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Principles of Metallurgy
Industrial Metallurgists, LLC
Northbrook, IL 60062
847.528.3467
www.imetllc.com
Copyright 2012 Industrial Metallurgists, LLC
Course content
Consists of text, graphics, animation, videos, and voiceover
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Introduction
Overall goal: understand how manufacturing processes enable us to modify
mechanical properties of metals.
Cover fundamental metallurgical concepts
Explain methods for strengthening metals
Course Learning Objectives
1. Explain the relationship between a metals properties and its composition,microscopic structure, and the manufacturing processes used to fabricate the
metal.
2. Describe three types of microscopic structures present in metals.
3. Explain how cold working, alloying, and heat treating are used to strengthen ametal.
4. Explain the microstructure and property changes that occur in cold worked
metals, steels, and precipitation hardened alloys when they are heat treated.
5. Relate the heat treatment time and temperature to the microscopic structures
and properties of precipitation hardened alloys, steels, and cold worked metals.
Continue
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Course Content
1. Material properties
2. Composition
3. Microscopic structures
a. Crystal structureb. Grains and grain boundaries
c. Metallurgical phasesd. Crystal structure defects
4. Diffusion
5. Examplesa. Cold working
b. Annealing cold-worked metalsc. Solution hardening
d. Steel heat treating
e. Precipitation hardening
Concepts applicable to components
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Concepts applicable to Non-Mechanical Joints
Solder and braze joint
Weld joints
Materials Properties
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Module learning objective
Explain the relationship between properties, composition, microscopic structures, and
processing.
Properties
Composition
Microscopic
structure
Manufacturing
defects
Continue
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Materials Properties
PhysicalDensity
MechanicalHardnessYield strength
Tensile strength
Modulus of elasticityFatigue strength
Fracture toughnessCreep strength
ThermalMelting point
ConductivitySpecific heat
Coefficient of thermal expansion
Thermal coefficient of resistance
ElectricalConductivity
Coercive force
Magnetic hysteresisMagnetic permeability
ElectrochemicalElectrochemical potentialCorrosion resistance
ManufacturingFormability
WeldabilityMachinability
Composition refers to elements that make up a metal
Steel - iron, carbon, manganese, and silicon
Brass - copper and zinc
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Microscopic structures
10 mm
0.0004
Phases
(Courtesy of Aston Metallurgical Services)Courtesy of Aston Metallurgical Services
Grains
Arrangement of atoms
Manufacturing defects
Alter metal properties
Try to minimize defects
What manufacturing defects can be tolerated?
How can the level of defects be controlled?
No more discussion of manufacturing defects in this course
Forging lap
0.125 mm
Gas porosity in die casting
0.5 mm
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Properties
Composition
Microscopic
structure
Can process an alloy different ways
Obtain different microscopic structures
Obtain different properties
Properties
Composition
Microscopic
structure
Manufacturing
Processes
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Courtesy of Aston Metallurgical Services Courtesy of Aston Metallurgical Services
Can process a metal different ways
Obtain different microscopic structures
Obtain different properties
Critical concept
Alloy and process selectionControl variation
Harder and stronger
For any particular design, want to
Select materials that have the desired properties
Use manufacturing processes capable of transforming a material into desired
shape with desired properties.
Properties
Composition
Microscopic
structure
Manufacturing
Processes
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Questions for further thought
1. What metals are used in your products?
2. What are the desired properties of the metals?
3. What manufacturing processes are used to obtain the desired properties in themetals?
Continue
Composition
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Learning objectives
At the end of this module learners will be able to do the following:
1. Explain how the composition of a metal is expressed
2. Explain the difference between an alloying element and impurity3. Describe the effects of composition changes on the properties of two alloys
4. Explain why there are a large number of different alloys
Continue
Metal Composition
Elemental make up
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Metals can be single element or alloys
Examples of single element
Copper
Aluminum
Gold
Alloys
Intentional mixtures of various elements
Carbon Steel (iron + carbon + manganese)
Brass (copper + zinc)
Die cast aluminum (aluminum + silicon + copper + iron)
Impurities
Unintentionaladdition of elements
Steel: sulfur and phosphorous are often impurities
Eliminating all impurities impossible
Some level of impurities tolerable
What amount of impurities is tolerable?
Keep impurities below maximum allowable amounts
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Alloy composition usually in weight percent
Al 10.5 wt.% Si 2.5 wt.% Cu
100 gram sample
10.5 grams Si2.5 grams Cu
87 grams Al
Steel Alloys
Plain carbon steels
Alloy steels
Stainless steels
Continue
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Plain carbon steel
These alloys contain iron, carbon, and manganese. The amount of carbon
and manganese added depends on the desired mechanical properties.
Sulfur and phosphorous might be present as impurities that should not
exceed a specified amount. In free-machining alloys phosphorous and
sulfur are added intentionally to improve the ease of machining
components.
Continue
Alloy steels
This class of steels contain iron, carbon, and manganese. Additionally theyalso contain nickel, chromium, and/or molybdenum. These elements are
added to improve mechanical and corrosion properties, and improve the
ease of obtaining certain mechanical properties during heat treatment.
Continue
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Stainless steels
Stainless steels contain at least 12 percent chromium in addition to iron,
manganese, and carbon. The addition of the chromium gives greatly
enhanced corrosion properties compared to the other steels. Other elements
like nickel and molybdenum are added in a wide variety of amounts to modifystrength, corrosion resistance, formability, and weldability.
Continue
Carbon (Wt. %) Maximum hardness
(Rockwell C)
0.1 37
0.2 46
0.3 53
0.4 58
0.5 62
Effect of carbon content on steel maximum hardness
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Aluminum alloysAluminum alloys are categorized according to the major alloying
element present. Copper, manganese, silicon, magnesium, or zinc are
added as major alloying elements. In addition, other elements are
added in smaller quantities. The designations for wrought alloys are
shown in the table. The first digit indicates the group, with the alloysbeing grouped by the major alloying element.
In the 1xxxgroup, the series 10xxdesignates unalloyed compositions
that have natural impurity limits. The last two of the four digits indicate
the minimum aluminum percentage. Designations having second digits
other than zero indicate special control of one or more individual
impurities.
In the 2xxxthrough 8xxxalloy groups, the second digit in the
designation indicates alloy modification. If the second digit is zero, it
indicates the original alloy. Integers 1 through 9 indicate modifications
of the original alloy. The last two digi ts have no special significance, butserve only to identify the different aluminum alloys in the group.
In all cases the specific alloying elements present and their quantities
are meant to provide specific performance and reliability properties. In
addition, alloying elements are added to modify processing properties.
Continue
Aluminum, 99.00% 1xxx
Copper 2xxx
Manganese 3xxx
Silicon 4xxx
Magnesium 5xxx
Magnes ium and silicon 6xxx
Zinc 7xxx
Other elements 8xxx
Copper Alloys
This list shows some of the different categories of copper alloys.
Coppers: >99% copper
High copper alloys: >96% copper + alloying elements
Brasses: Copper-zinc alloys
Leaded brasses: Copper-zinc-lead alloys
Phosphor bronzes: Copper-tin-phosphorous alloys
Nickel silvers: Copper-nickel-zinc alloys
Continue
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Weightpercent zinc
Yield strength(OS025 temper),
MPa (ksi)
Electricalconductivity
(relative to purecopper)
0 76 (11) 100%
10 97 (14) 44%
15 110 (16) 37%
30 130 (19) 28%
Brass alloys (Copper zinc)
Cost decreases as the amount of zinc increases
Huge variety of alloys
Modify properties of an alloy by altering composition
Properties of materials within a product
Properties as they relate to ease of manufacturing
Small changes in composition can alter properties substantially
Costs are different for different alloys
Select materials that optimize
Cost
Ease of manufacturing
Product performance and reliability
Variety of alloys increases likelihood of finding optimum choice
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Module review
1. Single elements, alloys, and impurities
2. Composition usually in weight percent
3. There are different classes of alloys for different metals based uponthe alloying elements to the main element.
4. Composition modifications can have a large influence on the
properties of an alloy.
Questions for further thought
1. What are some of the alloys used at your company?
2. What are their compositions?
3. Why were those compositions selected?
4. Have there ever been problems where an alloys composition was not
what it was supposed to be? What problems arose from this?
Continue
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Microscopic Structures
Learning objectives
At the end of this module learners will be able to:
1. Describe three microscopic structures present in metals
2. Explain why there are many grains in metals
3. List three examples of metal phases
4. List three characteristics of phases that influence the properties of ametal.
Overall outcome: Recognize metal objects as being an assembly of
microscopic structures. Not just an abstract slab of material.
Continue
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Practical importance of microscopic structures
1. They are present inside every metal component and joint
2. They directly influence the properties of a material
3. Manufacturing processes affect microscopic structures, which causes
changes in the metal's properties.
Will discuss how manufacturing processes are used to modify microscopicstructures
Produce desired mechanical properties
Microscopic structures
Need a microscope to see these structures
Dimensions typically less than 0.1 millimeters (0.004 inches)
Crystal structure
Arrangement of
atoms within a metal
20 mm
Phases
Materials
within a metal
0.005
Grains
Individual crystalswithin a metal
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Consider 1 mm (0.040 inch) thick sheet metal
Each grain about 0.020 mm (0.0008 in.) diameter
About 50 grains across the sheet thickness
0.625 mm (0.025 )
0.005
Atoms about 0.3 nm (0.0000003 mm) diameter
Over 3 million atoms across the sheet thickness
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Crystal Structure
Refers to the specific arrangement of atoms within a metal
Metals are typically crystalline materials
Atoms are arranged in a periodic manner
Periodic arrangement of atoms in metals
Crystal lattice structure specific arrangement of atoms within a crystal
Unit cell
Repeated in all three directions throughout a metal crystal
Lattice sites
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ActualModel
A crystal lattice continues unbroken throughout a crystal
Metals comprised of many crystals called grains
Within a crystal lattice there is a position where each atom belongs
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Body centered cubic
ChromiumIron
Tungsten
Face centered cubic
CopperAluminum
GoldSilver
Hexagonal
TitaniumZinc
Magnesium
Total of 14 possible lattice structures
Most pure metals body centered cubic, face-centered cubic, or hexagonal
There are metal compounds and alloys that have one of the other 14 lattice
structuresCrystal structure constant for a pure metal, metal alloy, or metal compound
Grains
Most metals polycrystallineComprised of many grains
Each grain a crystal
Each grain (crystal) forms independently
Crystal lattices are tilted in different directions from grain to grain
Courtesy of Aston Metallurgical Services
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Grain boundaries
Regions between grains where atoms try to accommodate crystal latticemisalignment
(Reprinted with permission of ASM International. All rights reserved.)
Grain boundaries
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Practical significance of grains and grain boundaries
1. Grain size affects mechanical properties
Metal strength increases as grain size decreases
Influences creep strength, fatigue strength, and forming properties
2. Grain size affects magnetic and electrical properties
Grain boundaries interfere with current flow if grains very small
Grain boundaries can degrade magnetic permeability
Grain size can be modified using one or more of the following methods
Thermal treatment Mechanical treatment
Alloying
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Orange Peel
Surface roughness in areas that have undergone significant deformation
Deep drawn metal
Surface has appearance of the peel of an orange
Occurs in sheet metal with large grains
Individual grains tend to deform independently of each other
Grains stand out in relief on surface
Can be corrected by using finer grain metal
Individual grains difficult to distinguish by eye
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125 mm
125 mm
Metallurgical Phases
Physically distinct material
Within an alloy more than one phase can be present at one time
Depends on composition and how alloy was processed
Each phase is a mixture or compound formed from alloying elements
Phase 1
Phase 2
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Phase 1 Phase 2
1060 steel
Ferrite a mixture of iron and carbon
Cementite (iron carbide) is Fe3C
Courtesy of Aston Metallurgical Services
20 m
0.0008
Ferrite(light color)
Cementite(dark color)
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Nickel + 20 weight percent vanadium
An alloy can be heat treated to obtain different size and shape of the phases
Long time at 980o CShort time at 980o C
2 mm
Ni3V (dark)
mixture of nickel andvanadium (light)
20 mm
Tin-Lead Solder Joint
Tin + 37% lead
Tin-LeadSolder
Componentlead
Circuit board
Light phase = tin with lead atoms mixed in
Dark phase = lead with tin atoms mixed in
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Phases present and their amounts, shape, size, and location depend on
Alloy composition
How the alloy was processed (i.e. thermal and/or mechanicaltreatment)
Possible to process a particular alloy in different ways
Obtain different amounts, size, and shape of the phases
Obtain different phases
Phase 1
Phase 2
Courtesy of Aston Metallurgical Services Courtesy of Aston Metallurgical Services
10 mm
0.0004
CementiteFerriteFerrite
Harder and stronger
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The phases within a metal affect its properties
1. Different phases have different properties
In steels, cementite is hard and brittle and ferrite is soft
2. Properties of an alloy depend on
Phases present
Relative amount of the phases
Size and shape of the phases
Location of the phases
3. Can modify phases present and their amounts, size, shape, and location
Alloy composition
Thermal treatment
Thermal + mechanical treatment
Microstructure
Microstructure a description of grains and phases present in a metal
Grain size and shape
Phases present
Shape, size, location, and relative amounts of different phases
Uniform distribution of lead
particles in a tin matrixFine grains of ferrite
Courtesy of Aston Metallurgical Services
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Module review
1. Microscopic structures present in metals
Crystal lattice structure
Grains
Phases
2. Grains and phases can be modified to obtain desired properties
3. Metals are an assembly of microscopic structures
Questions for further thought
1. What is the desired grain size in the metal components used in yourcompany s products?
2. What phases are supposed to be present in the alloys used in your
company s products?
3. What properties do the phases give to the components in which thealloys are used?
Continue
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Crystal Defects
Learning objectives
At the end of this module learners will be able to:
1. Describe four types of crystal defects present in metals
2. Explain why crystal defects are important for metal properties
Continue
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Crystal lattice defects
Not the same as manufacturing defects such as voids, cracks, and pits
Enable materials to be modified and manipulated
Add alloying elements
Heat treating to soften and harden metals
Shape metals using processes like stamping and forging
Vacancy
Vacancy
Number of vacancies increases as temperature increases
25 C: About one vacancy per trillion atoms
Just below melting point: one vacancy per 1000 atoms
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Substitution
Atom of a different element that occupies a lattice site of the parent metal
Intentionally added or an impurity
Substitution
Substitutions enable us to form alloys
Zinc in copper (Brass)
Manganese in iron (Steel)
Copper in aluminum
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Substitution solid solution
An alloy that has alloying elements present as substitutions
Alloying elements dissolved into crystal lattice of another element
Analogous to salt (solute) dissolved in water (solvent)
Solvent atom
Solute atom
Amount of an element that can be dissolved in another element depends on
Solvent and solute elements involved
Metal temperature
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Interstitial
Atoms must be small enough to fit in interstitial sites
Hydrogen, carbon, nitrogen, boron, oxygen
Carbon in iron (steel)
Interstitial
Interstitialsite
Interstitial solid solution
An alloy that has alloying elements present as interstitials
Amount of an element that can be dissolved in another element depends on
Solvent metal and solute element
Metal temperature
Solvent atom
Solute atom
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Vacancies, substitutions, and interstitials enable property modifications
Interstitials and substitutions enable alloying
Carbon interstitials in iron enables steel to be hardened
Chromium and nickel substitutions improve steel corrosion resistanceZinc substitutions improves brass strength
Copper substitutions enables aluminum to be hardened
Vacancies and interstitial sites enable atoms to move through a metal
Annealing after cold working
Metal hardening
Dislocation
Screw dislocations
Present in all metals
Edge Dislocation
A.G. Guy, The Essentials of Materials Science, McGraw-Hill, 1976.(Courtesy of Irene Guy)
1 2 3 4 5
1 2 3 4 5 6
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Electron microscope micrograph of dislocations in a metal
0.00004
Influence of dislocations on metal strength
Metal strength depends on ease of dislocation movement through a metal
Easier for dislocations to move in annealed copper or aluminum compared to
hardened steel
Therefore, the steel is stronger than the copper and aluminum
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Modify metal strength by controlling ability of dislocations to move
Heat treating Mechanical treatment
Alloying
Ease of dislocation motion depends on
Alloy composition
Grain size
Phases present
Number of dislocations present
Size and shape of the phases
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Dislocations in a metal with second phase particles
dislocation
matrix
particle
0.00025 mm (0.25 mm)0.00001 inch
Practical Considerations
1. Dislocations enable metals to be deformed
Metals can be bent, compressed, and stretched to the desired shape
2. Can intentionally modify stresses required for dislocations to move
Modify a metal to obtain the desired strength
Annealing brass
Heat treating steel
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Module review
Crystal lattice defects present in metals
Vacancies, substitutions, interstitials, and dislocations
Enables us to modify the properties of a metal
Diffusion
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Learning objectives
By the end of this module learners will be able to:
1. Explain the two mechanisms by which diffusion occurs
2. Relate heat treating time and temperature to the extent of changes in metalmicrostructure and properties
Continue
Diffusion
Vacancies and interstitial sites allows atoms to move through a solid metal
Diffusion is movement of atoms through crystal lattice of a solid metal
See changes in microstructure and properties
VacancyInterstitial sites
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Diffusion important for many processes
Heat treatment for alloy hardening
Annealing to increase metal ductility
Soldering and brazing
Steel surface treatments (e.g., carburizing and nitriding)
Thermally activated process
Metal must be heated for atoms to have the energy to move
Required temperature depends on metal and heat treatment objectives
Low melting point metals do not have be heated up as much as metals with
high melting points.
Practical reasons for understanding diffusion
Improve ability to select and control manufacturing process conditions
Process development
Supplier evaluation and selection
Root cause analysis
Poor control of atom motion within a metal will result in not obtaining the desired
properties in the item being fabricated
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Diffusion processes Vacancy Diffusion
(b)(a)
Diffusion processes Interstitial Diffusion
(a) (b)
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Vacancy Diffusion: Self Diffusion
Start Finish
Atoms vibrate because of their thermal energy
T1 T2 T3
Increasing temperature
Vacancy Diffusion: Substitution
Start Finish
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Interstitial Diffusion
Start Finish
For metallurgical process that involves diffusion
Concerned with speed and extent of atom motion through a metal
Speed with which atoms move depends on
Energy required for atoms to jump from site to site
Metal temperature
Extent of atom motion depends on
Speed of atom motion
Time at temperature
Cannot influence energy required for atoms to jump from site to site
Can control the temperature and time
2 equations to help understand effects of temperature and time
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Temperature
Diffusivity
D = Diffusion coefficient or diffusivity
Do= Frequency factor
Q = Activation energy
k = Boltzmans constantT = Temperature
Diffusivity increases exponentially with temperature
x = diffusion distance
D = diffusivityt = time
As time increases
Diffusion distance increases
Increase in extent of metallurgical changes
Increase in extent of changes in properties
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Example: Annealing a cold rolled metal
The larger the grains the softer and more ductile the metal
As diffusion distance increases
Grain size increasesMetal hardness decreases and ductility increases
Temperature
Time
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Temperature too low or time too short
Diffusion based process will not proceed far enough
Temperature is too high or time too long
Diffusion based process will go too far
Selection and control of the temperature and time is critical
Diffusion to proceed as needed
Obtain desired microstructure and properties
Problem 2: Carburized steel blade
Steel heated in a furnace along with a gas that contains carbon atomsCarbon atoms diffuse into the surface of the steel
Additional carbon enables surface layers to be hardened
Hardened surface layer forms when the steel is cooled
(Courtesy of Aston Metallurgical Services)
0.010
Hardened layer
Steel core
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Module review
1. Diffusion involves the motion of atoms through crystal lattice
2. Vacancy and interstitial diffusion
3. Diffusion requires thermal energy
4. Diffusion rate increases exponentially with temperature
5. Diffusion distance generally proportional to square root of time
6. Control diffusion to obtain the desired microstructure and properties
Questions for further thought
1. What heat treating processes are used at your company or by suppliers to yourcompany?
2. What are the purposes of the heat treatments?
3. What changes occur in the metal as a result of the heat treatment?
Continue
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Applications of Metallurgy Principles
Learning objectives
By the end of the next 5 modules learners will be able to:
1. Explain why cold working strengthens metals
2. Relate the effects of annealing a cold worked metal on its grain size and strength.
3. Explain how alloy composition is used to strengthen a metal.
4. Relate the effects of aging time and temperature to the strength and hardness of
a precipitation strengthened alloy.
5. List the different metallurgical phases that can be present in steel and their effects
on steel strength.
6. Explain the effects of carbon content on steel strength.
7. Explain the effects of tempering time and temperature on quenched hardened
steel.
Continue
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Phases
Courtesy of Aston Metallurgical Services
Diffusion
Temperature
Time
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Examples of common approaches used to modify properties of metals
Alloying, mechanical treatments, and heat treatments
Used to modify metal properties by modifying microscopic structures
Each example is in its own module
Cold working
Annealing
Solid solution strengthening
Precipitation
Steel quenching and tempering
Quiz will be given after all examples have been covered
Define three properties
Hardness
Yield strength
Tensile strength
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Hardness
Resistance to deformation due to an indenter pushing against metal
For a test load, indentation depth decreases as hardness increases
Resistance to deformation directly related to metal yield strength
Depends on ease of dislocation motion
Annealed aluminum vs. hardened steel
Aluminum has a lower strength
For the same test load, indentation in aluminum will be larger
Aluminum hardness reading will be lower
Aluminum Steel
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Yield Strength
Stress required for permanent deformation to begin
radius
Area = x radius2
width
thickness
Area = width x thickness
Tensile strength
Maximum stress that a metal can support
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Example 1: Cold Working
Learning objectives
By the end of this module learners will be able to:
1. Explain how cold working strengthens metals
Continue
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Cold Working
Plastic deformation of a metal at low temperatures
Less than about one-third melting point temperature
Cold rolling Sheet metal forming
Wire drawing
Most of the energy expended in cold work appears in the form of heat
A portion of the energy goes into the creation of more dislocations
Increase number of dislocations 10,000 to 1,000,000 times
Soft, annealed metal: 106 to 108 dislocations per cm2
Heavily cold worked metal: 1012 dislocations per cm2
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As metal deforms, number of dislocations increases
Dislocations generate new dislocations
Dislocations interfere with each other and become entangled
Increased stress to move dislocations through the metal
Increase in yield strength, tensile strength, and hardness
Dislocations in a metal
(Reprinted with permission of ASM International. All rights reserved.)
(amount of cold work)
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Temper
Minimum
tensile strength
Minimum
yield strength
MPa ksi MPa ksi
Annealed 515 75 205 30
hard 860 125 515 75
hard 1035 150 760 110
hard 1205 175 930 135
Full hard 1275 185 965 140
Cold-Rolled Tempers for 301 Stainless Steel
Temper refers to the amount of cold working
Temper
Minimum
tensile strength
Minimum
yield strength
MPa ksi MPa ksi
Annealed 515 75 205 30
hard 860 125 515 75
hard 1035 150 760 110
Full hard 1275 185 965 140
Cold-Rolled Tempers for301 Stainless Steel
Different metals with same temper designation have different strengths
Temper
Minimum
tensile strength
Minimum
yield strength
MPa ksi MPa ksi
Annealed 300 44 75 11
hard 370 54 275 40
hard 425 62 360 52
Full hard 525 76 435 63
Cold-Rolled Tempersfor Cu-30Zn Brass
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Effects of cold working applies to all metals
Degree to which a metal can be cold worked depends on the metal
Ductile metals like aluminum and copper alloys can withstand a significant
amount of cold working.
High strength steels cannot withstand much cold working.
Questions for further thought
1. What cold-formed tempers are specified for the metals used in your companysproducts?
2. What are the reasons for specifying those tempers?
3. What cold-formed tempers are specified for the metal products your company
supplies to its customers? Why?
Continue
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Example 2: Annealing Cold Worked Metals
Learning objectives
By the end of this module learners will be able to:
1. Relate a metal s strength to its grain size
2. Describe the processes used to modify metal grain size
3. Relate recrystallization annealing temperature and time to grain size
Continue
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Recrystallization anneal
Heat treatment used with cold worked metals
Improve metal ductility
Reduce metal strength and hardness
Purpose
Enable further cold working
Meet specific mechanical property specifications
Metallurgical effects
1. Eliminate dislocations and crystal lattice damage from cold working
2. New grains form and grow
Healing of microscopic structures is a diffusion based process
Cu-30 Zn brass cold-rolled and annealed
50% reduction550 C anneal75 MPa yield strength
50% reduction650 C anneal60 MPa yield strength
B CA50% reduction
550 MPa yield strength
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50% reduction
550 C anneal75 MPa yield strength
50% reduction
650 C anneal60 MPa yield strength
50% reduction550 MPa yield strength
Decrease in strength during anneal
Recovery and recrystallization processes occur inside metal
Recovery: Reduction in number of dislocations formed by cold working
Recrystallization: Form new grains
Diffusion based processes
50% reduction
550 C anneal75 MPa yield strength
50% reduction
650 C anneal60 MPa yield strength
50% reduction
550 MPa yield strength
Difference in strength between 550 C and 650 C samples
Due to difference in grain sizeAfter recrystallization, continued annealing results in grain growth
Larger grains grow at the expense of smaller grains
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Grain size and metal strength
Grain boundaries influence motion of dislocations in a way that makes areas neargrain boundaries stronger.
Yield strength and hardness increases as grain size decreases
Ductility decreases as grain size decreases
Strengthened area
Grain boundary
550 C anneal 650 C anneal
B CA
Difference in grain size between annealed samples makes sense
Diffusion rate and diffusion distance greater at higher temperatureMore grain growth at higher annealing temperature
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Applications and annealing temperature
Recrystallization annealing can be used with any cold worked metal
Specific annealing temperature depends on particular metal
Temperature increases as melting point temperature increases
AnnealingTemperature
Melting PointTemperature
Aluminum About 260 to 440 C(500 to 825 F)
About 630 to 660 C
(1166 to 1220 F)
1010 steel About 700 C(1292 F)
1535 C
(2792 F)
Questions for further thought
1. Are there specifications for the grain size of the metals used in your products orfor the metals your company supplies to its customers?
2. How is the grain size verified? Who does the analysis?
3. What happens if the grain size does not meets the specifications? On
manufacturing processes? On component performance? How is the problem
fixed?
Continue
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Example 3
Solid Solution Strengthening
Learning objectives
By the end of this module learners will be able to:
1. Explain how alloying elements as substitutions and interstitials can strengthen
an alloy.
Continue
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Solid Solution Hardening
Increase in metal strength due to presence of substitution or interstitial atoms.
Alloying elements (solute) dissolved in the crystal lattice of a solvent metal.
The solute atoms distort the crystal lattice.
Strain
Strain energy is associated with the lattice distortion
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Strain energy associated with a dislocation
Edge Dislocation
A.G. Guy, The Essentials of Materials Science, McGraw-Hill, 1976.
(Courtesy of Irene Guy)
Solute atom strain energy interacts with dislocation strain energy
Increased force to move dislocation past solute atoms compared to solute
free metal
Force increases as number of solute atoms increases
Stress to deform an alloy increases compared to a solute free metal
dislocation
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Effect of magnesium content on yield strength of aluminum alloys
% Magnesium
YieldStrength(MPa)
0
20
40
60
80
100
120
140
160
0 1 2 3 4 5 6
1050
5005
5050
5052
5086
5083 5056
Weight percent
zinc
Yield strength,
MPa (ksi)
0 76 (11)
10 97 (14)
15 110 (16)
30 130 (19)
Cu alloys, OS025 temper
Effects of zinc added to copper in brass alloys
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Example 4
Steel Heat Treating
Learning objectives
By the end of this module learners will be able to:
1. Describe the metallurgical phases that can be present in steel and their effects
on steel strength.
2. Describe the process steps for through hardening a steel alloy.
3. Explain the effects of quench temperature on martensite formation.
4. Relate steel strength and hardness to steel carbon content and tempering
temperature and time.
Continue
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Steel metallurgy and heat treatment
Focus on plain carbon, low alloy, and tool steels
Not stainless steels
Variety of steel microstructures can be created
Possible to heat treat a steel alloy to obtain different microstructures
Resulting strength and hardness are different
Phases of interest
Austenite
Ferrite
Cementite (Fe3C)
Martensite
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Austenite
Iron solid solution
Substitutions: manganese, chromium, or nickel
Iron atom
Carbon interstitial
Substitutions
Austenite normally a high temperature phase
Circumstances when austenite exists at room temperature
For strengthening, steel first heated to form austenite
When steel is cooled it transforms to one or more of the other phases
Approximate austenite formation temperatures
Austeniteregion
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Ferrite
Soft, low strength phase
Iron solid solution
Very li ttle carbon can dissolve in ferrite
About 0.02% at 727 C
Less than 0.005% at room temperature
Iron atom
Carbon interstitial
Substitutions (e.g. Mn, Cr, Ni)
Grains of ferrite in a low carbon steel
0.005 inches
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Cementite
Hard, brittle phase
Fe3C
Three iron atoms for every carbon atomAlso called iron carbide
Courtesy of Aston Metallurgical Services Courtesy of Aston Metallurgical Services
Cementite can be present in one of three different shapes
Cementite shape depends on how steel was heat treated
Pearlite (lamellae) Spheroidized cementite
Cementite
Ferrite
Cementite
1060 Steel (0.60% carbon)
HARDER AND STRONGER
10 mm
0.0004
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1. Heated to 860 C (1580 F)2. Air cooled
1. Heated to 860 C (1580 F)
2. Air cooled3. Heated to 650 C (1202 F) for
several hours
Possible to take an alloy and process it in different ways to obtain different
microstructures and properties.
Courtesy of Aston Metallurgical Services Courtesy of Aston Metallurgical Services
10 mm
0.0004
Ferrite-Pearlite
Alloys with less than 0.78% carbon
Heat to form austenite
Slow cool
1050 Steel
Courtesy of Aston Metallurgical Services
Ferrite
Pearlite
0.002
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Solid solution of carbon interstitials in iron
Martensite strength and hardness
Crystal lattice contains more carbon in solid solution than steel wants
Excess carbon atoms in solid solution strains the crystal lattice
Iron
Carbon
Lattice strain impedes dislocation motion
Increase in steel strength and hardnessAmount of strain increases as carbon content increases
Iron
Carbon
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Forming martensite
1. Heat steel to form austenite
About 775 to about 950 C (1427 to 1742 F)
2. Rapidly cool (quench)
Reason for quenching
Prevent formation of ferrite, cementite, or pearlite.
Not enough time at elevated temperatures for atoms to diffuse through steelto form ferrite, cementite, or pearlite.
Instead, groups of iron atoms shift slightly at once to form martensite
Austenite to martensite transformation
Non-equilibrium processProcess does not allow atoms to move to where they want to move
Extent of transformation depends only on temperature, not time
Martensite formation does not depend on diffusion
As temperature decreases more martensite forms
Transformation stops if cooling stops
Continued transformation if cooled to lower temperatures
For some alloys, cooling below room temperature required to obtain 100%martensite
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Martensite hardness governed primarily by carbon content
As-quenched martensite hardness
Tempering
Martensitehas poor toughness in the as-formed condition
Increase toughness by heating between 125 and 705 C (257 and 1292 F)
Martensitedecomposes
Carbon diffuses out of martensite
Forms iron carbide particles
Hardness and strength decreases while toughness increases
Temperature and time selected to obtain desired microstructure and properties
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(Reprinted with permission of ASM International. All rights reserv ed.)
Hardness(RockwellC)
Tempering time (seconds)
%
CarbonMicrostructure
Hardness,
Rockwell C
Hardness,
Brinell
Yield
Strength,
MPa (ksi)
Tensile
Strength,
MPa (ksi)
0.20
Spheroidized --- 111 295 (43) 395 (57)
Ferrite-pearlite --- 131 345 (50) 440 (64)
Martensite* 30 286 552 (80) 724 (105)
0.40
Spheroidized --- 149 350 (51) 520 (75)
Ferrite-pearlite --- 170 370 (54) 595 (86)
Martensite* 50 481 676 (98) 910 (132)
0.60
Spheroidized --- 179 370 (54) 625 (91)
Ferrite-pearlite 21 229 420 (61) 775 (113)
Martensite* 60 654 814 (118) 1104 (160)
Typical plain carbon steel properties
*As-quenched martensite
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Questions for further thought
1. What steel alloys does your company use, process, or produce?
2. What are the required properties of the steel alloys?
3. What manufacturing processes are used to achieve those properties?
4. What happens if the metal stock or components do not have the required
properties?
Continue
Example 5: Precipitation Strengthening
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Learning objectives
By the end of this module learners will be able to:
1. Describe the heat treating process for precipitation strengthening.
2. Explain why precipitation strengthens a metal.3. Relate metal strength to precipitation aging temperature and time.
Continue
Precipitation Strengthening
Also referred to as aging
Particles of a different phase form within a matrix phase
Particles often less than 1 mm (0.00004 )
Particles are referred to as precipitates
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Aluminum 5 weight % copper alloy with precipitates
Solution treated at 545 C for 1 week
Fast cooled to 20 C
Aged at 300 C for 12 hours
DoITPoMS Micrograph Library, University of Cambridge.
Precipitates obstacles to dislocation motion
Greater stress required to move dislocations through metalStrength and hardness increases
dislocation
precipitate
0.00025 mm
0.00001 inch
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Alloys for Precipitation Strengthening
Aluminum alloys: Al-Cu, Al-Mg-Si, Al-Mg-Zn, Al-Mg-Zn-Cu
2xxx, 6xxx, 7xxx wrought Al alloys
Some 2xx, 3xx, 5xx, 7xx, and 8xx cast Al alloys
Copper alloys: Cu-Be, Cu-Zr, Cu-Cr
Some stainless steel alloys: 13-8 PH, 15-5 PH, 17-4 PH and 17-7 PH
Some nickel alloys
Some magnesium alloys
2) Fast cool
Form supersaturatedsolid solution
Microstructure
1) Solution heat treatA (solvent)
B (solute)
Precipitation strengthening process
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3) Reheat alloy
Intermediate temperature
Excess solute atoms form precipitates with solvent atoms
Aging
Size and number of precipitates depends on aging time and temperature
Microstructure
after aging
Onset of coarsening
Large precipitates grow at expense of small precipitates
Maximum
strength and
hardness
Aging a supersaturated solid solution
a b c
d e f
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2014 Aluminum: Al 4.4Cu 0.8Si 0.8Mn
Maximum yield strength increases as aging temperature decreases
Number of precipitates increases as aging temperature decreases
Greater number of precipitates presents more obstacles to dislocations
(Reprinted with permission of ASM International. All rights reserved.)
Time to reach maximum strength increases as aging temperature decreases
Diffusion rate decreases as temperature decreasesTime required for precipitate formation and growth increases
(Reprinted with permission of ASM International. All rights reserved.)
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Questions for further thought
1. What precipitation strengthened alloys does your company use, process, or
produce?
2. What are the required properties of the alloys?
3. What happens if the metal stock or components do not have the requiredproperties?
Continue
Course Review
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Properties, composition, microstructure, and processing
Microscopic structures
Modified by alloying and mechanical and thermal processes
Effects of modification on mechanical properties
Properties
Composition
Microscopic
structure
Manufacturing
Processes
Discussed microscopic structures
Crystal lattice Grains Phases
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Discussed crystal defects
Vacancy, Substitution, Interstitial
A.G. Guy, The Essentials of MaterialsScience, McGraw-Hill, 1 976. (Courtesy of
Irene Guy)
1 2 3 4 5
1 2 3 4 5 6
Dislocations
Diffusion temperature and time dependence
Temperature
Time
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Presented examples
Common approaches to modify properties of metals
Cold-work metal
Increase number of dislocations
Increase metal strength and hardness
Decrease metal ductility
(amount of cold work)(Reprinted with permission of ASM International. All rights reserved.)
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Anneal cold-worked metal
Eliminate many dislocations
New grains form and grow
Increase ductility and reduce strength
Solid solution hardening
Impede dislocation motion
Increase metal strength and hardness
dislocation
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Through hardening and tempering steel
Form martensitein steel
Increase strength and hardness
Temper to reduce lattice strainImprove toughness
Martensite in 1040 steel
20 mm0.0008Courtesy of Aston Metallurgical Services
Precipitation hardening
Precipitates interfere with dislocation motion
Increase metal strength and hardness
0.010 mm0.0004
DoITPoMS Micrograph Library, University of Cambridge.
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Many other things that we can do to alter other properties
Electrical conductivity
Corrosion behavior
Fatigue behavior
Many more
Further Information
1. ASM Handbook Volume 1, Properties and Selection: Irons, Steels, and High-Performance Alloys
2. ASM Handbook Volume 2, Properties and Selection: Non-Ferrous Alloys andSpecial Purpose Materials
3. ASM Handbook Volume 4, Heat Treating, ASM International
4. Introduction to Materials Science for Engineers, J.F. Shackelford
5. Physical Metallurgy Principles, R. Abbaschian, L. Abbaschian, R. E. Reed-Hill
6 Annual Book of ASTM Standards ASTM