Heat Treatment of metal alloysHeat Treatment of metal alloys

Mohamed KablMohamed Kabl12/11/201512/11/2015

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ContentContentIntroductionIntroductionCategoriesCategoriesHeat treatment methodsHeat treatment methodsReferencesReferences

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IntroductionIntroduction

What is metal alloys heat treatment ? What is metal alloys heat treatment ?

Heat treatmentHeat treatment is a method used to alter the physical, is a method used to alter the physical, and sometimes chemical properties of a material. The and sometimes chemical properties of a material. The most common application is metallurgicalmost common application is metallurgical

It involves the use of heating or chilling, normally to It involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such extreme temperatures, to achieve a desired result such as as hardeninghardening or or softeningsoftening of a material of a material

It applies only to processes where the heating and It applies only to processes where the heating and cooling are done for the specific purpose of altering cooling are done for the specific purpose of altering properties intentionallyproperties intentionally

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Reasons for using heat Reasons for using heat treatmentstreatments

Improves properties of metal alloysImproves properties of metal alloysModifies the microstructureModifies the microstructureImproves formabilityImproves formabilityImproves machinabilityImproves machinabilityIncreases strength & hardnessIncreases strength & hardnessService performance improved such Service performance improved such

as in gearsas in gears

Cross section of gear teeth showing induction-hardened surfaces. Cross section of gear teeth showing induction-hardened surfaces. Source: Source: TOCCO Div., TOCCO Div., Park-Ohio Industries, IncPark-Ohio Industries, Inc..

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The properties and behavior of metals (and alloys) depend The properties and behavior of metals (and alloys) depend on their:on their:

StructureStructure

Processing historyProcessing history

CompositionComposition

How to Strengthen MetalsHow to Strengthen Metals

Increase dislocation density via Cold working (strain Increase dislocation density via Cold working (strain hardening)hardening)

Add alloying elements to give – SOLID SOLUTION Add alloying elements to give – SOLID SOLUTION HARDENING.HARDENING.

DISPERSION HARDENING – fine particles (carbon) DISPERSION HARDENING – fine particles (carbon) impede dislocation movement. (Heat treatment)impede dislocation movement. (Heat treatment)

Key: prevent dislocations from moving through crystal structure!!!Key: prevent dislocations from moving through crystal structure!!!

Metals

Valence electrons of 1,2 or 3

Primary bonding between electrons called metallic bonding

Valence electrons not “bonded” to particular atom but shared and free to drift through the entire metal

Properties include: good conductors of electricity and heat, not transparent, quite strong yet deformable!

Crystalline structures (i.e. metals) atoms are arranged in unit cells – 4 common cells shown above

How do Metal Crystals Fail??

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Pure MetalsPure Metals

In In PURE METALSPURE METALS, atoms are all the same , atoms are all the same type, except for rare impurity atomstype, except for rare impurity atoms

Pure Metals & AlloysPure Metals & Alloys

lead copper

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AlloysAlloys

ALLOYSALLOYS are composed of 2 or more chemical are composed of 2 or more chemical elements, at least one of which is a metalelements, at least one of which is a metal

Tungsten copper Bronze

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Classification of alloysClassification of alloys

Classification of alloysClassification of alloys

FerrousFerrous: containing iron, second most abundant : containing iron, second most abundant element (5% earth's crust).element (5% earth's crust).

Non-ferrousNon-ferrous: no iron, usually more expensive than : no iron, usually more expensive than ferrous metals.ferrous metals.

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Solid SolutionsSolid Solutions

Solute: the minor element that is added to the Solute: the minor element that is added to the solventsolvent

Solvent: the major elementSolvent: the major element Substitutional solid solutions: the size of the solute Substitutional solid solutions: the size of the solute

atom is similar to the solvent atom (example: brass atom is similar to the solvent atom (example: brass alloy of zinc & copper)alloy of zinc & copper)

Interstitial solid solutions: the size of the solute atom Interstitial solid solutions: the size of the solute atom is much smaller than that of the solvent (example: is much smaller than that of the solvent (example: steel alloy iron & carbon)steel alloy iron & carbon)

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Substitutional Solid SolutionsSubstitutional Solid Solutions

Must have similar crystal structures (e.g. Must have similar crystal structures (e.g. FCC with FCC).FCC with FCC).

Difference between atomic radii less than Difference between atomic radii less than 15% (same size atoms).15% (same size atoms).

Brass (zinc + copper).Brass (zinc + copper).Copper Grains

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Interstitial Solid SolutionsInterstitial Solid Solutions Interstitial Solid Solution - solvent atom has Interstitial Solid Solution - solvent atom has

more than one valence electron (easier to more than one valence electron (easier to control solute).control solute).

Atomic radius of solute atom is less than 59% Atomic radius of solute atom is less than 59% of solvent (atom sizes differ greatly).of solvent (atom sizes differ greatly).

Example = Steel (iron + carbon)Example = Steel (iron + carbon)

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Intermetallic CompoundsIntermetallic Compounds Complex structuresComplex structures

Solute atoms present among solvent atoms = Solute atoms present among solvent atoms = atomic bonding.atomic bonding.

Strong, hard, and brittleStrong, hard, and brittle

TiTi33Al, NiAl, Ni33Al, FeAl, Fe33Al.Al.

Aluminum Grains

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Two-phase SystemsTwo-phase Systems

Most alloys Most alloys consist of consist of twotwo or more solid phases (alloy contains or more solid phases (alloy contains particles of single element particles of single element OROR grains are differentgrains are different).).

Limited solubility (just as with sugar in water Limited solubility (just as with sugar in water Mechanical Mechanical

mixture).mixture).

Clear boundaries, mixture - each with its own properties.Clear boundaries, mixture - each with its own properties.

Stronger and less ductile than solid solutions.Stronger and less ductile than solid solutions.

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Phase DiagramsPhase Diagrams

Pure metals have clearly defined melting or freezing points, Pure metals have clearly defined melting or freezing points,

and solidification takes place at a constant temperature.and solidification takes place at a constant temperature.

Tool for understanding the relationship among Tool for understanding the relationship among temperaturetemperature, ,

compositioncomposition, and , and phasesphases present in a particular alloy present in a particular alloy

system. system.

Alloys solidify over a Alloys solidify over a range of temperaturesrange of temperatures, based on the , based on the

composition of the mixture.composition of the mixture. As the alloy cools the mixture begins to freeze, changing As the alloy cools the mixture begins to freeze, changing

gradually to a solid (liquid/solid phases).gradually to a solid (liquid/solid phases).

(a) Cooling curve for the solidification of pure metals. Note that freezing takes (a) Cooling curve for the solidification of pure metals. Note that freezing takes place at a constant temperature; during freezing, the latent heat of solidification place at a constant temperature; during freezing, the latent heat of solidification

is given off. (b) Change in density during cooling of pure metals.is given off. (b) Change in density during cooling of pure metals.

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Binary Phase DiagramsBinary Phase Diagrams

Composition

Tem

pera

ture

L

S

L+S

Tem

pera

ture

0% B100% A

100% B0% A

A

B

Complete Solid Solubility

Solid Solution - Single Phase

Two Phases

Phase DiagramsPhase Diagrams

Alloys solidify over a range of temperaturesAlloys solidify over a range of temperatures

Liquidus Liquidus - solidification occurs when the - solidification occurs when the temperature drops belowtemperature drops below

Solidus Solidus - solidification is complete- solidification is complete

Between liquidus and solidus the alloy is in a Between liquidus and solidus the alloy is in a mushy or pasty statemushy or pasty state

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Nickel-Copper DiagramNickel-Copper Diagram

(Black & Kohser, 2008, p. 75)

Lever RuleLever Rule• Used to determine the composition of various phases in Used to determine the composition of various phases in

the phase diagramthe phase diagram• Example: Copper NickelExample: Copper Nickel

– At 1288 degrees C, a mixture of solid/liquidAt 1288 degrees C, a mixture of solid/liquid– Solid is 42% Cu, 58% NiSolid is 42% Cu, 58% Ni– Liquid is 58% Cu, 42 % NiLiquid is 58% Cu, 42 % Ni• The completely solidified alloy is a The completely solidified alloy is a solid solid

solution solution because Cu completely dissolves because Cu completely dissolves in Ni and each grain has the same in Ni and each grain has the same compositioncomposition

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A + Liquid

B + Liquid

A + B

A

B

0% B100% A

Eutectic point

100% B0% A

AB

Two-Phase DiagramsTwo-Phase DiagramsLimited solubilityLimited solubility

Two Phases

Solid Solution - Single Phase

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Two-Phase Lead-Tin DiagramTwo-Phase Lead-Tin Diagram

(Black & Kohser, 2008, p. 75)

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Two-Phase Iron-Carbon DiagramTwo-Phase Iron-Carbon DiagramMost important phase diagram in manufacturing applications, since Most important phase diagram in manufacturing applications, since steels, cast irons, and cast steels are the most common engineering steels, cast irons, and cast steels are the most common engineering materials (versatile properties and relative low cost).materials (versatile properties and relative low cost).

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Iron-Carbon DiagramIron-Carbon Diagram

Solid Phases of the Iron-Carbon DiagramSolid Phases of the Iron-Carbon Diagram

Ferrite (Ferrite (-iron)-iron)

Austenite (Austenite (-iron)-iron)

Cementite (iron-carbide)Cementite (iron-carbide)

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FerriteFerrite

((-iron-iron))

Soft, ductile, magnetic.Soft, ductile, magnetic.

BCCBCC

Solid solution (0.022% carbon) almost pure iron.Solid solution (0.022% carbon) almost pure iron.

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AusteniteAustenite ((-iron)-iron)

FCC - higher density than BCC, ductile at elevated FCC - higher density than BCC, ductile at elevated temperatures (good formability)temperatures (good formability)

Interstitial Solid Interstitial Solid

Solution (2.11% carbon)Solution (2.11% carbon)

Non-magneticNon-magnetic

AUSTENITIC MANGANESE STEEL

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CementiteCementite

Iron carbide (Fe3C) 6.67% carbonIron carbide (Fe3C) 6.67% carbon

Hard & brittle Intermetallic Compound.Hard & brittle Intermetallic Compound.

Fe3C CategoriesFe3C Categories

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3434Engr 241Engr 241

Schematic illustration of the microstructures for an iron–carbon alloy of Schematic illustration of the microstructures for an iron–carbon alloy of eutectoid composition (0.77% carbon), above and below the eutectoid eutectoid composition (0.77% carbon), above and below the eutectoid

temperature of 727°C (1341°F).temperature of 727°C (1341°F).

Eutectoid System

3636Engr 241Engr 241

Hardenability and Weldability are influenced Hardenability and Weldability are influenced by four factorsby four factors

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Carbon content –Carbon content – Weldable Weldable .35% C .35% C HardenableHardenableHeating Cycle – maximum temperatureHeating Cycle – maximum temperatureCooling Cycle – minimum temperatureCooling Cycle – minimum temperatureSpeed of coolingSpeed of cooling

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Heat Treatment Heat Treatment ProcessesProcesses

AnnealingAnnealing: general term used to refer to the restoration : general term used to refer to the restoration of properties after cold work or heat treatment.of properties after cold work or heat treatment.

Full annealingFull annealing austenitizing and furnace cool.austenitizing and furnace cool. It is used in low- and medium carbonIt is used in low- and medium carbon

steels that need extensive machining or plastic deformationsteels that need extensive machining or plastic deformation

NormalizingNormalizing: cooling cycle done in still air to avoid : cooling cycle done in still air to avoid excessive softness in the annealing of steels. excessive softness in the annealing of steels.

SpheroidizingSpheroidizing: improve properties of high-carbon steels.: improve properties of high-carbon steels.

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Heat Treatment Heat Treatment ProcessesProcesses

Stress RelievingStress Relieving: reduce or eliminate residual stresses.: reduce or eliminate residual stresses.

TemperingTempering: reduce brittleness and residual stress, and : reduce brittleness and residual stress, and increase ductility and toughness of previously hardened increase ductility and toughness of previously hardened steels.steels.

HardeningHardening: heating and cooling rapidly (quenching): heating and cooling rapidly (quenching)

Case HardeningCase Hardening: complete alteration of the : complete alteration of the microstructure and properties of just the surface of the microstructure and properties of just the surface of the material by heating within a particular atmospherematerial by heating within a particular atmosphere

Heat treatment rangesHeat treatment ranges

4040Engr 241Engr 241

MartensiteMartensite Named after the German metallurgist Adolf Named after the German metallurgist Adolf

Martens (1850–1914)Martens (1850–1914) very hard form of steel crystalline structurevery hard form of steel crystalline structure

  formed in carbon steels by the rapid cooling (quenching) formed in carbon steels by the rapid cooling (quenching) of austenite at such a high rate that carbon atoms do not of austenite at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large have time to diffuse out of the crystal structure in large enough quantities to form cementite (Feenough quantities to form cementite (Fe33C)C)

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TTT DiagramTTT Diagram

4242Engr 241Engr 241

Ability of an alloy steel to be hardened by the formation of martensite.

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Steel MicrostructuresSteel MicrostructuresPerlite (eutectoid steel) - alternating layers of Ferrite and CementitePerlite (eutectoid steel) - alternating layers of Ferrite and Cementite

fine or coarse perlitefine or coarse perlite

  Spheroidite (spherical cementite) - tougher and harder than perliteSpheroidite (spherical cementite) - tougher and harder than perlite

Bainite (very fine ferrite-cementite) - stronger and more ductile than Bainite (very fine ferrite-cementite) - stronger and more ductile than perlite, same hardnessperlite, same hardness

Austempring and Martempering Austempring and Martempering

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References References

George E. Totten,1997, “George E. Totten,1997, “Steel Heat Treatment Handbook” , Steel Heat Treatment Handbook” , New New York: McGraw-HillYork: McGraw-Hill,1997.,1997.  ISBN  0-07-042366-0

Colin J. Smithell, 1990, “Colin J. Smithell, 1990, “Metals Reference Book”, Metals Reference Book”, London : London : Prentice-Prentice-Hall International,Hall International,19911991. . 634 p634 p. ISBN 0- 13-014502-5. ISBN 0- 13-014502-5

AZO Materials, 2015, “AZO Materials, 2015, “Steels - An Introduction to Heat TreatmentSteels - An Introduction to Heat Treatment”” “ “http://www.azom.com/article.aspx?ArticleID=313” http://www.azom.com/article.aspx?ArticleID=313”

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