PHASE DIAGRAMS &

HEAT TREATMENT OF CARBON STEEL

Phase Diagram Phase

Transformation Heat

Treatment 2

Phase is a homogeneous portion of a system having the same composition and the same state of aggregation throughout its volume, and separated from the other portions of the system by interfaces.

Phase

For instance, a homogeneous pure metal or alloy is a single phase system. A state in which a liquid alloy (or metal) coexists with its crystals is a two-phase system.

3

Gibbs Phase Rule

F = degrees of freedom of the system (or independently variable factors)

C = number of components forming the system

P = number of phases in equilibrium in the system

n = number of external factors(e.g. temp., pressure etc.)

F = C P- + n

The number of degrees of freedom i.e. the variance of the system is the number of factors, such as temperature, pressure and concentration that can be independently varied without changing the number of phases in equilibrium.

Now three types of equilibrium are possible: Invariant equilibrium (F=0) Univariant equilibrium (F=1) Bivariant equilibrium (F=2)

Phase Diagram

A “map” that will guide us in answering the general question:

“What microstructure should exist at a given temperature for a given metal composition?”

So phase diagram, the graphical representation of the state variables associated with microstructures, can be explained with three types based on Gibbs Phase Rule:

Unary Diagrams (put C = 1 in Gibbs phase rule) Binary Diagrams (put C = 2 in Gibbs phase rule) Ternary Diagrams (put C = 3 in Gibbs phase rule)

Binary Phase diagrams can be classified on the basis of liquid solubility and solid solubility. So before going into details, let’s first discuss about solid solution.

8

Unary Phase diagram

Melting point Boiling point

Triple point

Solid Solution

Solid solution is a solution within a solid where solute and solvent both are solid.

Substitutional Solid solution

In the substitutional solid solution alloy the involved solute and solvent atoms are randomly mixed on lattice sites. The single-phase solid alloys that extend across the entire phase diagram in the Cu-Ni and Ge-Si systems are good examples of random substitutional solid solutions. Both atoms are randomly distributed on FCC lattice sites in Cu-Ni and on diamond cubic sites in Ge-Si.

Contd.A necessary condition for single phase solid solution formation across the entire phase diagram is that both components have the same crystal structure. It is not a sufficient condition, however, because there are combinations like Ag-Cu (both FCC) and Fe-Mo (both BCC) that do not form an extensive range of solid solutions. Instead terminal solid solutions, so named because they appear at the ends of the phase diagram, form. The terminal α and β phases in the Pb-Sn diagram are examples. They are both substitutional solid solutions and display the limited solubility often exhibited by such phases. Introduction of foreign atoms into the lattice, whether by design (as dopants or solutes) or accident (as impurities), will always create dilute solid solutions which are often substitutional in nature.

The lattice parameter of substitutional solid solutions is usually an average of the interatomic spacing in the pure components weighted according to the atomic fractions present. This observation is known as Vegard's law. It predicts, for example, that the lattice parameter of a 25% Cu-75% Ni alloy will be approximately 0.25 x 0.3615 nm + 0.75 x 0.3524 nm = 0.355 nm.

Interstitial Solid Solution

When undersized alloying elements dissolve in the lattice they sometimes form interstitial solid solutions. Important examples include carbon and nitrogen in BCC iron, the former being the a phase on the Fe-Fe3C phase diagram.

Intermediate Solid Solution

Unlike terminal solid solutions that extend inward from the outer pure components, intermediate phase fields are found within the phase diagram. Two phase regions border either side of an intermediate phase. In the case of Cu-Zn, α and η are terminal phases, and β, γ, δ and ε are intermediate phases. These single phases are generally stable over a relatively wide composition range.

Ordered Solid Solution

Atoms within certain solid solutions can, surprisingly, order and give every appearance of being a compound. In the alloy 50% Cu-50% Zn β'-brass we can imagine a CsCl-like structure populated by atoms of Cu and Zn. Above 460°C the ordering is destroyed and a random solid solution forms. The order-disorder transformation, interestingly, bears a close resemblance to the loss of magnetism exhibited by magnets that are heated above the Curie temperature.

Binary Phase Diagram

On the basis of Liquid and Solid Solubility, Binary Solid Solution can be classified as

100 % liquid solubility & 100 % solid solubility

100 % liquid solubility & 0 % solid solubility

100 % liquid solubility & partial solid solubility

0 % liquid solubility & 0 % solid solubility

100 % liquid solubility & 100 % solid solubility

20% A

80% A

60% A

40% A

100% A

0% A

16

Liquidus

Solidus

17

18

Pure copperPure Nickel

35% Cu & 65% Ni

100 % liquid solubility & 0 % solid solubility

80%A

100%A

60%A

20%A

0%A

19

40%A

Liquidus

Solidus

20

100 % liquid solubility & partial solid solubility

100% A

80% A

60% A 40% A

21

0% A

Solvus

Liquidus

Solidus

22

0 % liquid solubility & 0 % solid solubility

100% A

100% B

23

24

Lever ruleWhyWe want know the phase composition at a particular temperature and alloy composition

Draw a horizontal line (tie line) passing through the point

Find the overall alloy composition on the tie line.α

WL Wα =

Cα C0 CL

Now calculate, =Cα – C0

Cα – CL

C0– CL

Cα – CL

Fe-Fe3C Phase Diagram

Alloys with a carbon content upto 2.0% are called STEEL.

Alloys with a carbon content exceeding 2.0% are called CAST IRON.

25

Cooling Curve for pure Iron1538

1401

1130

910

768

723

a=2.93 A

a=3.63 A

a=2.90 A

a=2.86 A

26

27

BCC

FCC

BCC

28

29

Hypoeutectoid and Hypereutectoid steel

30

31

CHT & CCT with reference to Fe-Fe3C metastable binary diagram

Phase Transformation

Simple diffusion-dependent transformation

Diffusion-dependent transformation

Diffusionlesss transformation

Metastable phase is produced.

Martensitic Transformation

No change in either the number or composition of the phases present.

Solidification of pure metal, allotropic transformations, recrystallization and grain growth.

Some alteration in phase compositions and often in number of phases present.

Eutectoid reaction

32

Kinetics of Phase Transformation

Nucleation Grain Growth

33

Nucleation

Homogeneous Nucleation Heterogeneous Nucleation

34

Mechanical Behavior of Iron-carbon Alloys

Fine Pearlite

Coarse Pearlite

Bainitte

Martensite

35

36

37

38

39

Phase Transformation Diagram

Isothermal Transformation Diagram (TTT)

(Time –Temperature – Transformation)

Continuous Cooling Transformation Diagram

(CCT)

40

450 C400 C

250 C700 C

% of Transformation

41

TTT diagram for Eutectoid Steel

42

43

TTT diagram for Hypoeutectoid and Hypereutectoid Steel

Hypoeutectoid Steel Hypereutectoid Steel

Austenite to pearlite

1150 sec

400 sec

1450 sec

4000 sec

1320 sec

44

Austenite to Bainite

400 sec

2500 sec

850 sec

900 sec

500 sec

45

Continuous Cooling Transformation Diagram

46

47

Martensitic Transformation of Steel

48

FCC unit cell

BCT unit cell

Heat Treatment

WHY?

Primary concern is to increase the strength of the material

WHIISKERING

HEAT TREATMENT

Theoritical Strength

Whiskers

Pure metal

Heat treated

(costly)

(cheaper)

50

Heat Treatment

Annealing

Normalizing Hardening

Tempering

51

Annealing

To obtain softness Improve machinability Increase / restore

ductility / toughness Relieve internal stress Reduce / eliminate

structural homogeneity Refine grain size

Incomplete Annealing

Isothermal Annealing

Spherodising

Diffusion Annealing (Homogenising)

Full Annealing

52

Heating a hypoeutectoid steel 30-50o C above the critical point A3, holding at this temperature and slowly cooling (@

30-200o C /hour)

Full Annealing

53

Heating steel to a temperature somewhat above the critical point A1, holding it at this temperature and slowly cooling

Incomplete annealing associated with only partial recrystallisation; excess ferrite of hypoeutectoid steel or excess cementite of hypereutectoid steel does not pass over into solid solution and is not recrystalised.

Incomplete Annealing

Incomplete annealing is applied chiefly to eutectoid and hypereutectoid steel.

54

Steel is heated as for ordinary annealing and then cooled comparatively rapidly (in air or by a blast in a furnace) to a temperature 50o to 100o C Eutectoid temperature.

Advantage:

Reduces time required for heat treatment

Reduce hardness

Application:

Produces good results in treating relatively small charges of rolled stock or small forgings.

Isothermal Annealing

55

Spheroidising is performed by heating the steel slightly above 730o-770o C with subsequent holding at this temperature followed by slow cooling @ 25o to 30o /hour to 600o C.

Spheroidising

56

Homogenising

Application

Alloy steel ingots and heavy complex castings for eliminating the chemical inhomogeneity within the separate crystals by diffusion.

Homogenising is carried out at temperatures from 1100o to 1200o C (optimum temperature is 1150o C) at which diffusion proceeds quite easily and to some extent equalises the composition of steels having developed dendritic segregations.

Scaling is very intensive at high temperatures and this leads to excessive losses of metal. Holding time, therefore should be minimum.

Cooling with the furnace for 6 to 8 hours to 800o-850o C and then further cooling in air.

After homogenising, Castings undergo full annealing to refine their structure.

57

NormalisingHeating steel to a temperature from 40o to 50o C above A3 , holding at this temperature for a short time and subsequent cooling in air.

58

NormalisingWhy it is done

To eliminate coarse grain structures obtained in previous working(rolling, forging or stamping)

To increase the strength of medium carbon steels

to a certain extent (in comparison with annealed steel)

To improve the machinability of low carbon steels

To improve the structure in welds

To reduce internal stress

To eliminate the cementite network in hypereutectoid steels

59

HardeningSteel is heated to a temperature above the critical point, held at this temperature and then quenched (rapidly cooled) in water, oil, or molten salt baths.

Why it is done

To increase hardness and wear resistance retaining sufficient toughness at the same time

60

Effect of hardening of HYPOEUTECTOID steel

Initial structure: pearlite + ferrite After quenching from the range

770oC: martensite + ferrite

After hardening at a normal temperature (840o C): martensite

Overheated (1000o C): coarse acicular martensite

61

Effect of hardening of HYPEREUTECTOID steel

Initial structure: granular pearlite

Structure after properly conducting hardening: martensite and cementite

Hardening with overheating

62

Precaution

Oxidation and decurburization may be prevented if a protective gaseous medioum is introduced into the furnace,called controlled or protective atmosphere.

Reaction of furnace gases (combustion products and air) with the surface of articles heated in flame and electric furnaces will lead to oxidation and decarburization of steel.

Oxidation in the heating process results in irretrievable losses of metal, detoriation of in the condition of the ordinarily most highly stressed layers of metal and necessity for subsequent descaling.

Decarburisation of the surface layers of the steel reduces the hardness in the as quenched condition as well as the water resistance and fatigue strength.

63

Quenching MediaQuenching medium must provide for a cooling rate above the critical value to prevent austenitic decomposition in the pearlitic and intermediate regions.

In the martensitic transformation temperature range, cooling should be slower to avoid high internal stress, warping of the hardened part and cracking.

64

Stages of Quenching

A thin vapor film or blanket surrounds the hot metal. Cooling proceeds by film boiling, the cooling rate is relatively slow and is determined by the radiation and conduction of vapor.

Vapor film breaks up and liquid boils with bubbles on the surface of the metal being cooled. During this period , liquid wets the metal surface in direct contact and cooling is accomplished by vapor generation on this surface. Since all quenching media have a high latent heat, this is the fastest stage of cooling. At temperature below the boiling point, cooling is much slower as heat is extracted mainly by convection. The cooling rate decreases as the temperature of metal falls.

65

66

Effect of different Quenching media

Hardening Procedure

Conventional quenching in a single medium

Stepped Quenching (Martempering)

Isothermal quenching (Austempering)

67

Effect of Hardening

Quenching in Water

Quenching in oil

Cooling in air after forging

Annealing at 730o-760oC

Annealing at 900o C and cooling in the furnace

68

Tempering

Low-temperature tempering

(150o to 250o C)

Medium-temperature tempering

(350o to 450o C)

High-temperature tempering

(500o to 650o C)

Almost completely eliminates internal stress and provides the most favorable ratio of strength to toughness for structural steels.

The tempered steel has a sorbite structure after this treatment

It is employed for coil and laminated springs and provides the highest attainable elastic limit in conjunction with ample toughness.

Steel has a troosite structure after this tempering procedure.

After this type of tempering, the martensite produce by quenching is transformed into tempered martensite

The purpose of this tempering is to reduce internal stress and to increase the toughness without any appreciable loss in hardness

69

70

Sub-zero TreatmentWhy it is done

A certain amount of retained austenite may always be found in hardened steel. Retained austenite reduces the hardness, wear-resistance and thermal conductivity of steel and makes its dimensions unstable.

A sub-zero treatment has been devised to reduce the retained austenite in hardened steel. It consists in cooling the metal being treated to sub-zero temperatures. Such treatment is suitable only when the temperature, at which the martensitic transformation is complete( Mf), is below zero.

71

Defects due to Heat Treatment

The main types of rejects in annealing and normalising are due to faulty regulation of heat temperature, including overheating, burning, underheating etc.

The chief cause of quenching defects in high residual (internal) stresses occurring in hardening articles. These stress may cause distortion, warping and even cracking.

72

Surface HardeningSurface hardening is a selective heat treatment in which the surface layers of a metal are hardened to a certain depth while a relatively soft core is maintained.

Types of Surfece hardening Hardening with high frequency induction heating Hardening with electrical contact resistance heating Hardening with electrolytic heating Oxyacetylene flame hardening

Purpose To increase the hardness and wear resistance of the structures of metal articles To improve the reliability in operation of a machine component To increase fatigue limit 73

Carburising of SteelCarburisation is the process of saturating the surface layer of steel with carbon.

Purpose

To obtain a hard and wear resistant surface on machine parts by enrichment of the surface layer with carbon to a concentration from 0.75 to 1.2 % and subsequent quenching.

Carburised and Quenched (case-hardened) steel has a higher fatigue limit.

Mechanism of Carburisation Dissociation of the carbonaceous gases with the evolution of atomic carbon

Enrichment of surface layer with carbon (degree of saturation depends on carburizing temperature and carburizer composition)

Diffusion of carbon, absorbed by the surface, deep into the metal.74

Microstructure of Steel after Carburisation

75

Variation of carbon contents depends on the temperature of the process, the holding time, the steel composition and the activity of the surrounding medium which supplies carbon atoms to the surface.

This results in the formation of “soft spots” on the surface of the part after quenching.

This may be eliminated by heating to higher hardening temperature and by using a highly effective quenching medium.

1 hr 5 hr 10 hr

76

Classification according to carbon source

Pack carburising with solid carbonaceous mixtures (carburisers)

Gas carburizing Liquid carburizing

Heat treatment after carburizing

Parts of carbon steel, requiring high mechanical properties, are usually subject to double-hardening, followed by tempering, after carburizing.

To improve the structure of the core and to impart optimum properties to the surface layers, a single heating to one temperature will evidently be insufficient.

77

Stages

First hardening or normalizing is conducted at a temperature of 880o-900oC to improve the core structure of the work which has been overheated in carburization.

The second hardening operation is conducted at 750o-780oC to eliminate the effects of overheating and to impart a high hardness to the carburized case.

Heat treatment is completed by tempering at 150o to 180oC.

Normal structure

With an increased amount of retained austenite

78

NitridingProcess of saturating the surface of steel with nitrogen by holding for a prolonged period at a temperature from 480o to 650oC in an atmosphere at Ammonia (NH3).

Purpose Increases the hardness of the surface to a very high degree.

Increases the wear resistance.

Improve the fatigue limit under corrosive condition.

79

Mechanism

Solid solution of nitrogen in α-iron (α-phase) at the eutectoid temperature((591oC), nitrogen concentration in the alpha phase will be 0.42% and reduced to 0.015% at room temperature.

γ’ phase, a solid solution on the basis

of iron nitride Fe4N (5.5 to 5.95 % N)

ε-phase, a solid solution on the basis of iron nitride Fe2N (8 to 11.2% N)

ε + γ’

γ’

α + γ’ (eutectoid)

α + γ’(excess)80

Effect of alloying elements

Highly dispersed particles of these nitrides interlock the slip planes and thus considerably increase the hardness of the nitrided layer. Al, Cr, Mo and V increase the hardness to the greatest extent.

Al

Cr

WMn Si

Ni

Mo

81

Procedures

Hardeniung and tempering is performed to impart the required the mechanical prpoperties to the core of the work, i.e. to increase its strength and toughness.

All required machining ooperations are done.

All areas which are not to be nitride , are protected by a thin layer of tin applied by an electrolytic method.

Nitriding

Finishing grinding and lapping is applied in accordance with the specified tolerance on the work.

82

600o

500o

550o

Steel

Alloy structural Steel

Carbon steel

83

Thank You

84