IE 337 W10 Lecture 3 Alloys&Heat.treat

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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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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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