyramid shaped indenter
• Method: The average diagonal (d) of the indentation is measured. Hardness is calculated using the formula

​

​

​

92

Knoop hardness test

• Indenter: Pyramid shaped diamond indenter which forms long and short diagonals. This enables hardness measurement on narrow objects and thin sheets. Hardness is calculated as

​

​

​

​

​

• Where L is the larger diagonal length.
• The applied load is small (<500g) for Knoop hardness measurement

​

​

​

93

SURFACE TREATMENTS

• Numerous industrial applications demand that a component possesses different or even contradicting properties for the surface and the interior.
• For example service conditions of many components like gears, cams, crank shafts, rolls etc. require a hard wear resistant surface and a tough, shock resistant interior.
• Storage tanks and chemical processing equipments need a surface to resist corrosion/chemical attack together with enough strength and low cost.
• Decorative fittings, home appliances etc. require good surface finish and appearance together with adequate strength and low cost.
• Electrical conductors should have better conductivity, strength and low cost.
• In order to meet these conflicting requirements, surfaces of such components are given appropriate treatments to develop the desired properties.

​

94

CLASSIFICATIONS

• Mechanical treatments:
• Shot blasting-subjecting the surface with a high pressure jet of hard steel/ cast iron balls or sand particles using compressed air;
• Burnishing- pressing between rolls. Residual compressive stress developed at the surface will enhance the load bearing capacity of the component.
• Providing protective coatings:
• Painting, plating, galvanising, hot dipping, anodising, vapour deposition, metal spraying, metal cladding are some of the methods.
• Changing the chemical composition at the surface:
• Surfaces of components are brought in contact with suitable materials so that elements like carbon, nitrogen etc. are allowed to diffuse into the surface to develop a layer of changed chemical composition and thereby changed properties.
• Surface composition can also be changed by ion implantation.
• Giving differential heat treatment to the surface:
• Components made of materials responsive to heat treatments (particularly steels) can be subjected to heat treatments at the surface alone to change the properties while leaving the interior unaffected.

​

95

CASE HARDENING/�DIFFUSION METHODS

• CARBURISING
• CYANIDING
• NITRIDING
• CARBONITRIDING

96

Carburising

• Carburising is a process of adding Carbon to the surface.
• This is done by exposing the part to a Carbon rich atmosphere at an elevated temperature and allows diffusion to transfer the Carbon atoms into steel.
• Pack Carburising:
• Gas Carburising
• Liquid Carburising

​

​

​

97

Pack Carburising

• In this process, solid substances such as coke, coal, charcoal, carbon black, carbonates of barium, calcium or sodium are used.
• The steel object is cleaned and placed in a closed chamber amidst the surrounding solid fumes.
• This chamber is heated to a desired temperature for a long duration.
• Carbon gets soaked into the surface of steel.
• The depth of soaking depends on the time of exposure between steel and the solid fumes.
• Normally 0.1 mm penetration (soaking) takes place in an hour.

98

• The agent for carburising is solid carbonaceous materials like charcoal or petroleum coke, energised with CaCO₃, or BaCO₃, or Na₂CO₃, (about 20%).
• Heating is done in heat treatment furnaces at a temperature of 870-980°C for 5 to 10 hours for a satisfactory penetration .
• At this temperature the steel gets austenitised in which state, it can absorb carbon upto 1.2%.
• The carbonaceous material will give off carbon monoxide (CO) which combining with Fe will form Fe₃C.
• If any portion of the specimen is not required to be carburised, the same may either be electroplated with copper or packed with ash, fire clay and asbestos.
• Generally pack carburising is a slow process.
• Pack carburising involves minimal capital.

99

Gas carburising

• In this process, the component is heated in a furnace which is filled with carbon rich gas such as methane, propane, butane or carbon monoxide.
• The hydrocarbons in the carburising gas decompose at high temperature and carbon gets deposited.
• The thickness of hardened case will depend on the rate of gas flow and the time of heating.
• Gas carburising is faster than pack hardening

​

​

100

Liquid carburising

• In this case, a liquid hydrocarbon rich in carbon is used either in a bath or in a pressurised system.
• The heated steel is impinged upon by jets of liquid rushing out of nozzles.
• In due course of time, carbon gets deposited on the steel.
• The steel is heated by gas burners or by electrical means.
• A case depth of 0.1 to 0.5 mm can be suitably obtained by this method.
• This method is better and faster than pack carburising.
• The bath is prepared with 20-50% NaCN, 40% Na2CO3, & 30% BaCl2, or NaCl and heated to 850 to 950°C in pots.
• The specimens are immersed in wire baskets for a period of one hour.
• Salt baths are relatively small chambers or tanks and so it is not convenient to immerse many large, odd shaped parts into a liquid bath.
• Therefore liquid carburising is restricted to the case hardening of small parts only

101

Nitriding

• This process involves heating of steel to about 650°C, and holding in an atmosphere of ammonia (NH3) for some time.
• Anhydrous ammonia gas is passed into the furnace at about 550°C, where it dissociates into nascent nitrogen and hydrogen.
• The treatment time varies from 21 - 100 hours, depending on the desired case depth and size of the steel parts.
• After nitriding, the steel part is allowed to cool in furnace itself in the presence of ammonia.
• The furnace container is made of heat resisting alloy steel.

​

102

• The nitrogen from ammonia penetrates into the surface of steel and forms very hard nitrides on the surface.
• These nitrides then disperse inside the steel.
• All machining and grinding operations are to be finished before nitriding.
• The portions which are not to be nitrided are covered by a thin coating of tin deposited by electrolysis.
• Nitriding improves corrosion resistance, provides very high hardness, and components need not be machined after the process.
• It is a costly and time consuming process.
• Nitriding is performed on dies, mandrels, gauges, parts of pumps and internal combustion engines.
• Nitriding offers the hardest case of all surface hardening processes.
• Nitriding is generally restricted to very thin cases.
• A 0.25 mm case depth might take 10 hours.
• A 0.75 mm case depth would take several days.
• It is difficult to machine a part after it has been nitrided.

​

103

Cyaniding

• This is a special case hardening process which the mild steel absorbs carbon and nitrogen to obtain a hard surface.
• The parts to be heat treated are immersed liquid bath (800-960°C) of NaCN with the concentration varying between 25% to 90%.
• A measured amount air is passed through the molten bath.
• NaCN reacts with oxygen in the air and gets oxidized.
• Carbon and nitrogen so formed in atomic form diffuse into the steel and give thin wear resistant layer carbonitride phase.
• Usually, this process requires 30-90 minutes for completion.
• For obtaining a case depth from 0.5 to 2.0 mm, the process is carried out at higher temperature (950°C) bath containing 8% NaCN, 82% BaCl and 10% NaCl.
• This process takes 2-6 hours for completion.
• After cyaniding, the components are taken out and quenched in water or oil. For thick sections, mineral oil is preferred for quenching.
• The final operation is low temperature tempering.
• Cyaniding fairly inexpensive case hardening process.

104

Carbonitriding

• This process is specifically used for improving wear resistance of mild, plain carbon or very low alloy steel.
• Carbonitriding is carried out at lower temperatures in the range 800-870°C in a gas mixture consisting of a carburising gas and ammonia.
• A typical gas mixture contains about 15% NH3, 5% CH4, and 80% neutral carrier gas.
• Carbon and nitrogen are diffused at the same time into the surface of the steel in the austenitic-ferritic condition and the mixture gives a case thickness of the order of 0.05 to 0.75 mm.
• Nitrogen is more effective in increasing the hardness of the case compared to carbon.
• Nitrogen content of steel depends on ammonia content and temperature.
• After carbonitriding, quenching is done in oil to avoid cracking.
• This is followed by tempering at 150-180°C.
• In this process, surface hardenability, wear resistance and corrosion resistance are better than the carburising process.
• But the time required for heat treatment is longer than that of carburising.
• Slower cooling leads to reduced internal stresses, distortion and lesser cracks.

105

SELECTIVE HARDENING PROCESSES�

106

Flame Hardening

• In this process, some areas of a component is heated above critical temperatures by oxy-acetylene flame and simultaneously quenched by spraying water under pressure.
• The depth of hardened case depends on the temperature of the flame, heating time, temperature of the cooling water and time elapsed between heating and cooling of certain region.
• This process is used for local hardening of components such as gear tooth, cams, machine tool beds and cutting tools.
• This process is cheap and it’s equipments are portable.

107

• Features
• In flame hardening, an ordinary oxy-acetylene torch designed to suit the specimen is used to heat the part locally over the region to be hardened
• The torch is adjusted so that the temperature of the part does not exceed 850°C but above the critical range.
• The torch is moved so that the part is heated uniformly and this is followed by a quenching jet of water from a nozzle.
• For good results, the carbon content of the steel should be between0.3 to 0.6%
• Tempering is done to relieve the stresses.

108

Advantages

• Flame hardening is a very rapid and efficient method for providing deep cases up to about 6 mm.
• Zone hardening is possible with this method. With flame hardening, heat can be applied merely to the small critical "zone" and quenched.
• This method is suitable for hardening of large bulky parts that will not fit into a furnace or liquid case-hardening tank. Also, large, heavy parts that cannot be transported conveniently can be flame hardened on site because the torch can be taken to them.

​

Disadvantages

• The depth of heat penetration into the metal is very difficult to control accurately and hence flame hardening is not used for thin cases.
• Only certain steels can be case hardened by this method. Since no additional carbon or nitrogen is added to the surface, the source of the hardening must come from the metal itself. Low carbon steel cannot be hardened by this method. Medium carbon steel with 0.35 to 0.60% carbon are most commonly used.

​

Applications

• Common applications of flame hardening include cases wherein the depth of hardness is not critical or where only a small zone of the part requires hardening
• Examples are gear teeth, cylindrical pins, lathe beds, cam surfaces, engine push rods, pulleys and sprocket teeth.

​

109

Induction Hardening

• By this process, one can get a hard and wear resistant surface with a soft core in steel.
• The process involves induction heating using various shapes of induction heating coil.
• The job to be hardened is placed in an induction coil that comprises several turns of copper wire.
• A high frequency current is passed through a copper block which acts as primary coil of transformer.
• This sets up an alternating magnetic field and induces an alternating current.
• This current produces heating effect on the job's surface.
• Temperature produced is about 750- 800°C.
• The heated surface is immediately quenched by a spray of water.
• Induction hardening is a rapid process.
• Hardness to a depth of about 0.8 mm can be achieved within 1 to 5 seconds.
• The actual time taken depends on the power input, depth of hardening and frequency of current.
• Induction hardening is portable heat treatment process for camshafts, crankshafts, gears and various automobile parts

110

• Advantages
• Induction hardening is fast.
• No warm up time is required prior to the contact between the electrical coil and the part
• Irregular shapes can be handled quite readily with induction hardening.
• The thickness of the case can be controlled more accurately with this method than with any other. The depth can be controlled by varying the frequency, the current and the time that the coil is in contact with the part.
• Accuracy is possible with this process even if an unskilled operator runs the equipment
• Induction hardening offers outstanding resistance to warpage, distortion, oxidation and scaling. This is because of the short period of time involved in the heating process

​

• Disadvantages
• Since no carbon or nitrogen is added to the surface of the part, Medium carbon steel with 0.35 to 0.60% carbon is most often used.
• The hardness is dependent on the carbon content of the steel. Therefore, higher hardness cannot be obtained unless expensive high alloy steel is used.

​

• Applications
• Practical examples of induction hardening include: irregular shaped parts such as cams, gear teeth and shaft splines, bearing surfaces of automotive crankshafts, pump shafts, piston rods, ball and roller bearings, chain links etc

​

​

111

Laser Hardening

• Laser beam hardening is a surface hardening process which is a variant of flame hardening.
• A phosphate coating is applied over the steel component to facilitate absorption of the laser energy.
• The selected areas of the part are exposed to laser energy causing the outer layer heated to about 900-1400°C.
• By varying the power of the laser, the depth of heat absorption can be controlled.
• The parts are then self-quenched and tempered.

​

• Features
• Upon heating, the outer surface is heated up for a short time above austenitising temperature so that the component is austenitised and the rapid cooling converts it into the martensitic structure.
• Heat is generated by absorbing the laser radiation on the surface and the material is self-quenched by heat transfer to the cooler mass inside.
• The hardening depth of outer layer typically varies from 0.1 to 1.5 mm.

​

• Applications
• It is used exclusively on ferrous materials suitable for hardening including, steels and cast iron with a carbon content of more than 0.2 percent.
• As the heating is done selectively, distortion is minimised and the core remains soft.
• Since no quenching media is involved, the hardening process is clean and the workpieces need not be cleaned.
• Laser surface hardening is primarily used in the automobile industry for axle shaft housings and power steering gear housings.

112

Electron Beam Hardening

• Electron beam hardening is similar to laser beam hardening.
• The heat source is a beam of high-energy electrons.
• The beam is manipulated using electromagnetic coils.
• Austenitising occurs through the energy transferred by electron beams.
• The rapid cooling of the austenite required for martensite formation occurs through a self-quenching process that is dependent on the thermal conductivity of the material and starts after the energy transfer has ceased.

​

Features

• As in laser beam hardening, the surface can be hardened very precisely both in depth and in location.
• Typical hardening depths obtained by the process range from 0.1 to 1.5 mm.

​

113

STRENGTHENING OF MATERIALS

114

STRENGTHENING MECHANISMS

• Engineers usually may have to design materials having high strength along with some ductility and toughness.
• Different techniques are available for tailoring a material to the required mechanical properties.
• Basically, strength of a material is related to the mobility of dislocations within the material.
• When the dislocations can be moved easily, plastic deformation occurs at lower values of applied load.
• If it is easy to deform a material, it means the material is having lower mechanical strength.
• This leads to the situation that, if we can introduce obstructions to dislocation motion, the material can be strengthened
• All strengthening mechanisms rely on this simple principle.

​

• Single phase materials can be strengthened by grain size reduction, solid solution strengthening and work hardening.
• Precipitation and dispersion hardening are strengthening mechanisms applied to multi phase materials.

115

Grain Size Reduction

• On application of load, dislocations move along favourably oriented directions along the slip planes within a grain.
• As the moving dislocation reaches near a grain boundary, it will experience difficulty in moving forward to the next grain.
• This is due to the different atomic arrangements in the neighbouring grains.
• To overcome this barrier and to move to next grain, higher applied loads may be necessary.
• When the material is having a fine grained structure, the total grain boundary area will be larger and this will impose more obstructions to dislocation motion.
• To overcome this, more load needs to be applied.
• In other words, a fine grained material is having higher strength and hardness.
• Consequently, coarse grained materials will be having lower strength and hardness compared to a fine grained one.

​

116

Work Hardening or Strain Hardening

• Almost all ductile materials become stronger when deformed plastically by cold working.
• This increase in strength is called work hardening or strain hardening.
• Even, materials which do not respond to heat treatment can be strengthened by work hardening
• We know that plastic deformation is caused by the dislocation movement.
• But these dislocations interfere with other dislocations creating barriers to moving dislocations.
• When the material is deformed, there will be an increase in dislocation density and there will be more interactions between moving dislocations.
• The interacting dislocations lead to a decrease in their mobility and hence the material gets strengthened.
• The rate of strain hardening is higher in metals with cubic structure as compared to HCP metals.
• Lower temperature also promotes work hardening and this is why cold working leads to work hardening whereas hot working does not.
• Annealing is done to relieve the effects of strain hardening, if necessary.

​

117

• When a material deforms, there is some distortion of the lattice structure.
• This distortion is greatest on the slip planes and grain boundaries and increases with increasing deformation.
• This is manifested by an increase in resistance to further deformation.
• Now the material is said to be undergoing strain hardening or work hardening.
• The effect of strain hardening is illustrated using a stress - strain curve as shown in figure.
• Initially the metal with yield strength σyo in plastically deformed to point D.
• The stress is released completely and then re-applied.
• The yield strength is found to be increased to σy1 .
• The metal has thus become stronger during the process since σy1 is greater than σyo

118

Solid Solution Hardening

• This is another technique for strengthening a single phase material (a pure metal).
• Metals can be hardened by alloying with impurity atoms to form substitutional or interstitial solid solutions.
• The presence of impurity atoms induces lattice strains within the material
• These lattice strains interact with the dislocations and thereby dislocation movement is restricted.
• In this situation, greater loads are to be applied to initiate and continue dislocation movement and hence plastic deformation.
• This means the presence of impurities had enhanced the mechanical properties.
• The higher strength and hardness of a solid solution (as compared to a pure metal) are contributed by solid solution strengthening.

119

Precipitation Hardening

• Strength and hardness of some alloys may be enhanced by the formation of extremely small uniformly distributed particles of a second phase within the original matrix phase.
• This is accomplished by appropriate heat treatments.
• This process is called precipitation hardening because the small particles of the second phase are made to precipitate within the matrix by the heat treatment.
• Age hardening is also used to designate this procedure, because the strength develops with passage of time (or as the alloy ages), even when the alloy is not subjected to any heat treatment (when the alloy is maintained at room temperature).
• The alloy system which can be subjected to precipitation hardening should display two essential characteristics: one, an appreciable maximum solid solubility of one component in the other, and two, a solid solubility limit that decreases with fall in temperature.
• Examples of alloys that are suitable for precipitation hardening include: Al-Cu, Al-Si, Cu-Be, Cu-Sn and Mg-Al systems.
• Some ferrous alloys are also precipitation hardenable.

​

​

​

120

• Precipitation hardening is accomplished by two different heat treatments.
• The first is known as solution treatment in which all the solute atoms are made to dissolve in the solvent metal to form a single phase solid solution.
• Consider an alloy of composition Co in the figure
• Heat the alloy to a temperature (T_o) within the α- phase field and keep at that temperature for enough time so that the entire phase becomes α. Now the alloy is quenched to room temperature.
• Even though, at room temperature the alloy of composition Co should have α +β phase structure, quenching from T_o prevents the diffusion process and thereby formation of any β-phase.
• Thus at room temperature, a non-equilibrium situation exists in which the α-phase supersaturated with the Cu atoms is present.
• Diffusion rates at room temperature is so slow that this single phase is retained at this temperature for a long period.

​

121

• The second step is known as the precipitation treatment during which, the supersaturated α-solid solution is re-heated to a temperature (T) within the α+β phase region (ie, below the solvus line).
• At this temperature, the diffusion rates become appreciable and the β phase precipitates out as finely dispersed particles within the phase matrix.
• After an appropriate period of time, the alloy is cooled to room temperature.
• Thus after precipitation treatment, the structure consists of β -phase precipitate particles dispersed in a matrix of α -phase.
• The character of these β particles and the subsequent strength and hardness of the alloy depend on both the precipitation temperature and the holding time at this temperature.

​

• For any particular temperature, there is an optimum time at which maximum strength and hardness are obtained.
• Keeping the alloy beyond this period of time causes coarsening of the precipitate particles, resulting in loss of hardness and strength.
• This is known as over aging.

​

• In many cases, strength and hardness are found to be increasing with passage of time even when the alloy is kept at the room temperature.
• The Precipitate particles form as the alloy ages.
• This is why the process is also called as age hardening.

​

• In certain other alloys, appreciable precipitation can occur even at room temperature within short periods of time after quenching.
• This is known as natural aging.
• When the quenching is followed by heating in order to form the precipitates, the process is called artificial aging.

​

122

Dispersion Hardening

• In dispersion hardened alloys, fine particles of one phase are introduced into another phase which is weaker and ductile.
• The soft phase is continuous is called matrix and the hard and stronger phase is called precipitate or dispersed phase.
• Though the dispersed phase is present in smaller amounts, it strengthens the matrix which is present in larger amounts.
• The dispersed particles are to be small enough that they can impose obstacles to dislocation movement.
• Dispersion strengthening is actually produced through phase transformations

​

123

MECHANICAL WORKING OF METALS

• Mechanical working is defined as the process of effecting shape changes in materials by the application of an external force.

​

Cold Working

• A material is said to be cold worked, if its grains are in a distorted condition at the end of plastic deformation
• Tensile strength, yield strength, hardness, ductility, electrical conductivity and resistance to corrosion are some of the properties so affected.

​

Hot Working

• The mechanical working done above the recrystallisation temperature is known as hot working.
• Below this temperature, the metal is prone to strain hardening and becomes harder and less ductile.
• But, during hot working or soon after that the material will anneal itself and will remain soft and ductile after the processing

124

Bauschinger Effect

​

• Consider two pieces of the same material subjected to plastic deformation due to the application of tensile stress.
• After this, when the first one is subjected to tensile stress and the second one to compressive stress, it can be observed that the second one yields at a lower value of stress compared to the first one.
• This means the compressive yield stress is lesser than the tensile yield stress in magnitude.
• This phenomenon is known as Bauschinger effect, in which a material already subjected to tensile stress shows reduction in compressive strength.
• This effect is seen on stress reversal also.
• The Bauschinger effect plays an important role in mechanical processing of metallic materials.
• Here, metal forming processes involving compressive loading and tensile loading are to be used alternately to make use of this feature.

125

RECOVERY, RECRYSTALLISATION & GRAIN GROWTH

• Annealing is the process by which the distorted cold worked lattice structure is changed back to one which is strain free.
• Application of heat accelerates the process.
• Hence, the annealing process for changing the structure of a deformed metal involves heating it to the desired temperature and then cooling slowly.
• The transformation (or phase changes) taking place during annealing can be divided into three stages: Recovery Recrystallisation and Grain Growth.

​

126

Recovery

• This is primarily a low temperature process; and the property changes produced do not cause appreciable changes in the microstructure.
• When the deformed metal is heated to low temperatures, the atoms are able to move to positions nearer to equilibrium in the crystal lattice.
• Such small movements can reduce internal mechanical stresses without producing any visible alteration in the distorted shape of the cold-worked crystal.
• The principal effect of recovery is seen to be the relief of internal stresses due to cold working.

​

• At a given temperature the rate of decrease in internal stress (the strain hardening effect) is fastest at the beginning and drops off at longer times.
• Also the amount of reduction in residual stresses that occurs in a particular time increases with increasing temperatures.
• Mechanical properties, practically remain unchanged during recovery, however, electrical conductivity is increased appreciably.

​

• Hence the principal application of heating in the recovery stage is in stress relieving of the cold worked metal to prevent stress-corrosion cracking or to minimise the distortion produced by the residual stresses.
• Commercially this low temperature treatment in the recovery stage is known as stress relief annealing.

127

Recrystallisation

• As the temperature is raised further and the upper temperature of the recovery stage is reached, the vibrational energy of the individual atoms is increased.
• This causes the atoms to break loose from the strained lattice, leading to the formation of minute new unstrained crystals.
• These new crystals will have the same composition and lattice structure as the original undeformed metal.
• The new crystals generally appear at the most drastically deformed portions of the grains, usually at the grain boundaries and slip planes.
• The cluster of atoms from which the new grains are formed is called a nucleus.
• These nuclei grow in size until the whole material has a structure of unstressed polygonal crystals.
• This process is named recrystallisation.
• Thus recrystallisation takes place by the nucleation of strain free grains and their subsequent growth until the elongated grains of the deformed material is completely transformed into strain free equiaxed grains.
• The driving force for the recrystallisation process is the amount of strain energy stored in the zones of high dislocation density
• This change in structure is accompanied by almost complete release of internal energy (or strain energy); reduction in strength, hardness and brittleness, along with an increase in ductility and malleability
• The temperature at which recrystallisation occurs in pure metals, is in the range of 0.3 to 0.5 of the melting temperature in degree Kelvin (absolute temperature).

128

Factors Affecting Recrystallisation

• Amount of Cold work: Increasing the amount of prior cold work enhances the rate of recrystallisation, with the result, the recrystallisation temperature is lowered.
• Time of Annealing: Increasing the annealing time decreases the recrystallisation Temperature. That means complete recrystallisation of a deformed material can be effected by keeping the material at lower temperatures for longer periods
• Annealing Temperature: If recrystallisation is carried out at higher temperatures it gets completed soon.
• Composition: Recrystallisation proceeds more rapidly in pure metals than in alloys. Impurity atoms (or alloying elements) raises the recrystallisation temperature by preventing the movement of grain boundaries. Also finer the precipitate particles (or second phase), the higher the recrystallisation temperature.
• Initial Grain Size: For equal amount of cold working, more strain hardening is introduced into a fine grained structure than into a coarse grained material, A fine grain structure prior to cold working means more grain boundaries, and more distorted regions providing more nucleation sites, promoting rapid recrystallisation. Therefore, finer the initial grain size, the lower the recrystallisation temperature,
• Temperature of deformation: The lower the temperature of cold working, the greater the amount of strain introduced, which effectively decreases the recrystallisation temperature for a given annealing time.
• Amount of recovery prior to recrystallisation: If the deformed metal had been recovered for long, recrystallisation will be delayed, because more strain energy stored during cold work will have been released during the recovery and hence the driving force for recrystallisation becomes lower

​

129

Grain Growth

• After the recrystallisation is complete: if the metal specimen is kept at the elevated temperature, the strain free fresh grains will continue to grow and the grain size will become coarse (or larger).
• This phenomenon is called grain growth.

​

• This is because, larger grains have lower free energy compared to smaller grains.
• This is also associated with the reduction of the amount of grain boundary.
• Therefore, under ideal conditions, the lowest energy state for a metal would be as a single crystal.
• This is the driving force for grain growth.

​

• Opposing this force, is the rigidity of the lattice.
• As the temperature increases, rigidity of the lattice decreases and the rate of grain growth is more rapid.

​

• Since the transformations during annealing involve, nucleation and growth, factors that favour rapid nucleation and slow growth, results in a fine grained structure.
• Whereas, those which favour slow nucleation and rapid growth will result in a coarse grained material.

130

131

Factors affecting Grain Growth

• Degree of Prior Deformation: Increasing the amount of prior deformation favours nucleation and decreases the final grain size. At critical deformation, the grains will grow to a very large size upon annealing. This is due to the fact that, when the degree of deformation is low, the lesser will be the distorted regions that can act as nucleation sites. Hence, only a few number of grains will be formed during recrystallisation resulting in a coarse grained structure. With increasing amount of deformation, an increasing number of points of high stress (or high energy) are present. This leads to the formation of greater number of nuclei and finally a greater number of grains.
• Time at Temperature: Increasing the time at any temperature above the recrystallisation temperature favours grain growth and increases the final grain size. If the material is withdrawn from heating soon after the recrystallisation is complete, the fine grain structure will be retained.
• Annealing Temperature: The lower the temperature above the recrystallisation temperature, the finer the final grain size.
• Heating Time: The shorter the time of heating to the recrystallisation temperature (faster heating rate) the finer the final grain size. Slow heating will form few nuclei and less number of grains, thus favouring grain growth, resulting in coarse grains.
• Insoluble Impurities: The greater the amount and finer the distribution of insoluble impurities, the finer the grain size. They not only increase nucleation, but also act as barriers to grain boundary movement and thus to the growth of grains.

​

• Since full annealing restores the material to a strain-free lattice structure, it is essentially a softening process.

132