---
title: "Theory and Technology of Heat Treatment - MML214"
tags : "SEM4, MME"
---

# Theory and Technology of Heat Treatment - MML214

Dr. Yogesh Y Mahajan
[e-mail]
([phone]
Exam Slot - C


Thinwires - drawing

## Classification of steels

- Composition
- Carbon content
- Presence of other elements - Alloy steel
- Manufacturing methods
- Electric Arc Furnace
- Blast Furnace
- Product shape
- Bar, Plate, Strip, Tubing, or structural shape
- Oxidation Practice employed
- Rimmed, killed, semikilled, capped
- Microstructure
- Quality Descriptors and classification
- Forging quality, Commercial Quality

### Designation system

SAE 1040

1 - Indicates whether it is carbon or allow steel - 1(carbon steel), 2(alloysteel)
0 - Modification in allow(plain carbon)
40 - Carbon Content(0.40%)

## Steel

Steel has a wide range of properties depending on the composition
Thereby is also has a wide range of application

Steel - Alloy of Fe and C(0.008-2%) + (Cr,Mo,Mn,...)

S and P - Impurities

Plain C steel - Presence of alloying elements in small amount
Structure and properties - can be discussed by FeC or Fe-Fe~3~C equilibrium diagram

Alloy steel - is influenced by other elements - requires a modified FeC or Fe~3~C diagram

### Phases of Steel

#### α-Ferrite

- Interstitial solid solution of C in BBC α-Ferrite
- 0.025%C at 723°C
- 0.008% at RT
- Carbon tries to fit in the interstitial voids
- 80BHN - soft
- 95VPN
- Ferromagnetic upto 768°C
- Paramagnetic above 768° upto 910°C
- Although octahedral coids are smaller - the planar Fe atoms have enough place to accomodate C atom - it only has to distort 2 atoms
- Carbon Concentration is less than Austenite

#### γ-Austenite

Interstitial ss of C in FCC
- Soft, ductile, Malleable and non-magnetic
- 395VPN - Hard
- Toughness is high
- Radius Ratio - 0.596 - Octahedral
- Impurities prefer octahedral voids

#### δ-Ferrite

- Interstital solid solution of C in BCC - δ Ferrite
- Soublility Limit - 0.09wt%
- Formed at 1495°C
- High temperature phase
- Higher Lattice Parameter
- Not stable at room temperature

#### Fe~3~C (Cementite) or Iron Carbide

- Intermetallic Compound of Fe and C - with fixed C - 6.67% by weight
- Complex orthorhombic Structure
- Low tensitle strength and high compressive strength
- 12 Fe atoms + 4 C atoms in a unit cell
- 900VHN - Hard and Brittle
- Ferromagnetic upto 210°C
- Paramagnetic above 210°C

### Transformation

#### Eutectoid Transformation

727°C

$\gamma_{0.8\%C} \rightarrow \alpha_{0.0257\%C} + Fe_3C_{6.67\%C}$

Eutectoid Mixture is formed - Pearlite

Pearlite
- Mixture of α-Ferrite and Cementite forming distinct layers or bands in slowly cooled carbon steel
- 0.8%C - formed at 723°C

Amount of Ferrite = $\dfrac{6.67-0.8}{6.67- 0.008} = 88.1%$

Amount of Cementite = 11.9%

Ferrite lamilla is 7.4 times thicker than cementite lamilla

#### Peritectic Transformation


$\delta_{0.1\%C} + L_{0.55\%C} \overset{1492°C}{\longrightarrow} \gamma_{0.18\%C}$

Amount of $\delta$ = 82.2%
Amount of L = 17.8%

- No commercial heat treatment is done in this region

#### Eutectic Transformation

$L_{4.3\%C} \overset{1147°C}{\longrightarrow}\gamma_{2\%C}+ Fe_3C_{6.67\%C}$

Eutectic Mixture
- Austenite and Cementite
- Small islands of austenite dispersed in carbide phase
- Ledeburite
- Stable upto 727°C
- Below 727°C - Pearlite + Cementite
- Formed at 1147°C

Amount of P = 40.4%
Amount of Cementite = 59.6%

### Solidification and Microstructures of Slowly Cooled Steels



A~0~ - Subcritical Temperature
A~1~ - Lower Critical Temperature
A~3~ - Upper Critical Temperature
A~4~ - Eutectic temperature
A~5~ - Peritectic Temperature
A~cm~ - $\gamma/\gamma$+Cementite phase field Boundary

Fe-Fe~3~C Diagram

- A - Arrest
- c - chauffage - Heating - Few degrees above ideal arrest point
- r - refroidissement - Cooling - Few degrees below ideal arrest point

#### Mictrostructures of Hypoeutectoid Steels


For 0.008%C
- α - 100%
- P - 0%
For 0.8%C
- P - 100%

12.5%P for every 0.1%C


#### Microstructures of Hypereutectoid steels


α - 80BHN
Pearlite - 230BHN

Hypoeutectoid

Hardness (BHN) = 80 x α + 230 x P

Hypereutectoid

Hardness(VPN) = 900 x Fe~3~C + 240 x P


### Property Variation with Microstructure

Factors affecting
- Amount of phases
- Distrubution of phases or Morphology

Tensile Strength above 0.8%C is approximated

When cooling Fe~3~C comes form austenite and it gets around the austenite grain

Cracks grow very fast along the network of cementite - fails at once

#### Interlaminar spacing


- The distance between centre of one ferrite layer and other
- Affects toughness
- As it decreases the strength increases in the case of Cementite and α-ferrite


> Prove that as the interlaminar spacing decreases, strength of steel increases - Graphically, Mechanism

### Recovery, Recrystallization and Grain Growth

Cold work
- Rolling operation
- At room temperature(Cold working)
- At elevated temperature (hot working)
- Stresses are developed in the metal

Thermally Active and Overlap
- Recovery
- Dislocation which are scattered rearrange themselves to reduce electronic scattering
- Internal strain is relieved by dislocation motion
- Due to the compressive force the grains get elongated
- They want to regain thier shape
- Recrystallisation
- Growing new strain free grains
- The structure was thermodynamically unstable - Hence recrystallization would take place
- By small heating at recrystallization temperature - Temperature at which recrystallization is complete within 1 hour - 1/3 to 1/2 of melting temperature
- If the grains are less elongated the recrystallization temperature is a bit high
- Grain Growth
- Driving force for Grain Growth - Energy associated with the Grain Boundaries
- Holding at a particular temperature some new grains develope
- Small grains merge into larger ones


During Rolling
- Stress field at the top - Compressive
- Stress field at the bottom - Tensile
- Higher the degree of cold working more is the elongation in the grains
- In cold work material electrical conductivity first decreases then it increases in recovery

To have equiax structure all the grains should be hexagonal one


Arrhenious Equation

$\dfrac 1t = Ae^{-Q/RT}$

A - Constant
Q - Activation Energy for recrystallisation
t - Time of recovery at Temp T


Changes in volume during transformation

Density increases by 8.9% - BCC to FCC - Specific Volume decreases

## Effect of Alloying Elements

### Solid Solution Strengthening/Hardening

Mostly dissovle in ferrite


Fomration of Inclusion
- Combine with Oxygen
- Oxide
- Si, Al, Mn, Cr, V, Ti,...

Ni - Ferrite SS strengthener
- Dissolves in Ferrite
- Hardness Increases - Tensile Strength Increases
- Toughness Increases without Ductility Decreases
- Austenite Stabilizer
- Increase the range in which Austenite is stable
- Raising A~4~ and Lowering A~3~
- Slows the separation of Carbides

Ni Steel
- Ductile
- Soft
- Malleable
- Non-Magnetic

Ni
- Increases Impact Resistance
- Decreases Dutile-brittle Transition Temperature

Invar
- 36% Ni
- 0.5% Mn
- 0.2% C

S
- Forms FeS - hard breittle low mpt
- Increases brittleness - Freezes last
- FeS liquifies - increases difficulty to Hot Working
- Hot Shortness - decreased by Mn addition

Free Cutting steel
- MnS, FeS - promote chip formation
- By breaking continuity of the matrix - increases machinability

HSS - High speed steel

P
- Dissolves in ferrite and increases Tensile Strength
- Strong solid solution strengthener
- Reduces solubility of 'C' in adjacent areas
- Formation of banded structure - alternate layers of pearlite and ferrite

Si
- Ferrite Solid Solution Strengthener
- Dissolves in ferrite - Increases Strength and hardness
- Increases toughness without loss of ductility
- Increases permeability
- Lowers hysteresis losses

Formation of intermetallic compounds
- Increases brittleness

Cr
- Increases hardenability

### Shifting of Critical Temperature

- Shifting Transformation Temperature
- Ferrite Stabilizers
- Ti, Mo
- Increase Transformation temperture
- These elements have α-Phase BCC crystak structure
- Decrease the amount of carbon soluble in austenite - thus tend to increase the volume of the free carbide content
- Austenite Stabilizer
- Ni, Mn
- Decrease transformation Temperature
- These elements have $\gamma$-phase FCC crystal structure - thses elements are more stable in Austenite


- Carbide forming elements
- Increasing affinity for carbon - to form carbide
Fe -> Mn -> Cr -> W -> Mo -> V -> Ti -> Nb -> Ta -> Zr
- Graphitising Element
- Si, Ni, Cu, Al
- Graphitise steel, impare the properties of steel
- Eutectoid Composition
- All the elements lower the eutectoid carbon content
- Ti, Mo - most effective in
- Eutectoid Temperature
- Ni, Mn - Austentite stablizer - lower eutectoid temperature
- Cr, V, W - Ferrite stabilizers - raise the eutectoid temperature

### Limitation of Fe-Fe~3~C Phase

- Equilibrium Conditions - actual heat treatment of steels are normally under non-equilibrium condition
- N Lacks character of transformation - Bainite or Martesite
- For Martensite and Benite
- Lacks Metastable phase
- Lacks M~s~, B~s~
- No Kinetice
- Possibilities of suppressing the pearlitic or bainitic transformations

### Strengthening Mechanism in Steels

- Solid State Strengthening
- Number of impurities atoms in the lattice of basic element is small - incapable of forming stable and metastable phases under any thermal condition
- Influence that carbon exterts on plastic deformation resistance of the α-Phase is due to both its strong interaction with dislocations and pinning of the dislocations and elastic deformations - arising as tetragonal distortion
- Grain Size Refinement
- The smaller the austenite grain
- The finer the network of excess ferrite at their boundaries
- Smaller Pearlite colonies and martensite crystals
- Higher Strength
- Fine Crystal Fractures
- Greater the grain size lesser the strength
- Dispersion Strengening
- In case of abrupt of cooling - carbon has no time to precipitate
- At RT the retained amount of carbon can correspond to its maximum solubility of 0.18%C
- Storage at RT - Causes Natural aging - carbon tends to precipitate out of the solid solution
- Carbon enriched regions appear predominantly in defective sections of the matrix
- As the material age naturally the factors such as strength and hardness increases but the plastic properties such as reduction of area specific elongation and impact strength gets hamperd
- Prononunced yeild stress appear after aging
- Hardness increase by 50% of as of quenched stage
- Work hardening
- Deformation Strengthening
- Shape of stress-strin curve depends on
- Crystal Lattice type of the metal
- Purity
- Thermal Treatment
- Platic deformation stregnting depends on
- Interaction of dislocations and structural changes that impede the movement of dislocations

# Heat Treatment

A combination of heating and cooling operation, timed and applied to a metal or allow in the solid state in a way that metal or alloy will produce desired properties

Purpose

- Remove gases from casting
- Hydrogen Embrittlement
- Nacent form - Small diameter
- Moves from one location to another - forms H~2~
- Exerts tensile pressure in solid
- Lower internal stresses due to unequal contraction of castings
- Increase electrical properties
- Increase Magnetice properties
- Increase machinability, increase toughness

### Kinetics of Formation of Austenite


T~4~> T~3~ >T~2~ >T~1~ > Ac~1~

- Microstructure
- Property change
- Hardness
- Colume
- Magnetic
- Interal Stresses

Quench - Suppresses the formation of phases

Higher amount of retained Austenite, more non magnetic it will be

Inhomogeneous Austenite - Concentration Gradient of Carbon



Super heating - The start of transformation also changes from 723°C


If we have 100% perlite and we heat it at a faster rate then 723°C gets shifted to higher temperature

- Transformation is complete in shorter period at higher temp
- Higher heating rates, transformation will start at higher temperature

Formation of Austenite

Nucleation - depends on over heating


Austenite formation
- Function of T, t
- Function of interlamellar spacing ($\lambda$)
- Inversely Proportional
- Function of C%
- More C% - greater Interfacial area - easy to austenize
- Function of Alloying Elements
- Alloying element are difficult to austenize because of alloy carbide dissolve with greater difficulty


- If cooling rate is high eutectoid point shifts to the left
- High cooling rate - eutectoid transition takes place at a lower temp

#### Importance of Grain Size

Fine grained steel
- Hardness Increases
- Impact strength increases

Coarse Grains
- Easy to make martensite

Fine Grains
- Less number of dislocations
- Less pile up at Grain Boundary
- Less concentration gradient

Dislocations move through densely packed planes

#### Grain Coarsening Characteristics

Tendency of $\gamma$-grain to grow at upper critical temperature
- Fine Grained Steel - Resists
- Deoxidised with Al
- Al + N -> AlN
- Inhibits grain growth
- Coarse Grained Steel - Readily shows grain growth
- Deoxidised with Si


ASTM
- Used to determine $\gamma$ Grain Size
- Used for equiaxed grains

Etched steel surface ->

Finding out the Grain size
- when the grains are not equiaxed
- Heyn's Intercept mthod is used
- Method
- Count the Number of grains intercepted by a line of known length
- count at 10 diffrebt locations
- Take average
- Report I

No. of grains intercepted by 0.005" at 1000X = I

No. of grains intercepted by 0.005" at 1000X = $\frac I{0.005}$
No of grains/inch^2^ aat 100x = $(2I)^2 = n$

$n = 2^{N-1}$

$N = \frac{2\text{log }I}{\text{log }2} + 3$

- When steel is sloqly cooled from ɣ range (FCC->BCC)
- Carbon atoms are diffused as solubility of 'c' in BCC is lower than FCC
- Fe Changes from FCC to BCC
- This change is time dependent
- When cooled at faster rate(>critical value)
- time is less to diffuse out of solution
- FCC tries to convert into BCC but 'c' is trapped(inside) in solution
- Structure is BCT

### Martensite Trasnformation

$\gamma \rightarrow M$

- w/o diffusion
- diffusionless
- Shear transformation

Martensite start
If cooling stopped


Ms - lower with high austenizing temperature
$\ Ms \downarrow by \uparrow ring temp$

- dissolutiond of carbide
- removal of concentration gradient

$\ Ms \uparrow\ by\ \large Grain-Size\ \gamma$


Pure $\gamma$ iron - Lattice parameter 3.548 Angstrom



Up to 0.2% C - segregated to dislocations

### Types of Martensite

- Lath Martensite
- Low 'C' and low alloying content
- Higher Temperature(Martensite Start)
- Shape of strip
- Parallel Fashion
- Lower Hardness
- Plate Martensite
- High 'C' (>1%)
- Lower Temperature (Martensite Start)
- Varying size - lens
- Non Parallel fashion
- Higher Hardness
- Partitioning - $\gamma$ grains in several smaller pockets


There is a large difference in the solid solubility of 'C'
$\gamma_{0.8%C}\rightarrow α_{0.02%C}$

Carbon ideally tries to occupy the tetrahedral site in Iron - Tetrahedral hole is larger than octahedral hole in BCC iron

- C atom does not sit in tetreahedal hole
- it sits in octahedral hole in α-Fe as hole is ot symmetrical

Isothermal Heat Treatment

Cooling at once and soaking


Austenite -> Martensite - Volume Expansion

Thermal Stresses are high

As we quench below 727°C
- To have a stable nucleus

### Determination of TTT diagram for Eutectoid Steel

1) Salt Bath

- Maintained at 780°C
- For eutectoid steel
- A~C1~ + 20-40°C - Hypertectoid
- A~C3~ + 20-40°C - Hypoeutectoid
- Also
- A~1~+20-40°C - Eutectoid
- A~3~+20-40°C - Hypoeutectoid
- A~Cm~+20-40°C - Hypereutectoid steels

## Grossman's Method of Measuring Hardenability

Hardenability - ability of a steel to partially or completely transform from austenite to some fraction of martensite at a given depth below the surface, when cooled under a given condition

Hardenability depends on
- Composition - 'C' and alloying elements
- Usually shift the TTT curve to right
- Grain Size
- Nature of Coolant
- Geometry and size of the compound
- Thinner - Better hardenability
- Criterion of hardenability

Surface will cool faster hence they will have diffent grain structure

To bring 100% martensite structure in the core the TT graph needs to be shifted to the right

Hardenability is a function of property of steel

### Criterion of Measuring Hardenability

- Depth which contains
- 50% Martensite
- 90% Martensite


$\gamma$ - Quenched - Cut at 1/2 length - Center hardness - Steep fall in hardness - Critical Diameter

Critical Diameter - D~C~ - of a steel under a given quenching consdition is defined as the diameter of the cylindrical bar which hardens upto its centre

D~C~ depends on
- Nature of quencing medium
- Raio of heat removal
- severity of heat removal

#### Ideal Critical Diameter (D~I~)

Hardenability of two steels cannot be compared based on critical diamter if quenchants are different

Necessary to obtain a measure of hardenability independant


### Continous Cooling Temperature Diagra (CCT)


Banite - Needles aof ferrite and beads of cementite

- Bound to get Pearlite start or Martensite start

# Types of Heat Treatment

## Annealing

### Full Annealing

The steel is heated above A~3~ (hypo-eutectoid) and A~1~ (hyper-eutectoid) - hold - cooled to obtain coarse Pearlite

Not heated above A~cm~ - to avoid continous network of cementite

### Recrystallization Annealing

- Heat below A~1~ - sufficient time - Recrystallization
- Cold worked grain - New stress free grain
- Used in between processing steps (e.g Sheet Rolling)
- Better Ductility, Hardness

### Stress Relief Annealing

Residual stresses - Heat below A~1~ - Recovery - Annihiliation of dislocation

### Spheroidization Annelaing

- Heat below/above A~1~ (Prolonged holding)
- Cementite Plates -> Cementite spheroids - Ductility Increases
- Used in high carbon steel

### Diffusion Annealing

- Homogenizing Annealing
- Remove any structural non-uniformity
- Dendrite, Columnar grain and chemical inhomogeneities are generally observed in the case of ingots, heavy plain carbon steel casting and high alloy steel castings - defects - promote brittleness and reduce ductility and toughness of steel


### Partial Annealing

- Intercritcal Annealing or Incomplete Annealing

## Normalizing

- Heat above A~3~|A~cm~ - Austenization - Air cooling - Fine Pearlite (higher Hardness)
- Purpose
- Refine grain structure

## Hardening

Heat above A~3~|A~cm~ - Austenization - Quench (higher than critical cooling rate)

- Certain applications demand high tensile strength and hardness values so that the components may be
- hardening process consists of four steps.The first step involves heating the steel above a~3~ temperature for hypoeutectoid steels and above A~1~ temperature for hypereutectoid steels and Above A~1~ temperature for hypereutectoid steels by 50 $\degree C$
- The second step involves holding the steel components fir sufficient socking time for homogeneous oxidation.
- The third step uinvolveds cooling of hot steel components at a rate just exceeding the ciritical cooling rate of the fine

#### Salient Fratures of Harening of Steel

Proper queching medium should be used such that the compnenet gets cooled at a rate just exceeding the critical cooling rate of that steel

- alloy steels have less critical cooling rate and hence some of the alloy steels can be hardened by simple air cooling
- High carbon steels have slightly more critical cooling rate and has to be hardened by oil quenching
- Medium carbon steels have still higher cooling rates and hence water or brine quenching is necessary.

Factors affecting Hardening Processes

- Chemical Composition of steel
- Size and shape of the steel part
- Hardening cycle (heating/cooling rate, temp, soak time)
- Hommogeneity and grain size of austenite
- Quenching media
- Surface condition pf steel part

Hardening Methods
- Conventional or direct quneching
- Quenching in stages
- Spray Quenching
- Austempering or isothermal Quenching
- Martempering

## Retained Austenite (RA)

- Untransformed Austenite
- Present in the ferrous alloyes even after copletion of heat treatment
- Austenite transforms from M~s~ to M~f~ as it is essential in athermal transformation .However this transformation never goes to compleation i.e 100% martensite(M~f~ temperature line is illustrated as dotted line in TTT digrams)
- This is beacuse at M~f~,small amount of (~1%) of austenite is present in highly stressed state along with 99% martensite and can not transform to martensite because of unfavourable stress conditions
- Both M~s~ and M~f~ temperatures decrease with increase in carbon content - Retained Austenite increases with increase in carbon content
- All alloying elements, except Al and Co, lower the M~s~ temperature and hence enhance the amouunt of retained austenite

- The center has more RA than surface - restrictions are more in the center hence they don't transform

### Advantages

- Ductility of austenite can help to relive some internal stresses devloped due to hardening to reduce danger of distortion and cracks - 10 % retained austenite along with martensite is desireable
- The presence of 30-40% retained austenite makes straightening operation of the
components possible after hardening. Straightening increases the hardness slightly.
Non-distorting steels owe their existence to retained austenite. Here enough austenite is
retained to balance the transformational contracting during heating, on the formation of
austenite from ferrite carbide aggregate on the one hand, and the expansion corresponding

disadvantages
austenite from ferrite carbide aggregate on the one hand, and the expansion corresponding
to the formation of martensite during cooling, on the other, Here, the basis of dimensional
- As RA may transform - to lower bainite or martensite - increase in dimension - Stresses developed
- Affects magnetic properties

### Sub-Zero Treatment

The Retained austenite

Subzero treatement consists in cooling the hardened steel to a temperature below 0°C

Temperature depends on the position of M~f~ temperature of the steel

More effective - if carried out immediately

A steel can be cooled much below the Mf temperature, but it, evidently achieves nothing,because because it cannot bring about any additional additional increase increase of hardness, hardness, or any additional additional increase increase of martensite, because the Martensitic transformation ends at Mf temperature.

Sub-zero treatment is more effective, if it is carried out immediately after quenching
operation. Any lapse of time between hardening and the cold treatment causes the stabilization of austenite, makes the retained austenite resistant to further transformation.

The low-cooling unit consists of two vessels, the interior one of copper, where the parts or
tools to be deep frozen, are placed and the exterior one of steel provided with a good heat
insulation.


## Tempering

### Objectives:

- Relieve Internal Stresses
- Restore ductility and toughness
- To have dimensional Stability
- To improvemagnetic properties
<br>
- First stage - RT - 200°C
- Second stage - 200-350°C
- Third
- Fourth - 350-700°C

### Structure in Quenched state
- Highly Superstaurated martensite
- Retained austenite
- Undissolved carbides
- Rods, or plates of carbide particles produced during auto tempering
- segregation of carbon

Tempering temperatures are below 400°C

### First Stage

- RT to 200°C
- Carbon less than 0.2% - Carbon atoms have not yet segregated to dislocation - they are around dislocation and lath boundaries - No carbide forms
- Decomposition of martensite into low tetragonality martensite
- Diffusion is less so Fe~3~C is not formed - ε-Carbide is formed
- ε-Carbide - Separate phase not a preliminary step in thr formation of cementite
- Thr structure is refered as tempered maretensite - double pahase micture of low tetragonal martensite and ε-Carbide
- Volumne in this stage is low - Volume of martensite is less due to rejecting of C atoms

### Second stage

- Second stage of tempering temperature lies between 200-300°C. The amount of retained austenite in quenched steel depends upon temperature and composition
- RA transforms to lower Bainite (Carbide in Bainite is ε-Carbide)
- Matrix is cubic ferrite
- Volume Increases

### Third Stage of Tempering

- 200-350°C
- ε-Carbide dissolves in matrix and low tetragonal martensite losses it tetragonality

### Fourth Stage of Tempering

- 350-700°C
- Growth and speoridisation of cementite as well as revcovery and recrystallization of ferrite occur
- Groth of Cementite starts above 300°C
- Spheroidisation - 400-700°C
- It takes place due to reduction in interfacial energy of ferrit-cementite interfaces
-

## Martempering

- Given to oil hardenable and air hardenable steels and thin sections of water hardenable steels to produce martensite with minimal differential thermal and transformation stress to avoid distortion and cracking
- Primary requirement - Steel should have reasonable time for the nose of the TTT curve at the Bainitic bay
- Isothermally held at the bainite


- Transformation stresses are avoided

## Austempering

- Aim is to produce lower bainite without any distortion or cracking


- Ausforming - giving mechanical treatment to austenite before austempering

## Isothermal Annealing

## Patenting

- Thin wires - Piano Wires
- Eutectoid steel can have interlamellar spacing as small as 40

## Continous Cooling Transformation

- Lacks soaking
- The cooling rate may or may not be constant
- Practical utility of TTT curve is limited
- Diagram drawn for a given cooling rate
- Bainite is not formed


- CCT diagrams are determined by measuring some physical properties - Specific volume and magnetic permeability

### Tempering of Alloy Steels

- Silicone dissolves in ε - carbide - stabilize it
- 1-2% Silicone - ε-Carbide present even after 400°C

#### Secondary Hardening

- Alloys steels - larger amounts of strong carbide forming elements like Mo, Ti, V, Nb, W, Cr and Carbon - the hardness of the as-quenched matensite on tempering, decreases intitally as the tempering temperature is raised - Then hardness increases to often become higher

Classification of Cast Iron

- White CI
- Brittle
- No engineering application
- Catastrophic Failure
- Grey CI - Graphite flakes
- Ductile/Nodular - Circular Flakes
- Malleabilize - White CI - Malleable CI

Cast Irons Alloying Systems

Cast iron - High Castability
Carbon equivalent - a measutre of the rquivalency of carbon coupled with outher alloying elements to that of just carbon

Cementite - Metastable phase - tendency to decompose into iron and carbon

Gray Cast Iron - Greater Machinability (due to graphite flakes)

Hypereutectic - Cementite the form - Primary Cementite - Needle like

Two stages of Annealing cycle
- Converts primary carbides to temper carbon
- 940-960°C - All cemenite -> Graphite (rosette form)
-
- Carbon is dissolved in Austenite at the


# Copper Alloys

- Electrical Conductor
- There are diffrent grades of copper
- Arcenical Copper
- Free cutting copper
- Silver bearing copper
-
- Pure Copper Annealed at 800°C
- 40%Zn - $\alpha + \beta$

Cupronickels

- copper nickel

Types of Alloys

Aluminium alloy designation system


Precipitation Hardening

A Heat to 550 C ---> solid solution alpha
B Quench to RT
C Age reheat to 200 C ---> fine precipitates


Ageing with time at 180 C some copper atom will try to move at a pirticular plane and form a cluster (Gp zones) Thease particals are coherrent with the matrix after peak coherrence is lost and they have a specific shape

At 180C with change in time
- Increasing in size of precipate with increase interparticle spacing
$GP \rightarrow \theta'' \rightarrow \theta' \rightarrow \theta$




Duralumin highest hardness and strength

solid solid Interface


Semi Coherrnt vs Incoherrent