Section 3.1Section 3.1Introduction to MetalsIntroduction to Metals

Rev. 4Rev. 4

8/27/018/27/01

OverviewOverview

SolidificationSolidification Changing Metals in the Solid StateChanging Metals in the Solid State Recovery and RecrystallizationRecovery and Recrystallization Heat Treatment of SteelHeat Treatment of Steel Heat Treatment of Nonferrous MetalsHeat Treatment of Nonferrous Metals Corrosion & Corrosion ProtectionCorrosion & Corrosion Protection

Part 1 - SolidificationPart 1 - Solidification

Energy States of the AtomEnergy States of the Atom

High EnergyHigh Energy– Gaseous state;Gaseous state;– Repelling forces predominate;Repelling forces predominate;– Atoms tend to move apart.Atoms tend to move apart.

Energy States of the AtomEnergy States of the Atom

Moderate EnergyModerate Energy– Liquid state;Liquid state;– Atoms assume fixed distances (equilibrium Atoms assume fixed distances (equilibrium

is reached);is reached);– Materials have fixed volume.Materials have fixed volume.

Energy States of the AtomEnergy States of the Atom

Low EnergyLow Energy– Solid State;Solid State;– Mobility of atoms decrease;Mobility of atoms decrease;– Metals occur as crystalline solids.Metals occur as crystalline solids.

NucleationNucleation

Since the metal at the mold wall is Since the metal at the mold wall is coolest, nucleation starts first at the mold coolest, nucleation starts first at the mold walls.walls.

Each nuclei becomes a point of nucleation Each nuclei becomes a point of nucleation for the growth of a crystal. for the growth of a crystal.

Due to interference with neighboring Due to interference with neighboring crystals, these crystals take on a crystals, these crystals take on a columnar or dendritic shape, growing columnar or dendritic shape, growing inwards towards the center of the melt. inwards towards the center of the melt.

Crystal LatticesCrystal Lattices

Common crystalline structures:Common crystalline structures:– Body-centered cubic,Body-centered cubic,– Face-centered cubic, andFace-centered cubic, and– Hexagonal close-packed.Hexagonal close-packed.

The existence of more than one crystal The existence of more than one crystal structure in a metal is termed allotropy.structure in a metal is termed allotropy.

Grain BoundariesGrain Boundaries

When dendrite growth is restricted by When dendrite growth is restricted by an adjacent dendrite, the result will be an adjacent dendrite, the result will be the formation of a grain boundary.the formation of a grain boundary.

Atoms are perfectly lined up within each Atoms are perfectly lined up within each dendrite or grain, but the direction of dendrite or grain, but the direction of alignment varies between dendrite.alignment varies between dendrite.

Grain SizeGrain Size

Grain size is an important factor in Grain size is an important factor in determining mechanical properties such determining mechanical properties such as hardness, strength, and ductility. as hardness, strength, and ductility.

Dependent upon:Dependent upon:– Rate of nucleation, andRate of nucleation, and– Rate of growth.Rate of growth.

Part 2 - Changing Metals in Part 2 - Changing Metals in the Solid Statethe Solid State

Elastic Deformation Elastic Deformation

Load does not stress the material past Load does not stress the material past its elastic limit.its elastic limit.

Material returns to its original position Material returns to its original position upon removal of the load.upon removal of the load.

Plastic FlowPlastic Flow

Load exceeds the elastic limit.Load exceeds the elastic limit. Material des not completely return to its Material des not completely return to its

original position when the load is original position when the load is removed.removed.

Permanently deformed by plastic flow Permanently deformed by plastic flow within its crystalline structure.within its crystalline structure.

Plastic DeformationPlastic Deformation

Permanent deformation occurs in three Permanent deformation occurs in three ways:ways:– Slip,Slip,– Twinning, and/orTwinning, and/or– Rotational Deformation.Rotational Deformation.

SlipSlip

Sliding between the atomic planes Sliding between the atomic planes within a grain.within a grain.

Planes of greatest atomic population Planes of greatest atomic population and greatest distance are most subject and greatest distance are most subject to slip.to slip.

TwinningTwinning

Loads applied suddenly.Loads applied suddenly. Grain deforms by twisting or reorienting Grain deforms by twisting or reorienting

a band of adjacent lattice forms.a band of adjacent lattice forms.

Rotational DeformationRotational Deformation

Rotational deformation of portions of Rotational deformation of portions of crystal lattice.crystal lattice.

Fibering occurs after a large percentage Fibering occurs after a large percentage of grains has been reoriented.of grains has been reoriented.

Part 3 - Recovery and Part 3 - Recovery and RecrystallizationRecrystallization

Recovery (stress relief)Recovery (stress relief)

Rearrangement of some of the more Rearrangement of some of the more strenuous dislocations or imperfections.strenuous dislocations or imperfections.

Although the changes that take place Although the changes that take place have little or no effect on the external have little or no effect on the external forms of the crystal or grains, they have forms of the crystal or grains, they have a marked effect on some properties.a marked effect on some properties.

Electrical properties and corrosion Electrical properties and corrosion resistance are improved and residual resistance are improved and residual stresses are reduced.stresses are reduced.

RecoveryRecovery

Temperature chosen will be dependent Temperature chosen will be dependent upon the metal, and to some extent on upon the metal, and to some extent on the amount of cold work that has been the amount of cold work that has been performed previously.performed previously.

Objective is to regain electrical and Objective is to regain electrical and chemical properties without sacrifice of chemical properties without sacrifice of mechanical properties.mechanical properties.

RecrystallizationRecrystallization

Recovery of ductility from a material that Recovery of ductility from a material that has been cold worked can be obtained has been cold worked can be obtained only by elimination of deformed grains.only by elimination of deformed grains.

Takes place by nucleation of new grains Takes place by nucleation of new grains mainly about the high energy points of mainly about the high energy points of dislocation in a work hardened grain.dislocation in a work hardened grain.

These growths continue until they fill the These growths continue until they fill the old grain space and eliminate the old grain space and eliminate the existing strain of the atoms into a new existing strain of the atoms into a new crystal lattice.crystal lattice.

RecrystallizationRecrystallization

Occurs over a wide temperature range Occurs over a wide temperature range with the length of time required for with the length of time required for complete recrystallization inversely complete recrystallization inversely related to the temperature and to the related to the temperature and to the degree of strain present.degree of strain present.

Grain GrowthGrain Growth

If a metal is kept heated at or below its If a metal is kept heated at or below its recrystallization temperature after the recrystallization temperature after the new, unstrained grains have formed, the new, unstrained grains have formed, the tendency is for some of the new grains tendency is for some of the new grains to absorb others and grow to a larger to absorb others and grow to a larger size.size.

Grain GrowthGrain Growth

During processing, large grains are During processing, large grains are desired for:desired for:– greater ductility,greater ductility,– better machinability, andbetter machinability, and– less pressure for deformation.less pressure for deformation.

Final product should be of relatively fine Final product should be of relatively fine structure.structure.

Part 4- Heat Treatment of Part 4- Heat Treatment of SteelSteel

Heat Treatment of Steel Heat Treatment of Steel

Definition:Definition:– Intentional heating and cooling for control Intentional heating and cooling for control

of properties.of properties.– Unintentional heat transfer from a process Unintentional heat transfer from a process

such as welding or from service such as a such as welding or from service such as a gas turbine is also of importance.gas turbine is also of importance.

Heat- Treatment ProcessesHeat- Treatment Processes

Places the material in either a complete Places the material in either a complete or approximate equilibrium energy or approximate equilibrium energy condition.condition.

Processes include:Processes include:– Austenitizing,Austenitizing,– Annealing,Annealing,– Normalizing, andNormalizing, and– Spheroidizing.Spheroidizing.

AustenitizationAustenitization

Steel is heated to or above its critical Steel is heated to or above its critical temperature, and held for a period of time. temperature, and held for a period of time.

Carbon unites in solid solution with iron in the Carbon unites in solid solution with iron in the gamma or face-centered cubic lattice form.gamma or face-centered cubic lattice form.

With the formation of this new lattice, grain With the formation of this new lattice, grain refinement occurs, leaving new small grains, refinement occurs, leaving new small grains, if the temperature is not raised too high or if the temperature is not raised too high or maintained too long.maintained too long.

AnnealingAnnealing

Purpose of full annealing is to decrease Purpose of full annealing is to decrease hardness, increase ductility, and hardness, increase ductility, and sometimes improve machinability.sometimes improve machinability.

Also used to relieve stresses, refine Also used to relieve stresses, refine grain size, and promote uniformity of grain size, and promote uniformity of structure.structure.

NormalizingNormalizing

Similar to annealing, although steel is Similar to annealing, although steel is not reduced to its softest condition.not reduced to its softest condition.

Used to refine grain size, relieve internal Used to refine grain size, relieve internal stresses, and improve structural stresses, and improve structural uniformity and ductility.uniformity and ductility.

SpheroidizingSpheroidizing

Usually performed on normal steel.Usually performed on normal steel. Iron carbide forms in small spheres or Iron carbide forms in small spheres or

nodules in a ferrite matrix.nodules in a ferrite matrix. Improves machinability of high carbon Improves machinability of high carbon

steels and/or to pre-treat hardened steel steels and/or to pre-treat hardened steel to help produce greater structural to help produce greater structural uniformity.uniformity.

Part 5- Heat Treatment of Part 5- Heat Treatment of Nonferrous MetalsNonferrous Metals

Heat Treatment- Nonferrous MetalsHeat Treatment- Nonferrous Metals

Age/Precipitation HardeningAge/Precipitation Hardening– A process of increasing hardness and/or A process of increasing hardness and/or

strength by the precipitation of particles strength by the precipitation of particles from a supersaturated solid solution alloy.from a supersaturated solid solution alloy.

Heat Treatment- Nonferrous MetalsHeat Treatment- Nonferrous Metals

The hardening cycle usually consists of:The hardening cycle usually consists of:– Solution Heat Treating: heating or Solution Heat Treating: heating or

annealing at a temperature sufficiently high annealing at a temperature sufficiently high to maintain solid solution.to maintain solid solution.

– Quenching: rapid cooling to retain the Quenching: rapid cooling to retain the super saturated solution, andsuper saturated solution, and

– Aging: subsequent heating at or slightly Aging: subsequent heating at or slightly above room temperature to refine above room temperature to refine precipitant particle size and spacing.precipitant particle size and spacing.

Part 6 – Corrosion & Part 6 – Corrosion & Corrosion ProtectionCorrosion Protection

CorrosionCorrosion

Deterioration of metals by the chemical Deterioration of metals by the chemical action of some surrounding medium action of some surrounding medium (may be liquid or gas).(may be liquid or gas).

May occur through:May occur through:– direct chemical action, direct chemical action, – electrolytic reaction, or electrolytic reaction, or – a combination of the two.a combination of the two.

Direct Chemical ActionDirect Chemical Action

Used to describe those reactions where Used to describe those reactions where coupled anodes and cathodes in an coupled anodes and cathodes in an electrolyte are not identifiable.electrolyte are not identifiable.

Best illustrated by:Best illustrated by:– Chemical milling, andChemical milling, and– Steel pickling.Steel pickling.

Electrochemical ReactionElectrochemical Reaction

Involves the flow of an electric current Involves the flow of an electric current between two electrodes:between two electrodes:– Anode (positive) andAnode (positive) and– Cathode (negative).Cathode (negative).

Corrosion RateCorrosion Rate

Determined by the:Determined by the:– position in the electrochemical series of the position in the electrochemical series of the

metal(s);metal(s);– presence of residual stresses in the presence of residual stresses in the

metal(s);metal(s);– electrolyte present; andelectrolyte present; and– atmosphere.atmosphere.

Corrosion TypesCorrosion Types

GeneralGeneral PittingPitting IntercrystallineIntercrystalline

Corrosion ProtectionCorrosion Protection

Can be improved by:Can be improved by:– selection of most suitable metals;selection of most suitable metals;– controlling the presence of corroding controlling the presence of corroding

media;media;– coating the metal (metals, chemicals, coating the metal (metals, chemicals,

organic materials, or plastics); or organic materials, or plastics); or – stress relieving.stress relieving.