Heat treatmet of metal alloys

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  • Heat Treatment of metal alloys Mohamed Kabl12/11/2015

  • *ContentIntroductionCategoriesHeat treatment methodsReferences

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    IntroductionWhat is metal alloys heat treatment ?

    Heat treatment is a method used to alter the physical, and sometimes chemical properties of a material. The most common application is metallurgicalIt involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a materialIt applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally

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    Reasons for using heat treatmentsImproves properties of metal alloysModifies the microstructureImproves formabilityImproves machinabilityIncreases strength & hardnessService performance improved such as in gears

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

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

    Structure

    Processing history

    Composition

  • How to Strengthen MetalsIncrease dislocation density via Cold working (strain hardening)

    Add alloying elements to give SOLID SOLUTION HARDENING.

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

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

  • MetalsValence 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??

    *To move a dislocation, less energy involved since only one bond is broken. Note, the edge dislocaiton discussed here is only 1 of several different dislocations that have been characterized. This is the basis of Material Science!!

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    Pure Metals

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

    Pure Metals & Alloysleadcopper

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    AlloysALLOYS are composed of 2 or more chemical elements, at least one of which is a metalTungsten copperBronze

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    Classification of alloys

    Classification of alloys

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

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

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    Solid SolutionsSolute: the minor element that is added to the solventSolvent: the major elementSubstitutional solid solutions: the size of the solute atom is similar to the solvent atom (example: brass alloy of zinc & copper)Interstitial solid solutions: the size of the solute atom is much smaller than that of the solvent (example: steel alloy iron & carbon)

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    Substitutional Solid Solutions

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

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

    Brass (zinc + copper).Copper Grains

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    Interstitial Solid SolutionsInterstitial Solid Solution - solvent atom has more than one valence electron (easier to control solute).

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

    Example = Steel (iron + carbon)

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    Intermetallic CompoundsComplex structures

    Solute atoms present among solvent atoms = atomic bonding.

    Strong, hard, and brittle

    Ti3Al, Ni3Al, Fe3Al.Aluminum Grains

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    Two-phase SystemsMost alloys consist of two or more solid phases (alloy contains particles of single element OR grains are different).

    Limited solubility (just as with sugar in water Mechanical mixture).

    Clear boundaries, mixture - each with its own properties.

    Stronger and less ductile than solid solutions.

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    Phase DiagramsPure metals have clearly defined melting or freezing points, and solidification takes place at a constant temperature.Tool for understanding the relationship among temperature, composition, and phases present in a particular alloy system. Alloys solidify over a range of temperatures, based on the composition of the mixture.As the alloy cools the mixture begins to freeze, changing gradually to a solid (liquid/solid phases).

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

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    Binary Phase DiagramsCompositionTemperatureLSL+STemperature0% B100% A100% B0% AABComplete Solid Solubility

    Solid Solution - Single PhaseTwo Phases

  • Phase DiagramsAlloys solidify over a range of temperatures

    Liquidus - solidification occurs when the temperature drops below

    Solidus - solidification is complete

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

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    Nickel-Copper Diagram

    (Black & Kohser, 2008, p. 75)

  • Lever RuleUsed to determine the composition of various phases in the phase diagramExample: Copper NickelAt 1288 degrees C, a mixture of solid/liquidSolid is 42% Cu, 58% NiLiquid is 58% Cu, 42 % NiThe completely solidified alloy is a solid solution because Cu completely dissolves in Ni and each grain has the same composition

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    A + LiquidB + LiquidA + BAB0% B100% AEutectic point100% B0% AABTwo-Phase DiagramsLimited solubility

    Two PhasesSolid Solution - Single Phase

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    Two-Phase Lead-Tin Diagram

    (Black & Kohser, 2008, p. 75)

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

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    Iron-Carbon DiagramSolid Phases of the Iron-Carbon DiagramFerrite (-iron)Austenite (-iron)Cementite (iron-carbide)

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    Ferrite (-iron)

    Soft, ductile, magnetic.

    BCC

    Solid solution (0.022% carbon) almost pure iron.

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    Austenite(-iron)

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

    Interstitial Solid

    Solution (2.11% carbon)

    Non-magneticAUSTENITIC MANGANESE STEEL

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    CementiteIron carbide (Fe3C) 6.67% carbon

    Hard & brittle Intermetallic Compound.

  • Fe3C Categories

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  • *Engr 241

    Engr 241

  • Schematic illustration of the microstructures for an ironcarbon alloy of eutectoid composition (0.77% carbon), above and below the eutectoid temperature of 727C (1341F).Eutectoid System

  • *Engr 241

    Engr 241

  • Hardenability and Weldability are influenced by four factors

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    Carbon content

    Weldable .35% C HardenableHeating Cycle maximum temperatureCooling Cycle minimum temperatureSpeed of cooling

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

    Full annealing austenitizing and furnace cool. It is used in low- and medium carbon steels that need extensive machining or plastic deformation

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

    Spheroidizing: improve properties of high-carbon steels.

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    Heat Treatment ProcessesStress Relieving: reduce or eliminate residual stresses.

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

    Hardening: heating and cooling rapidly (quenching)

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

  • Heat treatment ranges*Engr 241

    Engr 241

  • MartensiteNamed after the GermanmetallurgistAdolf Martens(18501914)veryhardform of steel crystalline structure

    formed in carbon steels by the rapid cooling (quenching) ofausteniteat such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to formcementite(Fe3C)*

  • TTT Diagram*Engr 241 Ability of an alloy steel to be hardened by the formation of martensite.

    Engr 241

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

    fine or coarse perlite

    Spheroidite (spherical cementite) - tougher and harder than perlite

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

  • Austempring and Martempering *

  • References George E. Totten,1997, Steel Heat Treatment Handbook , New York: McGraw-Hill,1997.ISBN0-07-042366-0

    Colin J. Smithell, 1990, Metals Reference Book, London : Prentice-Hall International,1991. 634 p. ISBN 0- 13-014502-5

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

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    *To move a dislocation, less energy involved since only one bond is broken. Note, the edge dislocaiton discussed here is only 1 of several different dislocations that have been characterized. This is the basis of Material Science!!