IE 337: Materials & Manufacturing
Processes
Lecture 3:
Metal Alloys and
Heat Treatment
Chapters 3, 6 and 27
Last Time
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The nature of metals
 Different crystalline structures
 Different crystalline defects that affect properties
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The properties of metals
 Mechanical properties
 What they are and what they mean
2
Stress-Strain Relationships
Figure 3.3 Typical engineering stress-strain plot in a tensile
test of a metal.
3
True Stress-Strain Curve
Figure 3.4 - True stress-strain curve for the previous
engineering stress-strain plot in Figure 3.3.
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This Time
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Hardness (Chapter 3)
How can we modify mechanical properties in
metals? (Chapter 6 and 27)
Different types of metal alloys and how are
they used (Chapter 6)
Assignment #1
Hardness
 Resistance to permanent indentation
 Good hardness generally means material is
resistant to scratching and wear
 Most tooling used in manufacturing must be
hard for scratch and wear resistance
6
Brinell Hardness Test
 Widely used for
testing metals and
nonmetals of low to
medium hardness
 A hard ball is
pressed into
specimen surface
with a load of 500,
1500, or 3000 kg
Figure 3.14 Hardness testing methods: (a) Brinell
7
Brinell Hardness Number
Load divided into indentation area = Brinell
Hardness Number (BHN)
HB 
2F
Db (Db  Db2  Di2 )
where HB = Brinell Hardness Number (BHN),
F = indentation load, kg; Db = diameter of ball,
mm, and Di = diameter of indentation, mm
8
Rockwell Hardness Test
Figure 3.14 Hardness testing methods: (b) Rockwell:
(1) initial minor load and (2) major load.
9
Why Metals Are Important
 High stiffness and strength - can be alloyed for
high rigidity, strength, and hardness
 Toughness - capacity to absorb energy better
than other classes of materials
 Good electrical conductivity - Metals are
conductors
 Good thermal conductivity - conduct heat better
than ceramics or polymers
 Cost – the price of steel is very competitive
with other engineering materials
10
Metals: Periodic Table
26
Fe
55.847
11
Starting Forms of Metals used in
Manufacturing Processes
 Cast metal - starting form is a casting
 Wrought metal - the metal has been worked or
can be worked after casting
 Powdered metal - starting form is very small
powders for conversion into parts using powder
metallurgy techniques
12
Classification of Metals
 Ferrous - those based on iron
 Steels
 Cast irons
 Nonferrous - all other metals
 Aluminum, magnesium, copper, nickel,
titanium, zinc, lead, tin, molybdenum,
tungsten, gold, silver, platinum, and others
 Superalloys
13
Metals and Alloys
 Some metals are important as pure elements
(e.g., gold, silver, copper)
 Most engineering applications require the
enhanced properties obtained by alloying
 Through alloying, it is possible to increase
strength, hardness, and other properties
compared to pure metals
14
Alloys
An alloy = a mixture or compound of two or more
elements, at least one of which is metallic
Two main categories:
1. Solid solutions


Substitutional
Interstitial
2. Intermediate phases
15
Two Forms of Solid Solutions
Figure 6.1 Two forms of solid solutions: (a) substitutional
solid solution, and (b) interstitial solid solution.
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Equilibrium Binary Phase Diagram
Figure 6.2 Phase diagram for the copper-nickel alloy
system.
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Intermediate Phases
 There are usually limits to the solubility of one
element in another
 When the amount of the dissolving element in
the alloy exceeds the solid solubility limit of the
base metal, a second phase forms in the alloy
 The term intermediate phase is used to
describe it because its chemical composition is
intermediate between two phases
 Its crystalline structure is also different from
those of the pure metals
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Tin-Lead Phase Diagram
Figure 6.3 Phase diagram for the tin-lead alloy system.
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Melting in the Tin-Lead Alloy System
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Pure tin melts at 232C (449F)
Pure lead melts at 327C (621F)
Tin-lead alloys melt at lower temperatures
The diagram shows two liquidus lines that
begin at the melting points of the pure metals
and meet at a composition of 61.9% Sn
 This is the eutectic composition for the
tin-lead system
Eutectic Alloy
A particular composition in an alloy system for
which the solidus and liquidus are at the same
temperature
 The eutectic temperature = melting point of the
eutectic composition
 The eutectic temperature is always the
lowest melting point for an alloy system
 The word eutectic is derived from the Greek
word eutektos, meaning easily melted
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Metals: Classification
FERROUS
22
IMPORTANCE OF IRON
23

Steel: engineered alloys
based on iron (often
containing carbon): 10,000
compositions in common use

One of mankind’s most
popular engineering
materials: 750 million tons per
year

Fe melting temp. = 1537°C

Fe density = 7.87 g/cm3
Iron-Carbon Phase Diagram
Figure 6.4 Phase diagram
for iron-carbon system, up
to about 6% carbon.
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The Several Phases of Iron
 The phase at room temperature is alpha (),
called ferrite (BCC)
 At 912C (1674F), ferrite transforms to
gamma (), called austenite (FCC)
 This transforms at 1394C (2541F) to delta ()
(BCC)
 Pure iron melts at 1539C (2802F)
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Solubility Limits of Carbon in Iron
 Ferrite phase can dissolve only about 0.022%
carbon at 723C (1333F)
 Austenite can dissolve up to about 2.1%
carbon at 1130C (2066F)
 The difference in solubility between alpha
and gamma provides opportunities for
strengthening by heat treatment
26
Steel and Cast Iron Defined
Steel = an iron-carbon alloy containing from
0.02% to 2.1% carbon
Cast iron = an iron-carbon alloy containing
from 2.1% to about 4% or 5% carbon
 Steels and cast irons can also contain
other alloying elements besides carbon
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Fe-C: Properties
28
Annealing
Heating and soaking metal at suitable temperature
for a certain time, and slowly cooling
 Reasons for annealing:
 Reduce hardness and brittleness
 Alter microstructure to obtain desirable
mechanical properties
 Soften metals to improve machinability or
formability
 Recrystallize cold worked metals
 Relieve residual stresses induced by
shaping
29
Annealing of Steel
 Full annealing - heating and soaking the alloy
in the austenite region, followed by slow
cooling to produce coarse pearlite
 Usually associated with low and medium
carbon steels
 Normalizing - similar heating and soaking cycle
as in full annealing, but faster cooling rates,
 Results in fine pearlite, higher strength and
hardness, but lower ductility
30
Time-Temperature-Transformation Curve
Allotropic
transformation
- austenite to
martensite
Figure 27.1 The TTT curve, showing transformation of austenite into
other phases as function of time and temperature for a composition
of about 0.80% C steel. Cooling trajectory shown yields martensite.
31
Tempering of Martensite
A heat treatment applied to martensite to reduce
brittleness, increase toughness, and relieve
stresses
 Treatment involves heating and soaking at a
temperature below the eutectoid for about one
hour, followed by slow cooling
 Results in precipitation of very fine carbide
particles from the martensite iron-carbon
solution, gradually transforming the crystal
structure from BCT to BCC
 New structure is called tempered martensite
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Steels
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Alloy Steels
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Further refined
from carbon steels,
with elements
added to modify or
change the
mechanical
properties.
Alloy Steels
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Further refined from carbon steels, with
elements added to modify or change the
mechanical properties.
Tool Steels are special grades of alloy steels
used for a variety of tooling, with very close
control of the alloying element additions
 Highly wear-resistant
 Highly shock-resistant
 Heat-resistant
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Alloy Steels
36

Cr addition
improves
corrosion
resistance

So does Ni
Alloy Steels: Alloying Elements
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Boron
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 Increases wear-resistance
 Increases hot-hardness ability to keep shape at
elevated temperature
 Used in high speed steel
 Large increase in
hardenability with very
small addition of element

Chromium
 Increases depth hardness
 Increases corrosion
resistance
 Principle component in
stainless steel
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Cobalt
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Lead
 Reduces cutting friction,
improving machinability
 Good weldability
 Good formability
 Environmental concern
Alloy Steels: Alloying Elements
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Manganese
 Large amounts (1% 15%) gives good
hardness and wearresistance
 Small amounts useful for
purifying melt by
combining with impurities
and forming dross

Vanadium
 Also used to purify melt
 Produces fine-grained
steels
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
Tungsten
 Provides high wearresistance
 Adds hardenability and
strength at elevated
temperatures
 Used in tool steels
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Phosphorous / Sulfur
 Give excellent machining
characteristics
 Used in free-machining
steels
Alloy Steels: Alloying Elements
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Molybdenum




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Aids toughness
Used in tool steels
Improves depth-hardness
Improves strength at
elevated temperatures
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Nickel
 Provides corrosionresistance
 Improves resistance to
elevated temperatures
 Used in stainless steels
 Combined with
Molybdenum to provide
very tough steel for
aircraft applications
General Cast Iron Properties

Advantages:
 Very good compressive strength
 Good machinability
 Reasonable corrosion resistance

Disadvantages:
 Natural brittleness
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Cast Iron: 2-4.5 wt. % C
Gray Iron
1-3 % Si
cheap
used in compression
vibrational damping
(machinery housing)
White Iron
< 1% Si
brittle, wear resistant
malleable iron
precursor
(rollers)
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Ductile Iron
Mg, Ce, Ca, Li, Na, Ba
to gray iron
stronger and ductile
(valves, gears,
crankshafts)
Malleable Iron
< 1% Si
heat treat white iron
strong, malleable
(connecting rods,
transmission gears
flanges, fittings)
Non-Ferrous Metals
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Metals whose major element is not Iron (wow!)
Compared to Iron & Steel:
 Density (strength to weight ratio), non-corrosive
 Conductivity, fabricatability (machined, formed, cast)
 Cost (by weight)
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Major Materials:
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Aluminum Alloys
Copper & Copper Alloys
Magnesium
Nickel & Nickel Alloys
Refractories
Superalloys
Non-Ferrous Alloys
• Cu Alloys
• Al Alloys
Brass: Zn is subst. impurity -lower r: 2.7g/cm3
(costume jewelry, coins,
-Cu, Mg, Si, Mn, Zn additions
corrosion resistant)
-solid sol. or precip.
Bronze: Sn, Al, Si, Ni are
strengthened (struct.
subst. impurity
aircraft parts
(bushings, landing
& packaging)
gear)
• Mg Alloys
NonFerrous
Cu-Be:
-very low r: 1.7g/cm3
Alloys
precip. hardened
-ignites easily
for strength
-aircraft, missles
• Ti Alloys
• Refractory metals
-lower r: 4.5g/cm3
-high melting T
vs 7.9 for steel
• Noble metals -Nb, Mo, W, Ta
-reactive at high T -Ag, Au, Pt
-oxid./corr. resistant
-space applic.
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Aluminum Alloys
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Pure metal properties:
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Alloying elements:
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

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Low density, melting point
Ductile
Malleable
Good electrical / thermal
conductor
Copper
Magnesium
Silicon
Manganese
Zinc
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Typical uses of Al:
 High strength aircraft
structures
 Low pressure
hydraulic/pneumatic
fittings
 Jet engine parts
 Truck frames
Precipitation Hardening
Heat treatment that precipitates fine particles
that block the movement of dislocations and
thus strengthen and harden the metal
 Principal heat treatment for strengthening
alloys of aluminum, copper, magnesium,
nickel, and other nonferrous metals
 Also utilized to strengthen a number of steel
alloys that cannot form martensite by the
usual heat treatment
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Conditions for Precipitation Hardening
 The necessary condition for whether an alloy
system can be strengthened by precipitation
hardening is the presence of sloping solvus
line in the phase diagram
 A composition in this system that can be
precipitation hardened is one that contains two
equilibrium phases at room temperature, but
which can be heated to a temperature that
dissolves the second phase
46
Precipitation Hardening
Figure 27.5 Precipitation hardening: (a) phase diagram of an
alloy system consisting of metals A and B that can be
precipitation hardened; and (b) heat treatment: (1) solution
treatment, (2) quenching, and (3) precipitation treatment.
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Sequence in Precipitation Hardening
1. Solution treatment - alloy is heated to a
temperature Ts above the solvus line into the
alpha phase region and held for a period
sufficient to dissolve the beta phase
2. Quenching - to room temperature to create a
supersaturated solid solution
3. Precipitation treatment - alloy is heated to a
temperature Tp, below Ts, to cause
precipitation of fine particles of the beta phase
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Copper Alloys
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Pure metal properties:

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
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Very soft
Ductile
Malleable
Good electrical / thermal
conductor
Typical uses of Cu:
 Electronics production
 Electrical conductors
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Typical uses as Bronze:
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
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Alloying elements:
 Alloyed with Sn to make
Bronze
 Alloyed with Zn to make
Brass
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Machine parts
Bearings
Corrosion-resistant fittings
Electrical connectors
Typical uses as Brass:
 Hardware
 Marine corrosionresistance
 Ornamental applications
Magnesium Alloys

Pure metal properties:

 Aircraft components
(strength to weight ratio)
 Automobile wheels
 Racing frames
 Lightweight structural
parts
 Lightweight
 Strong (per unit volume)
 Flammable in fine sizes

Alloying elements:






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Aluminum
Bismuth
Copper
Tin
Lead
Iron
Typical uses of Mg:

Handling Magnesium
 Keep chips coarse
 Avoid chip accumulation,
mixing with other material
 Avoid water, water-based
coolants (explosive)
Nickel Alloys

Properties:
 Corrosion resistance
 Heat resistance

Alloy forms:




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Monel
K-Monel
R-Monel
Inconel

Typical uses of Ni:
 Plating of electronics
(pure form)
 Thermocouples
 Alloying element

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
Naval Brass
Steel toughness
Steel corrosion
resistance
Steel heat resistance
Effect of Temperature on Properties
Figure 3.15 General effect of temperature on strength and ductility.
52
Hot Hardness
Ability of a material to
retain hardness at
elevated temperatures
Figure 3.16 Hot
hardness - typical hardness
as a function of temperature
for several materials.
53
Superalloys


High-performance alloys for strength and
resistance to surface degradation at high
service temperatures
Many superalloys contain substantial
amounts of three or more metals,
Commercially important because they are
very expensive
See Tables 6.15 & 6.16
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Why Superalloys are Important


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55
High temperature performance is excellent tensile strength, hot hardness, creep
resistance, and corrosion resistance at very
elevated temperatures
Operating temperatures often in the vicinity of
1100C (2000F)
Applications: gas turbines - jet and rocket
engines, steam turbines, and nuclear power
plants - systems in which operating efficiency
increases with higher temperatures
Refractory Metals


Metals capable of enduring high
temperatures - maintaining high strength and
hardness at elevated temperatures
Most important refractory metals:
 Molybdenum
 Tungsten

56
Other refractory metals are niobium and
tantalum (used in capacitors)
Tungsten
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

57
Properties: highest melting point among
metals, one of the densest, also the stiffest
(highest modulus of elasticity) and hardest of
all pure metals
Applications typically characterized by high
operating temperatures: filament wire in
incandescent light bulbs, parts for rocket and
jet engines, and electrodes for arc welding
Also widely used as an element in tool steels,
heat resistant alloys, and tungsten carbide
Molybdenum
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Properties: high melting point, stiff, strong,
good high temperature strength
Used as a pure metal (99.9+% Mo) and
alloyed
Applications: heat shields, heating elements,
electrodes for resistance welding, dies for
high temperature work (e.g., die casting
molds), and parts for rocket and jet engines
Also widely used as an alloying ingredient in
steels and superalloys
Precious Metals

Gold, platinum, and silver
 Also called noble metals because chemically inert
 Available in limited supply


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Widely used in jewelry and similar
applications that exploit their high value
Properties: high density, good ductility, high
electrical conductivity and corrosion
resistance, and moderate melting
temperatures
Shaping, Assembly, and Finishing
Processes for Metals
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Metals are shaped by all of the basic
processes: casting, powder metallurgy,
deformation, and material removal
In addition, metal parts are joined to form
assemblies by welding, brazing and
soldering, and mechanical fastening
Heat treating to enhance properties
Finishing processes (e.g., electroplating and
painting) to improve appearance and/or to
provide corrosion protection
You should have learned:

How we can modify mechanical properties in
metals?




61
Alloying
Annealing
Allotropic transformation
Precipitation hardening

Different types of metal alloys and how they
are used?

Assignment #1
Next Time

How do we shape materials?
 Secondary operations  Material Removal

The fundamentals of metal cutting (Chapter 21)
 Orthogonal machining
 The Merchant Equation
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