L2 - Phase Diagram

8/17/2019 L2 - Phase Diagram

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1) Phase Diagram

2) Phase Transformation Under

Thermal Equilibrium3) Microstructural Constituents

4) Alloying Elements Effects

TOPICS

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Goals of this lecture

• Learn definitions and basic concepts of phase equilibria and

phase diagrams

• Interpret equilibrium phase diagrams

–Binary isomorphous

– Binary eutectic

– Intermediate compounds/phases

– Phases present

– Their compositions and amounts

– Development of Microstructure and their Mechanical

Properties

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Phase Diagrams

• Many materials systems can exist in a varietyof forms depending on the temperature,pressure and overall composition

• A “phase diagram” is a graphicalrepresentation which details the form(s) thematerial takes under specific conditions

• Assumes the system has achieved chemicalequilibrium

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• Changing T can change the

phases: path A to B.

• Changing Co can change thephases: path B to D.

Example: water-sugar system

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Definitions and Basic Concepts

• Equilibrium

– Thermodynamic definition: a system is at equilibrium

if its free energy is at a minimum, given a specified

combination of temperature, pressure and

composition.

– Characteristics of the system do not change with time,

i.e., the system is stable

– A change in T, P or C for the system will result in an

increase in the free energy and possible changes to

another state whereby the free energy is lowered.6


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• Sometimes (usually) the kinetics (rate of reaction) of the

system will not allow a phase change to take place

completely, or instantly, under certain conditions – Example: glass of ice water in the sun when 36

• Components:

– Pure Substances (s) required to express composition

of phases in the system

or

–The elements or compounds that are mixed initially(eg. Al and Cu)

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• Phases:

• Homogenous portion of a system

•Uniform chemical and physical properties

• Several phases may be present simultaneously

• In principal, phases are recognizable and separable

Aluminum-

Copper

Alloy


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Solubility

1. Unlimited Solubility

2. Limited Solubility

• Condition for unlimited solid solution

– Size Factor: similar size (±15%)

–Crystal Structure: Same crystal structure

– Valance: Same valence

– Electronegativity: Similar electronegativity

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Solubility Limit

• Solute and solvent

Solvent (component present in greatest amount) Solute (present in minor concentration)

• Solubility Limit:

• Maximum allowed concentration of solute in solvent• Depends on species, temperature and pressure

• Solution and mixture

• A mixture is heterogeneous (more than 1 phasepresent)

• A solution is a single homogeneous phase of variable

composition

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•Solidus Line- Temperature where alloy is completely

solid. Above this line, liquefaction begins.

•Liquidus Line - Temperature where alloy is completely

liquid. Below this line, solidification begins.

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Example :

Question:

a) What are the phases in this

system?

b) What is the solubility limit for

sugar in water at 20°C?

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Sugar/Water Phase Diagram

S u g a r

T e m p e r a t u r e

( ° C )

0

20

40

60

80

100

C = Composition (wt% sugar)

L

(liquid solution

i.e., syrup)

Solubility

Limit

L

(liquid)

+S

(solid

sugar) 20

4 0

6

0

8 0

10 0

W a t e r

This is a phase diagram of a two-component system, sugar and

water.

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Answer

a) Liquid (“syrup” or sugar in water) and a solid

(sugar)

b) 65 wt% sugar.At 20°C, if C < 65 wt% sugar: syrup

At 20°C, if C > 65 wt% sugar: syrup + sugar

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• Indicate phases relation as a function of temperature,

component and pressure.

• However, many useful diagrams are constructed for

constant pressure of 1 atm, so only composition and

temperature are variables

• It provides us with information needed for the control

of phase and microstructure in the materials we make

Phase Diagrams of Materials

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One Component Phase Diagram

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Variable: Pressure and temperature


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• Melting point of the system changes with wt% of Ni

• 2 phases present

• L (liquid)

• (FCC solid solution)

• 3 different phase fields

• L

• L +

•

Cu-Ni system

wt% Ni 20 40 60 80 100 0 1000

1100

1200

1300

1400

1500

1600

T(°C)

L (liquid)

(FCC solid

solution)

Two Component Phase Diagram


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• Changing T can change # of phases: path A to B.

• Changing Co can change # of phases: path B to D.

Effect of Temperature & Composition (Co)

wt% Ni 20 40 60 80 100 0 1000

1100

1200

1300

1400

1500

1600

T(°C)

L (liquid)

(FCC solid solution) A

B D

Cu

Cu-Ni

system


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Question:

a) Determine the phase (s) present at point A and Bb) What is the melting points for Cu and Ni

Cu-Ni

phase

diagram

Example :

• Rule 1: If we know T andCo, then we know:

--the phase(s) present


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Answer

(a)At point A (1100, 60) - 1 phase present, α

At point B (1250, 35) – 2 phases presesnt, L + α

(b) Melting points

Cu = 1085°C, Ni = 1453 °C

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Example

• Rule 2: If we know T and Co,

then we know:--the composition of each phase.

Cu-Ni

system

Question:

a) Determine the composition of A, B and D

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Answer

At TA

= 1320°C:

Only Liquid (L) presentCL = C0 ( = 35 wt% Ni)

At TB

= 1250°C:

Both and L present

At TD = 1190°C:Only Solid () present

C = C0 ( = 35 wt% Ni)

CL = C liquidus ( = 32 wt% Ni)

C = C solidus ( = 43 wt% Ni)


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Example

Question:

a) Determine the amount of each phase(s)present at point A, B and D in wt%

• Rule 3: If we know Tand Co, then we know:

--the amount of each

phase (given in wt%).

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Cu-Ni system

Answer

WL 43 35

43 32 73wt %

= 27wt %W


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• Sum of weight fractions:

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• Conservation of mass (Ni):

• Combine above equations:

WL W 1

Co WLCL WC

• A geometric interpretation: moment equilibrium:

1W

solving gives Lever Rule

WLR

WS

THE LEVER RULE:

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Gibbs Phase Rule

• Phase diagrams and phase equilibria are subject to thelaws of thermodynamics.

• Gibbs phase rule is a criterion that determines howmany phases can coexist within a system atequilibrium.

• It used to predicts the number of phases that willcoexist within a system at Equilibrium

• It does not apply in non-equilibrium situations

P + F = C + N

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P + F = C + N

Where,

P: # of phases present

F: degrees of freedom (temperature, pressure, composition)

C: components or compounds

N: noncompositional variables

*N = 1 or 2 for T (temperature) and p (pressure)

N = 1 if p = constant or T = constant

(HINT: p is usually constant so N is usually 1)

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For the Cu-Ag system @ 1 atm for a

single phase P:

N=1 (temperature), C = 2 (Cu-Ag), P=

1 (, b, L)

F = 2 + 1 – 1= 2

This result tells us that two variables(temperature and composition) canbe varied independently and thesystem will still remain a single

phase)

Example


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• If 2 phases coexist, for example, +L , b+L, b, thenaccording to GPR, we have 1 degree of freedom: F =2 + 1 – 2= 1. Thus, one variable can be changedindependly and still maintain a system with two

coexisting phases.

• If 3 phases exist (for a binary system), there are 0degrees of freedom. This means the composition andTemp are fixed. This condition is met for a eutecticsystem by the eutectic isotherm.

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Example

• A

• B• C

Question:

Please determine the value of the degree of freedom for

point A, B and C

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Answer

• Point A – C = 2 (A&B), P = 1 (L)

– F = C+1-P = 2+1-1 = 2

• Point B – P = 2

– F = 1

•Point C – P = 3 – F = 0

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Importance of Phase Diagrams

• There is a strong correlation between microstructureand mechanical properties, and the development of

alloy microstructure is related to the characteristics of

its phase diagram.

• Phase diagrams provide valuable information about

melting, casting, crystallization and other phenomena.


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Concept of Equilibrium

• What phases do we get as we cool off a molten metal?

• This depends very much on the rate of cooling. If we cancool slowly enough, we get phases that are close to whatthermodynamicists would call “equilibrium.”

• This is the basic phase balance that we get if we haveenough time and temperature for diffusion to do itswork.

• Diffusion of key species is essential to being able to getequilibrium.

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Microstructure

• In metal alloys, microstructure is characterizedby the number of phases, their proportions,and the way they are arranged.

• The microstructure depends on:

– Alloying elements

–Concentration

– Heat treatment (temperature, time, rate of cooling)


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Eutectic

• A eutectic or eutectic mixture is a mixture of two or

more phases at a composition that has the lowestmelting point.

• It is where the phases simultaneously crystallize from

molten solution.

• The proper ratios of phases to obtain a eutectic isidentified by the eutectic point on a binary phasediagram.

L(CE) (CE) + b(CbE)


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Eutectic Reaction and Microstucture

2#1# Solid Solid Liquid

b Liquid

On cooling

On cooling

Eutectic Temperature

Two solids have to form side by

side. They do so in layers. This

microstructure is calledlamellar.

Eutectic Microstructure

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• Where is the eutectic point?


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•The eutectic point is the point where the liquid phase bordersdirectly on the solid α + β phase; it represents the minimum meltingtemperature of any possible A B alloy.

•The temperature that corresponds to this point is known as theeutectic temperature.

•Not all binary system alloys have a eutectic point: those that forma solid solution at all concentrations, such as the gold-silver system,have no eutectic. An alloy system that has a eutectic is often referredto as a eutectic system, or eutectic alloy.

•Solid products of a eutectic transformation can often be identified bytheir lamellar structure, as opposed to the dendritic structurescommonly seen in non-eutectic solidification.

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• Eutectoid – one solid phase transforms to two other solid

phases

Solid1 ↔ Solid2 + Solid3

+ Fe3C (For Fe-C, 727C, 0.76 wt% C)

Eutectic, Eutectoid, & Peritectic

• Eutectic - liquid transforms to two solid phasesL + b (For Pb-Sn, 183C, 61.9 wt% Sn)

cool

heat

• Peritectic - liquid and one solid phase transform to a 2nd solid

phase

Solid1 + Liquid ↔ Solid2

+ L ε (For Cu-Zn, 598°C, 78.6 wt% Zn)

cool

heat

cool

heat


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Eutectoid & PeritecticA

B


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Limited Solubility

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Cooling Curves


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Cooling Curves• Used to determine phase transition temperature.

• Temperature and time data of cooling molten metal is recorded andplotted.

• Thermal arrest : heat lost = heat supplied by solidifying metal

• Thermal arrest is defined as a region of the cooling curve for a pure metalwhere temperature does not change with time representing the freezingtemperature.

• Alloys solidify over a range of temperature (no thermal arrest)

Pure Metal

Iron

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Cooling Curves for Iron

• Pure iron when heated

experiences 2 changes incrystal structure before itmelts.

• At room temperature thestable form, ferrite ( iron)has a BCC crystal structure.

• Ferrite experiences apolymorphic transformationto FCC austenite ( iron) at912 ˚C (1674 ˚F).


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Cooling Curve of Pure Iron

• At temperature above

1540 ˚C, iron is in liquidform.

•At 1540 ˚C, iron start to

solidified.

•Delta (BCC) phase start

to form and stable within

the temperature of 1395

- 1540˚C.

•Delta change to gamma

(FCC) phase at around

1395 ˚C. 46


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• Next is the phase change at 910 ˚C, gamma tobeta/alpha (BCC).

• Beta phase is a non-magnetic version of alphairon, is identical to alpha iron in crystal structure,and exists from 910 ˚C to 768 ˚C.

•The temperature at which paramagnetic beta irontransform to ferromagnetic alpha iron is termedas Curie Temperature.

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Iron Iron Carbon Phase Diagram


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Iron – Iron Carbon Phase Diagram

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• Iron carbide (cementite or Fe3C) an intermediate

compound is formed at 6.7 wt% C.

• Typically, all steels and cast irons have carboncontents less than 6.7 wt% C.

• Carbon is an interstitial impurity in iron and forms asolid solution with the , , phases.

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Transformation Temperature


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Transformation Temperature

The following symbols are used for iron and steel

Acm – In hypereutectoid steel, the temperature at which thesolution of cementite in austenite is completed duringheating.

Ac1 – The temperature at which austenite begins to form

during heating.Ac2 – The temperature at which the α-iron changes to non-

magnetic β-iron. (This is not important for heat treatmentstudies).

Ac3 – The temperature at which transformation of α-iron to

austenite is completed during heatingAc4 – the temperature at which austenite transforms to delta

ferrite during heating.

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Example:


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pQuestion: Define all the transformation temperature

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Answer


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Answer

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Delta region of Fe-Fe carbide diagram

Liquid + ↔ austenite

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Ferrite region of

Fe-Fe Carbide

diagram

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IRON IRON-CARBON DIAGRAM

• A map of the temperature at which different phase

changes occur on very slow heating and cooling in

relation to Carbon, is called Iron- Carbon Diagram.

• Iron- Carbon diagram shows

– the type of alloys formed under very slow

cooling,

– proper heat-treatment temperature and

–how the properties of steels and cast irons canbe radically changed by heat-treatment.

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Steel and Cast Iron in diagram

Ferrite

Austenite

Steel Cast iron

Pearlite

Pearlite and

Cementine

Pearlite and

Carbide

Eutectic

eutectoid

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Solid Phases in Iron-Carbide Phase


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Solid Phases in Iron Carbide PhaseDiagram

1. ferrite

•Solid Solution of carbon in iron• BCC Structure

• Carbon only slightly soluble in the matrix

• Max solubility of 0.02%C at 723oC to about 0.008%C at

room temperature

2. austenite

• Solid solution of carbon in iron

• FCC structure

• Can accommodate more carbon than ferrite

• Max of 2.08%C at 1148oC decreases to about 0.8%C at

723oC

• Different in C solid solubility between and is the

basis for hardenin of most steels57


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3. ferrite

• Solid Solution of carbon in iron• BCC Structure

• Max solubility of 0.09%C at 1495oC

4. Cementite (Fe3C)

• Intermetallic Fe-C compound

• Fe3C : 6.67%C and 93.3%Fe

• Orthorhombic cryrstal structure : hard and brittle

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P li


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Pearlite

is often said to be a two-phased, lamellar (or

layered) structure composed of alternating layers of

alpha-ferrite (88 wt%) and cementite (12 wt%) that

occurs in some steels and cast irons.

Ledeburite

a mixture of 4.3% carbon in iron and is a eutectic

mixture of austenite and cementite. Ledeburite isnot a type of steel as the carbon level is too high

although it may occur as a separate constituent in

some high carbon steels, it is mostly found with

cementite or pearlite in a range of cast irons. 59

Iron carbide (Cementite or Fe C)


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Iron carbide (Cementite or Fe3C)

• Forms when the solubility limit of carbon in ferrite is

exceeded at temperatures below 727 ˚C.

• Mechanically, cementite is very hard and brittle.

• For ferrous alloys there are 3 basic types, based on

carbon content:

Iron (ferrite phase):


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Three important horizontal lines

The diagram shows three horizontal lines which indicate

isothermal reactions (on cooling / heating): • First horizontal line is at 1492°C, where peritectic

reaction takes place:

Liquid + ↔ austenite

• Second horizontal line is at 1147°C, where eutecticreaction takes place:

liquid ↔ austenite + cementite

• Third horizontal line is at 727°C, where eutectoid

reaction takes place:

austenite ↔ pearlite (mixture of ferrite &

cementite)

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R ti i I bid Ph Di


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Reactions in Iron-carbide Phase Diagram

1. Peritectic Reaction

– This alloy containing 0.15% carbon

– The initial crystals of -solid solution and the whole liquid phase iscompletely transformed to ɤ at 1492oC – (almost no engineeringimportance).

+

2. Eutectic Reaction – Carbon content up to 4.33%

– The liquid is transformed into ɤ and cementite at 1147oC

– Cementite contains 6.67wt% carbon.

– The eutectic of austenite and cementite is known as ledeburite.

-- They are cast irons

+ 3

cool

heat

cool

heat

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3. Eutectoid reaction

– Iron –carbon alloy with 0.8% carbon

– ɤ is transformed into ferrite and cementite byeutectoid reaction at 723oC

– They are steels. + 3

cool

heat

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Summary


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Summary

Peritectic L + =

Eutectic L = + Fe3C

Eutectoid = + Fe3C

Phases present

L

Reactions

BCC structure

Paramagnetic

austenite

FCC structure

Non-magnetic

ductile

ferrite

BCC structure

Ferromagnetic

Fairly ductile

Fe3C cementite

Orthorhombic

Hard

brittle

Max. solubility of C in ferrite=0.022%

Max. solubility of C in austenite=2.14%64

Steel


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Steel

Three major categories of steels

1. low carbon steels (carbon up to 0.3%)

2. Medium carbon steels (carbon from 0.3% to

0.6%)

3. High carbon steels (carbon more than 0.6%)

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What is steel?

• A vast family of materials

• Usually composed of ferrite + cementite. Thereare some exceptions.

• The way that the cementite is distributed in theferrite is very important. For example,

1. Very coarse roundish cementite particles widelyseparated soft ductile steel.

2. Very fine roundish cementite particles closelyspaced hard strong steel. (Has some ductility)

3. And so on – there are many ways to distributecementite in ferrite.

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Phase transformation in steel according to Fe-C diagram

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l h f l d h h


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Principal phases of steel and their Characteristics

PhaseCrystal

structureCharacteristics

Ferrite BCC Soft, ductile, magnetic

Austenite FCC

Soft, moderate

strength, non-

magnetic

CementiteCompound of Iron

& Carbon Fe3CHard &brittle

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The Austenite to ferrite / cementite


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The Austenite to ferrite / cementitetransformation in relation to Fe-C diagram

In order to understand the transformationprocesses, consider a steel of the eutectoid

composition. 0.8% carbon, being slow cooled along

line x- x‘

. • At the upper temperatures, only austenite is

present, with the 0.8% carbon being dissolved in

solid solution within the FCC. When the steel coolsthrough 723°C, several changes occur

simultaneously.

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h i f i / i


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The Austenite to ferrite / cementite

transformation in relation to Fe-C diagram

• The iron wants to change crystal structure

from the FCC austenite to the BCC ferrite,

but the ferrite can only contain 0.02%carbon in solid solution.

• The excess carbon is rejected and forms

the carbon-rich intermetallic known as

cementite.

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P liti t t


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Pearlitic structure

•The net reaction at theeutectoid is the formation of

pearlitic structure.

• Since the chemicalseparation occurs entirely

within crystalline solids, the

resultant structure is a finemixture of ferrite and

cementite.

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Schematic picture of the formation and

growth of pearlite

Ferr ite

Cementite

Austeniteboundary

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Nucleation & growth of pearlite

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Transformations in Hypoeutectoid Steels


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Transformations in Hypoeutectoid Steels

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• Line GS shows the temperatures where transformation of ɤ to


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• Line GS shows the temperatures where transformation of ɤ to

starts.

• Line ES shows the temperatures where transformation of ɤ toFe3C starts.

• Point S is called eutectoid point, where degree of freedom,F = 0

• When point S is reached during cooling, the ɤ decomposes to

100% lamellar pearlite at 723oC.

• Line PQ shows the variation in solubility of carbon in iron with

the temperature. It corresponding to starting of precipitation ofsurplus cementite out of ferrite.

• The limiting composition for getting pearlite is 0.025% carbon.

With carbon content less than this, no pearlite will be formed.75

Case Study – Point X


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Case Study Point X1• At temperature T1 , it cuts the GS line, (proeutectoid

) starts to form.

• The composition of the ɤ moves along line GS.

• At T2

, the composition of ferrite is indicated by point Mand austenite by point O.

• At eutectoid temperature, the carbon content in theferrite is 0.025% and that of austenite 0.8%.

(0.8) →

0.025+

(6.67)

Pearlite 76


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Example Problem


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Example Problem

For a 99.6 wt% Fe-0.40

wt% C steel at atemperature just belowthe eutectoid,determine the

following:a) The compositions of

Fe3C and ferrite ().

b) The amount of

cementite (in grams)that forms in 100 g ofsteel.

Answer


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80

Answer

W Fe3C

R

R S C

0 C

C Fe3C

C

0.40 0.022

6.70 0.022 0.057

b) Using the lever rule with

the tie line shown

a) Using the RS tie line just below the eutectoid

C = 0.022 wt% C CFe3C = 6.70 wt% C

F e 3

C ( c e m e n t i t e )

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

(austenite)

+L

+ Fe3C

+ Fe3C

L+Fe3C

C , wt% C

1148°C

T(°C)

727°C

C0

R S

CFe C 3C

Amount of Fe3C in 100 g

= (100 g)WFe3C

= (100 g)(0.057) = 5.7 g

Transformation in a Steel Containing


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less than 0.025% Carbon – X2

• Carbon content is less than 0.025%.

• Austenite transform to at T3 and transformation iscompleted at T4

• No change in this structure between T4 and T5

• At T5, the vertical composition cuts the solvus line PQ.Excess carbon will be separates our as Fe3C.

• If cooling rate is higher than the equilibrium rate, themicrostructure is 100% ferrite, this is a ferrite which issupersaturated as its carbon-content

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Transformation in Hypereutectoid Steel


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– X3• Cooled from T6 to T7, no phase changes occurs.

• Below T7 , cementite begins to separate out.

• At T8 the composition are ɤ and cementite.

• Further cooling, the entire amount of austenite willtransform to pearlite.

• Final microstructure consists of pearlite andproeutectoid cementite.

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E l


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Example:

• Consider 1.0kg of austenite contanining1.15wt% C. cooled to below 727oC,

a) What is the proeutectoid phase?

b) How many kilograms each of total ferrite andcementite form?

c) How many kilograms each of pearlite and the

proeutectoid phase form?

d) Schematically sketch and label the resulting

misrostucture.

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Answer


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This problem asks us to consider various aspects of 1.0 kg of

austenite containing 1.15 wt% C that is cooled to below theeutectoid.

(a)The proeutectoid phase will be Fe3C since 1.15 wt% C is

greater than the eutectoid composition (0.76 wt% C).

(b) Application of the appropriate lever rule expression yields

0.83(1.0kg) = 0.83kg for total ferrite

88

6.7 - 1.150.83

1.150 17


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0.17 (1.0kg) = 0.17kg

(c)

0.93 (1.0kg) = 0.93kg

Proeutectoid

0.07 (1.0kg) = 0.07kg

89

0.17

1.150.93

1.150.07


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Eutectoid

(e) Schematically, the microstructure would appear as:

90

0.17 0.07 0.10 kg

Transformation in Eutectoid Steel –X4


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4

• On cooling a eutectoid point S, all austenite

will transform into 100% pearlite.

• Microstructure at room temperature will be

pearlite.

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• Below eutectoid temperature,


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p ,

layers of ferrite and cementite

are formed. Pearlite.

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PearliteEutectoid reaction:

F C


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↔ + Fe3C

Austenite – 0.76 wt% C

Ferrite - 0.022 wt% C

Cementite - 6.70 wt% C

Redistribution of carbon by diffusion

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Attention !!!

• It should be noted that the transitions as

discussed, are for equilibrium conditions,

as a result of slow cooling.

• Upon slow heating the transitions will

occur in the reverse manner.

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Attention !!!


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Attention !!!

•When the alloys are cooled rapidly, entirelydifferent results are obtained, since sufficient time

may not be provided for the normal phase

reactions to occur.

• In these cases, the equilibrium phase diagram is no

longer a valid tool for engineering analysis.

•Rapid-cool processes are important in the heattreatment of steels and other metals (to be

discussed later in H/T of steels).

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Alloying Steel with more Elements


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• Teutectoid changes: • Ceutectoid changes:

Alloying Steel with more Elements

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Cast Irons


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Cast Irons-Iron-Carbon alloys of 2.11%C

or more are cast irons.

-Typical composition: 2.0-

4.0%C,0.5-3.0% Si, less than

1.0% Mn and less than 0.2%

S.

-Si-substitutes partially for C

and promotes formation of

graphite as the carbon richcomponent instead Fe3C.

**Formation of graphite or cementite

(iron carbide) is depends on cooling rate

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Cooling rate dependent of cast irons


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microstructures

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Cast Irons


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• Can be divided into two main groups, depends on thecarbon content. There are white and grey cast irons.

• In white cast irons, carbon is present in the combined formas cementite.

•In grey cast irons, it is present in free form as graphite.

• Under very slow cooling rate, carbon atoms get sufficienttime to separate out in pure form as graphite, or add ingraphitizing element (Si).

• 3 groups of cast irons in iron carbide phase diagram – Eutectic, Hypoeutectic and hypereutectic.

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Gray Cast Iron


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Gray Cast Iron

•Composition of 2.5% to 4% carbon and 1% to 3%) silicon.

•Graphite exists largely in the form of flakes.

•Properties of gray iron :

o Low (negligible) ductility

o Weak in tension

o Strong in compression

o Good vibration damping

•Products from gray iron include automotive engine blocks and heads,

motor housings, and machine tool bases.

Nodular Cast Iron (Ductile Iron)


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• This is an iron with the composition of gray iron in which the molten metal

is chemically( added with magnesium) treated before pouring to cause the

formation of graphite spheroids rather than flakes.

• shock resistant , stronger

and more ductile iron.

• Applications include machinery components requiring high strength and

good wear resistance.

White Cast Iron


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•Due to large amounts of iron carbide presence, the structure of

white iron is very hard, wear resistance and brittle.

•It is obtained either by cooling gray iron rapidly or by adjusting the

composition by keeping the carbon and silicon content low.

•Products from white iron include railway brake shoes.

Malleable Iron


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•Obtained by annealing white iron in an atmosphere of carbon monoxide and

carbon dioxide, between 800oC~900oC, for several hours.

•2 types of malleable iron :

•Pearlite malleable(white malleable) – upon fast cooling of white iron

•Ferrite malleable (black malleable) – upon slow cooling of white iron

•The structure has good ductility, strength and shock resistance.

•Typical products include pipe fittings and flanges, railroad equipment parts.

Transformation in Hypoeutectic Cast


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Iron -

• A structure just below 1147 ˚C consist ofproeutecitc austenite and ledeburite (eutecticmixture consisting of austenite and cementite).

• Further cooling, the excess carbon comes out ascementite from proeutectic and eutecticaustenite.

• Both will contain 0.8% carbon and woulddecompose by the eutectoid reaction to pearlite.

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Problem 1


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Problem 1

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Answer


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Answer


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Answer


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Problem 2


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Answer


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Answer


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Answer


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Formation of graphite or

cementite (iron carbide) is

depends on cooling rate

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Iron- Iron

Carbide system

Iron-Graphite

system


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Problem 3


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Answer


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Answer


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Answer


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Answer


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Answer


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Transformation in Hypereutectic Cast Iron


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• A structure just below 1147 ˚C consist of proeutectic cementite andledeburite.

• Further cooling, the excess carbon comes out as cementite fromeutectic austenite.

• Proeutectic cementite will remain as it is since it does not

undergoes any further change on cooling.

• the excess carbon comes out as cementite from eutectic austenite.

• At 723 ˚C, eutectic austenite transforms into pearlite

• Final microstructure will reveal proeutectic cementite present asplate and transformed eutectic consisting of pearlite pluscementite.

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Transformations in Eutectic Cast Iron


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• Eutectic cast iron solidifies at 1147 ˚C.

• As the temperature decreases, the solubilityof carbon in austenite iron decreases, as

indicated by the cementite line.

• On further cooling, proeutectoid cementiteseparates out of the austenite at 723˚C.

• Final structure is ledeburite.

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Element Content in Cast Iron


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Element Function

Carbon Increase graphite content

Silicon Help formation of graphiteSulfur + Manganese Stabilize cementite and combination

of sulfur and manganese form

manganese sulfide.

Phosphorous Lower down melting point

Microstructure of cast irons


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ELEMENTS INFLUENCE IN FERROUS

ALLOYS PROPERTIES


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ALLOYS PROPERTIES

Manganese (Mn) improves hardenability, ductility and wear resistance. Mn,

increasing strength at high temperatures.

Copper (Cu) improves corrosion resistance.

Chromium (Cr) improves hardenability, strength and wear resistance, sharply

increases corrosion resistance at high concentrations (> 12%).

Sulfur improves machinability.

Silicon (Si) improves strength, elasticity, acid resistance and promotes

large grain sizes, which cause increasing magnetic

permeability.

Nickel (Ni) increases strength, impact strength and toughness, impart

corrosion resistance in combination with other elements.

Molybdenum

(Mo)

increases hardenability and strength particularly at high

temperatures.


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Aluminum

(Al)

deoxidizer, limits austenite grains growth.

Vanadium (V) increases strength, hardness, creep resistance and impact

resistance due to formation of hard vanadium carbides,

limits grain size.

Tungsten (W) increases hardness particularly at elevated temperatures due

to stable carbides, refines grain size

Titanium (Ti) improves strength and corrosion resistance, limits austenite

grain size.

From a phase diagram

• We know


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• We know

– Overall Composition

– Solidus line

– Liquidus Line

– Limits of Solid Solubility

– Chemical Composition of Phases at anyTemperature

– Amount of Phases at any Temperature

– Invariance Reaction

– Development of Microstructure

– Chemical Activity

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Determination of Phase Diagrams


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• Cooling Curves

• Differential Scanning Calorimetry

• Thermomechanical Analysis

• Metallography

• Energy Dispersive X-ray Spectroscopy

• Electron Microprobe Analyzer

•X-ray Diffraction

• Transmission Electron Microscopy

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Cooling Curves


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Usage of Phase Diagrams in

Determining the Heat Treatability


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Determining the Heat Treatability

• Heat Treatment is based on “controlling” thesolid state transformation rate

– Heat treatment of steels control of the eutectoidreaction

–Age hardening (precipitation strengthening) ofaluminium alloys: control of precipitation reaction.

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Example: Another – A closeup of A

steel


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steel.The darker area is Fe

with small amount of

interstitial carbon.

The lighter standout

areas are the

compound cementite,

Fe3C. (Iron carbide.)

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What have we seen?


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•Multiphase materials, or alloys. Phases areseparate, they are clearly different materials. Butthey are mixed together, at times very finely.

• We do not always have multiphase alloys. There

are many useful single phase alloys. BUT• The presence of the second phase is very

important to…

BLOCK DISLOCATIONS! INCREASE STRENGTH.

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Summary


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• Phase diagrams are useful tools to determine:

-- the number and types of phases present,

-- the composition of each phase,

-- and the weight fraction of each phase

For a given temperature and composition of the system.

• The microstructure of an alloy depends on-- its composition, and

-- rate of cooling equilibrium


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Additional Information

Effect of Alloys

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Carbon Content in Steels• Carbon is the most important alloying element in steel.


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• Most steels contain less than 1% carbon.

• Plain carbon steel - carbon is the only significant alloying element

• Mild steel , or low carbon steel, are produced in the greatest quantity

because it is cheap, soft, ductile, and readily welded. Caution: it cannot be heat-treated

• Mild steels are used for car bodies, appliances, bridges, tanks, andpipe.

Name Carbon Content Examples

Low carbon (mild) 0.05% - 0.32% Sheet, structural

Medium carbon 0.35% - 0.55% Machinery

High carbon 0.60% - 1.50% Machine tools

Cast iron >2.00% Castings

Carbon Content Cold Working in Steels• Medium carbon steel - used for reinforcing bars in concrete,


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farm implements, tool gears and shafts, as well as uses in

the automobile and aircraft industries.

• High carbon steels - used for knives, files, machine tooling,hammers, chisels, axes, etc.

• A small increase in carbon has significant impact onproperties of the steel. As Carbon increases the steel:

–becomes more expensive to produce and less ductile,i.e., more brittle

– becomes harder and less machinable and harder to weld

– has higher tensile strength and a lower melting point


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•Cold working is used to enhance the properties ofsteel

– Reducing thickness by 4% raises the tensilestrength by 50%

– Cold working is plastic deformation at roomtemperature.

– Cold working produces dislocations in the metal’sstructure which block dislocations as they slide

along the slip planes

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Other Elements in Steels• Alloying elements are added to nullify undesirable

elements


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elements

– Carbon

– Manganese

• increases strength, malleability, hardenability, and

hardness• Sulfur reacts with the Mn which reduces the hot short

effect of the iron sulfide accumulating at the grainboundaries and reducing strength at Temp

– Silicon- reduces Oxygen negative effects

– Aluminum-• reacts with Oxygen versus iron (no sparks). Killed

steel


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• promotes smaller grain size which adds toughness

– Boron- increases the hardenability of steel (only withAl added)

– Copper- increases corrosion resistance

– Chromium- increases corrosion resistance andhardenability

– Nickel, Niobium, titanium, tungsten carbide, vanadium

• increase toughness and strength and impactresistance 150

Alloying steel with other elements changes the Eutectoid

Temperature, Position of phase boundaries and relative

A f h h


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Amounts of each phase

Stainless Steel• Corrosion of steels can be slowed with addition of Cr and

Ni


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Ni.

• Stainless steels have chromium (up to 12%) and Ni(optional)

– ferritic stainless: 12% to 25% Cr and 0.1% to 0.35%

Carbon• ferritic up to melting temp and thus can not form the

hard martensitic steel.

• can be strengthened by work hardening

• very formable makes it good for jewelry, decorations,utensils, trim

– austenitic stainless: 16% to 26% Cr, 6% to 23% Ni,


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0.35% Carbon• machinable and weldable, but not heat-treatable

• used for chemical processing equipment, food utensils,architectural items

– martensitic stainless: 6% to 18% Cr, up to 2% Ni, and 0.1%to 1.5% C•

hardened by rapid cooling (quenching) from austeniticrange.• Corrosion resistance, low machinability/weldability

used for knives, cutlery.

– Marging (high strength) steels: 18% to 25% Ni, 7% Co, with

others• heated and air cooled cycle with cold rolled

L2 - Phase Diagram

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