Post on 25-Oct-2014
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IE 337: Materials & Manufacturing Processes
Lecture 3:
Metal Alloys and
Heat Treatment
Chapters 3, 6 and 27
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Last Time
The nature of metals Different crystalline structures Different crystalline defects that affect properties
The properties of metals Mechanical properties What they are and what they mean
Stress-Strain Relationships
Figure 3.3 Typical engineering stress‑strain plot in a tensile test of a metal.
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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
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
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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
Brinell Hardness Test
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Brinell Hardness Number
Load divided into indentation area = Brinell Hardness Number (BHN)
)( 22
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ibbb DDDD
FHB
where HB = Brinell Hardness Number (BHN), F = indentation load, kg; Db = diameter of ball, mm, and Di = diameter of indentation, mm
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Rockwell Hardness Test
Figure 3.14 Hardness testing methods: (b) Rockwell:
(1) initial minor load and (2) major load.
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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
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26Fe
55.847
Metals: Periodic Table
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
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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
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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
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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
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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
Pure tin melts at 232C (449F) Pure lead melts at 327C (621F) 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
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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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FERROUS
Metals: Classification
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IMPORTANCE OF IRON
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 912C (1674F), ferrite transforms to gamma (), called austenite (FCC)
This transforms at 1394C (2541F) to delta () (BCC)
Pure iron melts at 1539C (2802F)
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Solubility Limits of Carbon in Iron
Ferrite phase can dissolve only about 0.022% carbon at 723C (1333F)
Austenite can dissolve up to about 2.1% carbon at 1130C (2066F) The difference in solubility between alpha
and gamma provides opportunities for strengthening by heat treatment
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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
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
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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
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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.
Time-Temperature-Transformation Curve
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Allotropictransformation- austenite to martensite
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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Low Alloy High Alloy
low carbon <0.25wt%C
med carbon 0.25-0.6wt%C
high carbon 0.6-1.4wt%C
Uses auto struc. sheet
bridges towers press. vessels
crank shafts bolts hammers blades
pistons gears wear applic.
wear applic.
drills saws dies
high T applic. turbines furnaces V. corros. resistant
Example 1010 4310 1040 4340 1095 4190 304
Additions noneCr,V Ni, Mo
noneCr, Ni Mo
noneCr, V, Mo, W
Cr, Ni, Mo
plain HSLA plainheat
treatableplain tool
austentitic stainless
Name
Hardenability 0 + + ++ ++ +++ 0TS - 0 + ++ + ++ 0EL + + 0 - - -- ++
increasing strength, cost, decreasing ductility
Steels
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Alloy Steels
Further refined from carbon steels, with elements added to modify or change the mechanical properties.
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Alloy Steels
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
Cr addition improves corrosion resistance
So does Ni
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Alloy Steels: Alloying Elements
Boron Large increase in
hardenability with very small addition of element
Chromium Increases depth hardness Increases corrosion
resistance Principle component in
stainless steel
Cobalt Increases wear-resistance Increases hot-hardness -
ability to keep shape at elevated temperature
Used in high speed steel
Lead Reduces cutting friction,
improving machinability Good weldability Good formability Environmental concern
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Alloy Steels: Alloying Elements
Manganese Large amounts (1% -
15%) gives good hardness and wear-resistance
Small amounts useful for purifying melt by combining with impurities and forming dross
Vanadium Also used to purify melt Produces fine-grained
steels
Tungsten Provides high wear-
resistance Adds hardenability and
strength at elevated temperatures
Used in tool steels
Phosphorous / Sulfur Give excellent machining
characteristics Used in free-machining
steels
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Alloy Steels: Alloying Elements
Molybdenum Aids toughness Used in tool steels Improves depth-hardness Improves strength at
elevated temperatures
Nickel Provides corrosion-
resistance Improves resistance to
elevated temperatures Used in stainless steels Combined with
Molybdenum to provide very tough steel for aircraft applications
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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 Iron1-3 % Sicheap
used in compressionvibrational damping(machinery housing)
Ductile IronMg, Ce, Ca, Li, Na, Ba
to gray ironstronger and ductile
(valves, gears, crankshafts)
Malleable Iron< 1% Si
heat treat white ironstrong, malleable(connecting rods,
transmission gearsflanges, fittings)
White Iron< 1% Si
brittle, wear resistantmalleable iron
precursor(rollers)
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Non-Ferrous Metals
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)
Major Materials: Aluminum Alloys Copper & Copper Alloys Magnesium Nickel & Nickel Alloys Refractories Superalloys
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Non-Ferrous Alloys
NonFerrous Alloys
• Cu AlloysBrass: Zn is subst. impurity
(costume jewelry, coins, corrosion resistant)Bronze: Sn, Al, Si, Ni are
subst. impurity (bushings, landing gear)Cu-Be:
precip. hardened for strength
• Al Alloys-lower : 2.7g/cm3
-Cu, Mg, Si, Mn, Zn additions -solid sol. or precip.
strengthened (struct. aircraft parts & packaging)
• Mg Alloys-very low : 1.7g/cm3 -ignites easily -aircraft, missles
• Refractory metals-high melting T -Nb, Mo, W, Ta• Noble metals
-Ag, Au, Pt -oxid./corr. resistant
• Ti Alloys-lower : 4.5g/cm3
vs 7.9 for steel -reactive at high T -space applic.
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Aluminum Alloys
Pure metal properties: Low density, melting point Ductile Malleable Good electrical / thermal
conductor
Alloying elements: Copper Magnesium Silicon Manganese Zinc
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
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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.
Precipitation Hardening
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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
Pure metal properties: Very soft Ductile Malleable Good electrical / thermal
conductor
Alloying elements: Alloyed with Sn to make
Bronze Alloyed with Zn to make
Brass
Typical uses of Cu: Electronics production Electrical conductors
Typical uses as Bronze: Machine parts Bearings Corrosion-resistant fittings Electrical connectors
Typical uses as Brass: Hardware Marine corrosion-
resistance Ornamental applications
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Magnesium Alloys
Pure metal properties: Lightweight Strong (per unit volume) Flammable in fine sizes
Alloying elements: Aluminum Bismuth Copper Tin Lead Iron
Typical uses of Mg: Aircraft components
(strength to weight ratio) Automobile wheels Racing frames Lightweight structural
parts
Handling Magnesium Keep chips coarse Avoid chip accumulation,
mixing with other material Avoid water, water-based
coolants (explosive)
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Nickel Alloys
Properties: Corrosion resistance Heat resistance
Alloy forms: Monel K-Monel R-Monel Inconel
Typical uses of Ni: Plating of electronics
(pure form) Thermocouples Alloying element
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.
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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.
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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
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 1100C (2000F)
Applications: gas turbines ‑ jet and rocket engines, steam turbines, and nuclear power plants ‑ systems in which operating efficiency increases with higher temperatures
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Refractory Metals
Metals capable of enduring high temperatures - maintaining high strength and hardness at elevated temperatures
Most important refractory metals: Molybdenum Tungsten
Other refractory metals are niobium and tantalum (used in capacitors)
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Tungsten
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
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Molybdenum
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
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Precious Metals
Gold, platinum, and silver Also called noble metals because chemically inert Available in limited supply
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
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Shaping, Assembly, and Finishing Processes for Metals
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
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You should have learned:
How we can modify mechanical properties in metals? Alloying Annealing Allotropic transformation Precipitation hardening
Different types of metal alloys and how they are used?
Assignment #1
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Next Time
How do we shape materials? Secondary operations Material Removal
The fundamentals of metal cutting (Chapter 21) Orthogonal machining The Merchant Equation