WL 112 Ch05 ch05 presentation

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Metallurgy
Fundamentals
Ferrous and Nonferrous

Physical and Chemical
Properties of Metals
Chapter 5

Copyright Goodheart-Willcox Co., Inc. May not be posted to a publicly accessible website.
• Understand how solidifying metal forms crystalline dendrites that
become grains to form a cast microstructure.
• Understand the development of metal microstructure during hot-
and cold-working.
• Explain why the hot bulk deformation of billets, called hot-working,
improves the properties of the resulting material.
• Name three things in a microstructure that inhibit the motion of
dislocations.
Learning Objectives

• Understand why inhibiting the motion of dislocations increases the
strength of a metal.
• Describe how cold-worked metal can be restored to a dislocation-
free condition.
• Explain why annealing improves the ductility of cold-rolled metals.
• Describe the three types of metallurgical bonding in major use.
• Understand why the corrosion protection of steel is important.
Learning Objectives

• As liquid metal cools, it solidifies (liquid becomes solid).
• Liquid metal has amorphous (shapeless) structure.
• Solid metal is crystalline, with repeating pattern.
• Determined by metal and temperature
• Microstructure that forms depends on
• Composition
• Amount and temperature of deformation
• Time-temperature history
Microstructure of Metals

• Most steel is poured in continuous casting machines.
• Molds are water-cooled.
• Causes liquid steel to solidify quickly
• Solid steel is cut to length into billets.
Solidifying Metal—Formation of Cast
Structure
Goodheart-Willcox Publisher

• Single-crystal dendrites grow from mold
walls.
• Dendrites resemble treelike structures.
• Dendrites grow until they become grains.
• Size of dendrites depends on cooling rate
in mold.
• Slower cooling produces larger metal
dendrites.
• Faster cooling produces smaller metal
dendrites.
Crystals, Grains, and Dendrites

• Cross sections show how dendrites grow.
• Start on mold wall and grow inward
• Dendrites form large grains in cast steel.
• Near melting point, steel has face-centered
cubic (fcc) structure.
• This is called austenite.
• Austenite contains some carbon.
• May be in solution
• May be separate particles of iron carbide
Grain Structure in Continuous Cast Steel

• Continuous cast metal must be
reduced in size.
• Hot work is work done with metal
while above temperature where
crystals re-form quickly.
• Steel hot-worked at about 2100°F
(1150°C)
Structures Formed by Hot-Working
strekalova/Shutterstock.com

• Rolling is most common working process.
• Steel passed through series of large rolls that reduce size
• Also includes extrusion and forging
Hot-Working: Rolling

• Large grains are deformed during hot-working.
• Each crystal has slip planes of atoms.
• Slip planes allow atoms to easily slide past each other.
• The fcc structure has many possible slip planes.
Hot-Working and Atomic-Level Slip

• At hot-working temperatures, disrupted atoms quickly re-form into
new crystals.
• This process is called recrystallization.
• Dynamic recrystallization occurs while metal is being formed.
• The metal recrystallizes as fast as deformation occurs.
Recrystallization during Hot-Working

• After considerable hot work, small uniform
grain size obtained
• Properties are much improved from
original cast structure.
• More uniform
• Higher strength
• Improved ductility
• Ductility is metal’s ability to bend and form
without cracking or tearing apart.
Uniform Grain after Hot-Working

• After further size reduction, steel cools.
• Called hot strip
• While cooling, steel undergoes further
transformations.
• Below about 1341°F (727°C), austenite
transforms into ferrite.
• Most carbon combines with iron to form
compound called cementite.
Structures Formed during Cooling
Allied High Tech Products, Inc.

• Cementite is hard and wear-resistant.
• Iron carbide and ferrite together are
called pearlite.
• Clusters of pearlite are called pearlite
islands.
• Pearlite’s layered structure visible at
high magnifications
Cementite and Pearlite
Allied High Tech Products, Inc.

• Pickling often done after cooling to room temperature
• Metal is dipped into acid bath to remove oxide scale and surface
impurities.
• Oxide-free metal is uncoiled and rolled between large rollers until
reduced to smaller thickness.
• Produces very smooth, shiny surface
• Forging or rolling while near room temperature is called cold work.
Structures Formed by Cold-Working

• Strength of metal increases during cold work.
• Ductility drops.
• Further reduction may cause cracking.
Properties of Cold-Worked Metal

• Ferrite grains are not perfect bcc arrays.
• Some planes of atoms extend only
partway through grain.
• Edge dislocations are one imperfection in
crystal lattice.
• Dislocations can be seen with
transmission electron microscope (TEM).
Dislocations
magnetix/Shutterstock.com

• If forces push lattice edge, dislocations can shift.
• Changes shape of metal by one atomic spacing (about 0.28 nm)
• Occurs in all metals
Dislocations and Work Hardening

• Edge dislocations can be stopped, or pinned, by obstructions.
• Force required to move next dislocation increases as they pile up
near each other.
Pinned Dislocations

• If pushed hard enough, dislocation moves off at angle to first.
• Forms second type of dislocation, called screw dislocation
• Difficult to move
• Strength increased
Pinned Dislocations (cont.)

• Commercial alloys usually not pure
• Additional elements with different-size atoms distort space lattice
• Elements may form compounds.
• These second-phase particles block dislocations.
• Example: iron carbide in steel
• Add to force required to move dislocations
• Increase strength of alloy
Inhibitors to Dislocation Motion:
Second-Phase Particles

• Dislocations also have difficulty moving past grain boundaries.
• Grain orientation differs from one grain to next.
Grain Boundaries

• When a second slip plane crosses an earlier one, dislocation motion
on both is impeded.
• Increases force needed to move them
• When many dislocations encounter crossed slip planes, they form a
dislocation tangle.
• Force required to deform metal is much greater.
Tangles

• Strength of metal increases.
• Ductility decreases.
• Further reduction becomes more difficult.
• Dislocations at nanometer scale form tangles that prevent further
deformation.
Dislocation Effects on Metal
Performance

• Cold-worked metal cannot be worked.
• Annealing is used to restore ductility and workability.
• It is a high-temperature treatment.
• It may take just a few minutes at hot-work temperatures.
• It may take hours at lower temperatures.
• During annealing, grains disrupted with tangles recrystallize into a
crystal structure with no tangles.
Recovery of Cold-Worked Metal—
Recrystallization

• Recrystallization temperature depends in
part on composition and amount of cold-
working.
• Commercial grades of metals recrystallize at
slightly higher temperatures.
• Compare carbon steel with pure iron.
• Plain carbon steel recrystallizes at 1250°F
(680°C).
• Laboratory-purity iron recrystallizes at 842°F
(450°C).
Recrystallization Temperature

• Low-carbon steel goes through series of forming steps.
• Continuous cast
• Hot-rolled strip
• Cold-rolled strip
• Annealed at a thickness of around 0.060″ (1.52 mm)
• Further cold-rolled to 0.040″ (1.02 mm)
• Annealed again before shipment to auto part fabricators
Making Auto Body Sheet Steel

• Smaller grain size increases strength.
• Cast billets that are not rolled or forged
have larger grains.
• May have less strength and ductility
Effect of Grain Size on Properties
Thomas E. Hoffman

• Physical properties can be measured
without applying force.
• Melting point is a physical property
measured by heating a
sample until it melts.
• Density is mass per volume.
• Controlled by atomic mass of atom and
atomic spacing
• Density of a perfect single crystal is called
theoretical density.
Physical Properties of Metals

• Ways materials respond to heat are their thermal properties.
• Important when working with and producing metal parts
• Key thermal properties
• Specific heat capacity
• Thermal expansion
• Thermal conductivity
• Melting point
• Heat of fusion
Thermal Properties

• Specific heat capacity is the heat
required to raise the temperature of a
unit mass of the material by one
degree.
• Units are Btu/lb⋅°F or J/kg⋅°K.
• Important factor when heating large
amounts of metal
Specific Heat Capacity

• When metal is heated, it increases in size.
• Size change is called thermal expansion.
• Coefficient of thermal expansion is the
change in length of a material for each
unit change in temperature.
• Different materials expand at different
rates.
• Units are μin/in⋅°F or μm/m⋅°C.
Thermal Expansion

• Machine shops doing precision
work show part dimensions for 68°F
(20°C).
• Machinists determine actual
machined dimensions for the shop
temperature.
• Use the following equation: L(part
temperature) = L(68°F) + ΔL
Calculating Length Changes
with Thermal Expansion Coefficients

• 28″ long steel rod with grooves to be cut on each end
• Shop/part temperature is 86°F
• Distance between grooves must be 27.5710″ at 68°F
• Alloy’s coefficient of thermal expansion is 6.28 × 10−6
• L(86°F) = 27.5710 + (6.28 × 10−6) × 27.5710 × (86 − 68)
• = 27.5710 + 0.0031
• = 27.5741″ distance between each cut
Accounting for Thermal Expansion
Practical Metallurgy

• Metals have high thermal
conductivity compared to
nonmetals.
• Very useful for heat exchangers
• Copper and aluminum have very
high conductivity.
Thermal Conductivity

• Pure metal elements change from
liquid to solid at single, exact
temperature.
• Alloys usually transform from liquid to
solid over temperature range, called
freezing range.
• Slush (solid and liquid) in this range
• Hot-working must be done below this
range.
Melting Point

• Specific heat of fusion (heat of fusion)
is amount of thermal energy required to
change solid to liquid metal when
melted.
• When melting scrap steel into new steel
• Three-fourths of input energy heats it to
melting point
• The rest melts it without a temperature
change
• Units are Btu/lb or J/kg.
Heat of Fusion

• Metals, such as silver, copper, and
aluminum, are excellent
conductors of electricity.
• Alloys for electrical conductors
have a minimum of additional
elements.
Electrical Conductivity

• Ferromagnetism is property that causes some materials to form
magnets or be attracted to magnets.
• Iron, cobalt, and nickel exhibit this property at room temperature.
• Some alloys have this property.
• Ability to be magnetized is called magnetic susceptibility.
• Materials lose their ferromagnetism if heated above their Curie
temperature.
Magnetic Properties

• Joinability is ease or difficulty of attaching two
pieces of metal together.
• Joining processes include welding, brazing,
soldering, and other metallurgical bonding
methods.
• To join, metals must form metallurgical bond.
• A metal oxide layer can prevent bonding.
• Flux, shielding gas, or vacuum help avoid this
problem.
Joinability
Jay Warner

• Weldability is the ability of a material to be welded.
• A weld is like a small casting.
• Grains are smaller than most castings.
• Grains are larger than worked metals.
• Properties of weld may differ from surrounding metal.
• Alloys with a long freezing range may crack on cooling.
• These alloys are more difficult to weld.
Welding

• Brazing joins by melting a filler metal
but not parent metal.
• When joining occurs below 800°F
(430°C), this is soldering.
• Products that cannot withstand high
temperatures can be joined this way.
Brazing and Soldering
Kurtz Ersa

• Hammer welding (old process) is like modern roll bonding.
• Aluminum powder reacting with iron oxide produces molten iron to
join steel products.
• Adhesive bonding
• Diffusion bonding
Other Forms of Joining

• Machinability is the ease or difficulty of
cutting metal.
• A rating system is used to compare
different alloys.
• Free-machining alloys have been
developed for better machinability.
• When machined, smaller chips are
produced.
• These alloys have reduced ductility.
Machinability

• Key chemical properties include corrosion and electrolysis.
• Corrosion is usually undesirable.
Chemical Properties of Metals

• Most metals corrode when exposed to moist air or immersed in
water.
• Return to their natural oxidized condition
• Corrosion resistance is ability of metal to remain in metallic
condition.
• Most common form of corrosion is surface corrosion.
• Metal oxide forms evenly across surface of metal.
• Rust (iron oxide) is example.
Corrosion

• Some metals resist corrosion very well.
• Special surface coatings are used to reduce corrosion in steel.
• Zinc
• Tin
• Organic coatings
Corrosion Protection

• Metals with high potential to corrode are
more electronegative than metals with less
tendency to corrode.
• If two dissimilar metals make electrical
contact in conductive medium, more
electronegative metal corrodes first.
• Protects less electronegative metal
• Done intentionally, this is sacrificial corrosion.
• Galvanized steel has been coated with zinc
for this reason.
Galvanic Corrosion
Joe Mabel

• In some alloys and processing procedures, grain boundaries have
different composition than rest of each grain.
• Intergranular corrosion can occur along grain boundaries when
boundaries are more electronegative than grains.
Intergranular Corrosion

• At high temperatures in air, oxidation occurs.
• Steel heated over 900°F (480°C) forms black oxide scale.
• This oxide is magnetite (Fe3O4), not the common reddish-brown
hematite (Fe2O3).
• Both oxides flake off easily, offering little protection.
High-Temperature Oxidation

• Highly electronegative metals placed in electric circuit with suitable
electrolyte produces electrical power.
• Basis of batteries
• Rechargeable batteries
• Lead-acid batteries for automobiles
• Lithium-ion batteries for many applications
Electrolysis

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