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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?
65
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:
9
• 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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83
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85
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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
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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
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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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107
Answer
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108
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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114
Iron- Iron
Carbide system
Iron-Graphite
system
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115
Problem 3
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Answer
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Answer
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118
Answer
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119
Answer
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120
Answer
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121
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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128
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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Microstructure of cast irons
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Microstructure of cast irons
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Microstructure of cast irons
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Microstructure of cast irons
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134
Microstructure of cast irons
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135
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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147
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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149
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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151
Amounts of each phase
Stainless Steel• Corrosion of steels can be slowed with addition of Cr and
Ni
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152
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