Download - Sept 4th Alloying and Heat Treatment 2
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Dr.Mullany
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R2180-001
Topics for today
Alloying
Phase Diagrams
Heat treatment
Dr.Mullany
MEGR2180-001
Al loying
The mixing of metals and semi-metals in the molten state iscalled alloying
An alloy is composed of two or more elements, the principlecomponent is a metallic element
Alloying is performed to change the physical properties of ametal
Commonly alloying is done to change
Strength
Modulus of Elasticity
Ductility Toughness
Corrosion Resistance
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Dr.Mullany
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Al loying
Alloys are typically prepared by melting a known mass ofmetal (solvent) in a crucible and then adding in weighedamounts of the other material (solute).
The liquid alloy is then cast and allowed to solidify. Theresulting structure depends on how the different types ofatoms behave around each other.
If the atoms are indifferent to each other they willcrystallize as a single set of crystals all the atoms willbehave as if they are similar. A single phase Solidsolution is said to form.
If the different elements crystallize separately to formdifferent crystals that meet at grain boundaries then theresulting structure is referred to as a Phase Mixture
Dr.Mullany
MEGR2180-001
Al loying - Sol id Solut ion
In a solid solution the crystal structure is the sameas that of the solvent (parent metal). The Soluteatoms are distributed through in crystal. Thesolution may be formed in two different ways
Substitution Intermetallic
Interstitial
Substitutional Interstitial
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Dr.Mullany
MEG
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Al loying - Sol id Solut ion -substi tution
Conditions for Substitutional alloying:
-Atoms of the two metals do not differ in diameter more than15%
- The two metals must have a similar crystal structure.
An example is Brass
Solute is Zinc
Solvent is Copper(Elements are beside each other on periodic table)
Another example is that of Monel;
A mixture of Copper and Nickel
(Elements are also beside each other on periodic table)
Substitutional
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Al loying - Sol id Solut ion -intermetal lic
Intermetallic Compound: these are substituitionalsolid solutions where the solute atoms are presentin specific proportions and geometric relationships
They have sharp melting points, often higher thaneither of the two alloying elements, very goodstrength, low ductility
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Dr.Mullany
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Al loying Sol id Solut ion -interst itial Interstitial: Solute atoms positioned between the
atoms in the solvent
Conditions Atomic radius of the solute must be less than about 60% of
the solvent radius
An example is Steel Solute is Iron
Solvent is Carbon
Amount of carbon significantly affects material properties
Interstitial
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MEGR2180-001
Solidifcation curves
The graph below shows the difference between the solidification curvesfor pure metals (one element) and alloys (several elements)
Pure metal
Al loy: the temperatureat which it solidifi es is
not sharply defined
Temperature
range for
solidification
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Dr.Mullany
MEG
R2180-001
Phase Diagrams
Definition:A Phase has a definable structure, a uniform
and identifiable chemistry (aka composition) anddistinct boundaries or interfaces that separate it fromother different phases.
Definition:A Phase diagram (also called an equilibriumdiagram) illustrates the relationship betweentemperature, composition and the phases present in aparticular alloy.
Note: Phase diagrams are only valid under equilibrium conditions ...i.e.slow heating and cooling
Dr.Mullany
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Copper-nickel phase diagram
Melting point
of pure copper
Definition of wt%: wa = wt of component a x 100
S wt of all components
Melting point
of pure nickel
After spaceflight.esa.int
80% Ni, 20% Cu
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Nickel Copper Phase diagram
Point a: 40% Cu - 60%Ni , temp >1350C, homogenous liquid form
Point b: 40% Cu - 60%Ni , temp
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Dr.Mullany
MEG
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Consider slow cooling of a 50%Cu liquid mixture: first solids 36% Cu(at liquidus, go left to the solidus line, then down to read composition of solids)
EQUILIBRIUM PHASE DIAGRAM!After Kalpakjian and Schmid, 5th ed
50-50% at solidification (diffusion)
Intermediate temp, go left for solid composition, right for liquid
What happens during solidification?
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Lever rule
Create a lever balanced at the
nominal composition, C0.CS represents solid composition.
CL represents the liquid
composition
Lever rule:
Wt fraction solid a distancebetween C0 and CL:
S = C0-CLS+L CS-CL
After Kalpakjian and Schmid, 5th ed
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Lever rule on Lead (Pb) Tin (Sn) phase diagram
At 250C what is the solid and liquid fraction of the alloy at 80% Lead (Pb)?
dd b
C
Solid fraction, Fs = C-db-dX 100%
= 80-64
87-64X 100%
= 69.5%
Liquid fraction, FL = b-Cb-dX 100%
= 87-8087-64
X 100%
= 30.5%
Dr.Mullany
MEGR2180-001
Terminology
Definition: Eutectic: An isothermal reversiblereaction in which a liquid solution isconverted into two or more intimately mixedsolids on cooling (number of solids dependon the number of elements in the system)
Definition: Eutectoid: An isothermalreversible reaction in which a solid phase isconverted into two or more intimately mixed
solids on cooling (number of solids dependon the number of elements in the system)
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Dr.Mullany
MEG
R2180-001
Terminology- Eutectic
There is no mixed liquid-solid at an eutectic point.On freezing at this specific composition an eutectic
mixture with the individual crystal in the form of
plates or rods or tiny particles are formed.
Note: Eutectic points have the lowest melting pointicrostructureof
an
eutecticmixture
Eutectic mix
Different structugrain
Dr.Mullany
MEGR2180-001
Iron Carbon (steel) phase diagram
(Ferrite)
L + Fe3C
2.14 4.20
6.70
0.022
0.8
C
B D
H
Cementite
(Fe3C)
0.25
z
1.2
a + cementite (Fe3C)
g + cementite (Fe3C)
723C
Liquid
g + Liquid
(Austenite)
Eutectoid
Solid
Eutectic
Liquid
After Kalpakjian and Schmid, 5th ed
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Iron Carbon Systems
Why we need to know about Iron-carbon
Systems:
Steel is an Alloy of Iron and Carbon
Different phases of the Iron Carbon diagramhave different structures, it is important to befamiliar with them and to understand whatinfluence they have with respect to materialproperties
% Carbon content of dif ferent materials: Pure Iron (Fe) = 0.008%
Steel up to 2.11%
Cast Irons up to 6.67%
Dr.Mullany
MEGR2180-001
Iron Carbon phase diagram
100% Fe 93.3% Fe
Steel Cast Iron
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Iron Carbon - Main Structures
AUSTENITE: Single phase FCCStructure
Ductile at elevated temperatures Good formability important for
manufacturing
Nonmagnetic
CEMENTITE (Fe3C): Also called Carbide
A hard and brittle intermetalliccompound that has a significantinfluence on the properties of steel
FERRITE: BCC Structure Only stable at high temperatures and
has little engineering relevance Soft, Ductile, and Magnetic
PEARLITE: lamellar aggregate ofFerrite and Cemetite
Cementite(White areas)
Ferrite(dark areas)
Austenite
Pearlite
Dr.Mullany
MEGR2180-001
Cast Iron versus Steel
Cast iron has more
silicon than steel
which makes the Fe3C
decompose to Ferrite
and Graphite
4
3
2
1
0 0 1 2 3 4
% Silicon
Steels
White
Cast Iron
Grey Cast Iron
Nodular
Cast Iron
%C
arbon
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Dr.Mullany
MEG
R2180-001
Cast Iron Main structures found
The type of cast iron found isdependant on the following: Carbon content
Alloy and impurity content
Cooling rate during and after f reezing
The Heat treatment after casting
White Cast Iron All carbon is in the combined form as
cementite
Gray Cast Iron Carbon is uncombined in the form of
graphite flakes
Nodular Cast Iron Carbon is largely uncombined in the
form of compact spheroids
Malleable Cast Iron Carbon is uncombined in the form of
irregular round particles known astemper carbon
Malleable cast iron (white cast
iron annealed to precipitate
out carbon
Gray Cast iron
White Cast iron as cast
Nodular cast iron
See also Chapter 5, Kalpakjian and Schmid, 5th ed
Dr.Mullany
MEGR2180
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Effect of Carbon on Steel Properties
Effect of carbon content on the
mechanical properties of carbon steel
Figure 3.33 from Kalpakjian and Schmid, 5th ed
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Dr.Mullany
MEG
R2180-001
Do you remember ?
1. A stress of 10MPa is applied to
a tensile sample, if the materials
stiffness is 10GPa, how muchstrain will the sample undergo?
a) 0.001b) 0.001%
c) 1000
d) 0.01%
2. Which is false of work hardened
materials ?a) Hardness > non work hardened
b) Strength > non work hardened
c) Have equiaxed grain sizesa) They are plastically deformed
3. Which is not true about stressa) Units = Pab) Units = psi
c) = force/area
d) = area*force
4. Which of the following statementsis true
a) No part of engineering stressstrain curves should be
compared to a true stress-
strain curveb) Stiffness is the ratio of stress to
strain in the linear part of thecurve
c) Once you have gone past the
yield point you can have noelastic recovery
d) It doesnt matter under whattemperature conditions a
tensile test is preformed
Section B questions:Explain why a metal with a yield
strength of 10GPa may not be astough as a metal with a yield strength
of 5GPa
Dr.Mullany
MEGR2180-001
Heat treatment
Heat treatments modify the microstructureof alloys to impart different mechanicalproperties
Effects of thermal treatment depend on The alloys composition and microstructure, The degree of cold work,
The rates of heating and cooling, The temperatures and temperature ranges,
etc.
It is a very complex subject and we will notcover it in detail, just an overview
Definition:A combination of heating and cooling
operations, timed and applied to a metal or alloy in the
solid sate in a way that will produce desired properties
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Dr.Mullany
MEG
R2180-001
Heat treatment - terminology
Annealing: heating to and holding at a suitable temperature above the
recrystalization temp and then cooling in the furnace at a suitable rate
(usually slow), for such purposes as reducing hardness, improving
machinability, facili tating cold working, producing a desired
microstructure or obtaining desired mechanical, physical or other
properties. Any process of annealing will usually reduce stresses.
Cold treatment: cooling to a temperature, often near -100F, for the
purpose of obtaining desired condit ions or properties such as
structural stability.
Hardening: Increasing the hardness by suitable treatment, usually
involving heating and cooling.
Under suitable cooling rates the carbon is able to diffuse out of the
austenite structure. When steel is cooled quickly the carbon becomestrapped in solution and is known as Martensite or Martenistic structure
Dr.Mullany
MEGR2180-001
Heat treatment - terminology
Normalizing: Heating a ferrous alloy to a suitable temperature abovethe transformation range (as in annealing) and then cooling in air to a
temperature substantially below the transformation range. It will
produce harder and stronger steel than annealing partially due to
faster cooling rates than used in Annealing
Quenching: Rapid Cooling of a material. This increases the hardness
of the metal. Quenching mediums are (listed in order decreasing
severity):
Brine (water and 10% Sodium Chloride)
Tap water
Soluble oil
oilAir
Tempering: Reheating a quenched hardened or normalized ferrous
alloy to a temperature below the Transformation temperature and then
cooling at the desired rate. It relieves internal stresses.
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Dr.Mullany
MEG
R2180-001
Tempering Martensite
Tempering is accomplished by heating a martensitic steel
to a temperature below the eutectoid (normally, between200-650C) for a specified time period.
By diffusion processes:
Martensite (BCT, single phase) Tempered Martensite(a + Fe3C)
Tempered martensite may be nearly as hard and strongas martensite, but with substantially enhanced ductility &toughness.
Dr.Mullany
MEGR2180-001
Effect of tempering Temperature
Tensile and yield
strengths andducti lity (%RA) versus
tempering
temperature for an oi l-
quenched alloy s teel
(type 4340).
Heat treatment variablesare temperature andtime, and mosttreatments are constant-temperature p rocesses.
(Carbon diffusion isinvolved in thetransformation.)
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Dr.Mullany
MEG
R2180-001
Jominy End Quench test
Explain why stress strain curves are so important, detailthe information that can be extracted from the curve and
from looking at the tensile sample after fracture
Hardenability curve is the dependence of hardness on distance from the
quenched end. The higher the hardness levels further away from the
quenched end the more hardenable the alloy.
Test standards:
American Society for testing and Materials (ASTM) Method A 255
Society of Automotive Engineers (SAE) standard J406
Dr.Mullany
MEGR2180
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Jominy End Quench test
Quenched end cools most rapidly, contains mostly martensite
Cooling rate decreases with distance from quenched end: greater C
diffusion, more pearlite/bainite, lower hardness
High hardenability means that the hardness curve is relatively flat.
Less Martensite
Quenched
end
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Dr.Mullany
MEG
R2180-001
Dangers of heat treatment
Heat treatments can cause problems
such as cracking, distortions etc.
Parts incorrectly case hardened (forexample through hardened instead) canfail due to lack of toughness
Distortions must be corrected onprecision parts by finish grinding (usually) Martensitic and quench cracks
Grinding cracks
Dr.Mullany
MEGR2180
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Case hardening
Examples of such are gears, bearings,cams,
tool, dies, etc.
This technique is called case hardening
Case hardening is performed by adding other
elements to the surface or by special heat
treatments.
Many industrial applications require a hard wear
resistant surface called the case and a relatively soft
tough inside called the core
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MEG
R2180-001
Case Hardening
Other method involve heatTreating: Flame Hardening heating
surface with a flame andquenching
Induction hardening heatingsurface with high frequencyinduced current and quenching
This photo shows ways on 16" vise
base being hardened utilizing flame
hardening.
Dr.Mullany
MEGR2180-001
Case hardening
One method involves addingsurface elements:
Carburizing adding carbon
Carbonitriding adding carbonand nitrogen
Nitriding adding nitrogen
Many recipes exist
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MEG
R2180-001
Homework due Wednesday the 11th Sept 13
1. A 200mm long, 10mm dia. tensile sample experienced plastic
deformation at a strain of 0.01. The applied load was 78.5 kN. What is the elastic modulus of the material?
By how many mm did sample elastically deform?
2. Looking at the Ni-Cu phase diagram answer the following
questions.
At what temperature is a Cu (20%) - NI(80%) fully solid?
What phases are present at 1300C and 50%Cu50%Ni?
3. Draw and label an engineering stress-strain diagram for a brittle
metal.