WL 112 Ch09 ch09 presentation

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Metallurgy
Fundamentals
Ferrous and Nonferrous

Processing Steel to
Finished Products
Chapter 9

Copyright Goodheart-Willcox Co., Inc. May not be posted to a publicly accessible website.
• Describe the effects of cold-rolling on steel strip, at both the microscopic
and macroscopic scales.
• Explain the effects of annealing on cold-worked steel, at both the
microscopic and macroscopic scales.
• Understand the differences between drawing and stretching steel sheet.
• Understand why steel wire must be drawn multiple times to make small-
diameter wire.
• Understand what a metallurgical bond is.
Learning Objectives

• State the three things necessary to form a metallurgical bond.
• Describe the major advantage of brazing over welding steel.
• Understand why free-machining steels are easier to cut than other
steel alloys.
• Explain why galvanizing steel protects the steel better than tinplate.
• List three ways to protect steel sheet from corrosion due to moisture
in the air.
Learning Objectives

• As metal is reduced in thickness, cold work increases.
• Dislocations in metal allow atoms to slide past one another.
• Metal changes shape without fracturing.
• Dislocation tangles develop.
• Strength increases.
• Elongation is reduced.
Cold-Working Steel

• Steel is not hot-rolled under 3/16″ (4.8 mm)
thickness.
• Too much rough oxide scale at hot-rolling
temperatures
• Thinner gages are made by cleaning hot
strip, then cold-rolling at room temperature.
Cold-Rolled Sheet and Strip
Goodheart-Willcox Publisher

• Steel for cold-rolling is etched in acid (pickled).
• Uncoiled and fed through acid tanks to remove scale
• Washed, dried, and recoiled for cold-rolling
• Iron oxide scale is returned for smelting.
Cold-Rolled Sheet and Strip (cont.)

• Cold-rolling is typically done in four-high
rolling mills.
• Operators adjust process to achieve
desired reduction.
• Roll reduction, rolling speed, and tension
are controlled.
• Each roll reduction is a single pass
through rolls.
Controlling Thickness
JETSADA POSRI/Shutterstock.com

• Noncontact X-ray gauges measure gage thickness.
• Front (nose) or end (tail) of coil may be outside specified thickness.
• Must be cut off and recycled (mill scrap)
Controlling Thickness—Noncontact X-ray
Gauges

• Thickness variation across width is cause for rejection.
• Drawing and forming dies for next step function within narrow range of
thicknesses.
• Mill operators learn to make uniform reductions across width of
strip.
• Minimizes scrap
Importance of Uniform Thickness

• As strip is cold-rolled, force needed to cold work increases.
• Ductility decreases.
• Small edge cracks can form.
• Cracks must be trimmed before further rolling.
• Strip with cracked edges cannot be shipped to customers.
• Trimmed edges become mill scrap.
Work Hardening

• Annealing is used to recover ductility in cold-rolled strip.
• Steel may be process annealed partway between initial gage and
final thickness.
• Usually heated to 1200°F (650°C) or above
• Then air-cooled
• Finally ready to roll to thinner gage
• Resulting microstructure is equiaxed ferrite and pearlite.
Annealing

• Advantage of 1200°F–1300°F (650°C–700°C) process anneal is
simplicity.
• Uses less complex ovens than for higher temperatures
• Disadvantage is longer time required.
• Process is slower at lower temperatures.
• Can take 15 to 24 hours
Lower-Temperature Process Anneal

• Advantage of 1400°F–1600°F (760°C–870°C) process anneal is
speed.
• Only one hour in furnace
• Requires expensive ovens for higher temperatures
• Higher energy input
• Controlled atmosphere
• Good door seals
Higher-Temperature Process Anneal

• Uncoiled steel strip can be annealed in a continuous furnace.
• Steel strip is rapidly heated to over 1400°F (760°C).
• Held at temperature for less than one minute
• Cooled back to room temperature
• Protected by nitrogen-hydrogen atmosphere during annealing
Continuous Furnace Annealing

• Stationary (coil) and continuous (strip) annealing furnace
differences
• Different requirements for operators
• Produce slightly different metallurgical results
• Batch annealing (in coils) produces larger grain size.
• Produces rougher surface after cold forming
• Applications use formed areas not visible to customer.
Stationary vs. Continuous Anneal

• Continuous strip steel annealing
produces smaller grain size.
• Sheet bends more uniformly.
• Has smoother and cleaner
surface
• Less preparation needed for
painting
Stationary vs. Continuous Anneal (cont.)

• Cold-rolled strip
• Shipped as fully or partly cold-worked
and annealed
• May be cut into flat sheet or coiled
• Based on annealing temperature and
time
• Different yield strength
• Different elongation
Semifinished Sheet and Strip
PhotoStock10/Shutterstock.com

• Several things may be specified by
customer.
• Processing and annealing conditions
• Edge condition
• Maximum weight of each coil
• Direction of coil core
• Steel strapping requires high strength but
low formability.
• Shipped in fully cold-worked condition
Cold-Rolled Steel Requirements
Zygalski Krzysztof/Shutterstock.com

• Strip and sheet applications involve
forming operations.
• Bending, stretching, drawing, coining,
or ironing
• Bending stresses below elastic limit will
not permanently change shape.
• Returns to original shape when stress
removed
Forming Sheet by Bending

• Bending stresses at levels above elastic limit cause permanent
change in shape.
• Sheet will return partway to original shape.
• Effect is called springback.
Springback

• Press brake machines bend sheet and plate.
• Setup must overbend slightly to allow for
springback.
• Springback is sensitive to variations in cold
work and annealing.
• May need to adjust press for each coil
• Sheet metal cracks if bent too far.
• Steel supplier provides tables showing
minimum bend radius.
Press Brake
Dmitry Kalinovsky/Shutterstock.com

• Stretching sheet increases surface area and reduces thickness.
• Example of coil cut into 8′ (2.4 m) lengths for flat sheet
• Sheet is stretched between two grips.
• Makes sheet very flat and puts set into it
• It is fixed in that state.
• Flat steel workpieces may be stretched in die to form useful parts.
Forming Sheet by Stretching

• Flat sheet is drawn (pulled) into die cavity.
• No change in thickness
• Common application is home appliances.
• Called “white goods” regardless of painted
color
• Sides and tops made from thin cold-rolled
annealed strip
• Scratches caused by worn forming tools
appear through paint.
• Causes customer rejection
Forming Sheet by Drawing

• Stretcher-strain marks are caused by forming.
• Cannot be removed
• Appear through paint
• Unacceptable for white goods
• Root cause occurs at atomic level.
• When metal is loaded to upper yield stress,
some dislocations break free.
• Forms marks as pinned dislocations move
Stretcher-Strain Marks

• Mild cold work moves dislocations that cause problem.
• Skin pass (temper roll) of sheet can reduce thickness about 2%.
• Bending sheet slightly through series of small rollers in roller leveling
pass brings no thickness change.
• Steel must be used quickly.
• Within about two weeks at room temperature
Preventing Stretcher-Strain Marks

• Thickness of flat workpieces can be changed by coining.
• Coining stamps a punch onto flat sheet.
• Back plate holds sheet in place.
• Impression of punch is permanent.
Forming Sheet by Coining

• Thickness of sheet can be reduced by ironing.
• Sheet is drawn through wedge-shaped ironing
die.
• Compresses and thins it as it is pulled through
• Beverage cans use this method.
• Requires very clean steel
• Uses special lubricants
Forming Sheet by Ironing

• Small-diameter wire is cold-drawn from hot-rolled coiled rod.
• Steel wire must be drawn multiple times to create small-diameter
wire.
Drawn Wire

• Frequently annealed between draws
• Reduces cold work and drawing force required
• Drawing dies for larger diameters are made of hard materials.
• Tungsten carbide
• Diamond for very small diameters
Drawn Wire (cont.)

• Springs undergo many fatigue cycles.
• Every time they are compressed and
released.
• Many springs must withstand millions of
cycles without failure.
• Small surface notches act as stress
risers.
• Technicians must be alert to avoid this.
Drawn Wire for Small Springs
Julian Rovagnati/Shutterstock.com

• For drawing tube, starting material is
extruded tube.
• Tube is drawn through a reducing die.
• An inside plug assures desired inside
diameter and finish.
Drawn Tube

• Room-temperature forging is called cold forging.
• Cold work develops dislocation tangles.
• Strength increases, and amount of deformation
is restricted.
• Metal is often forced into closed dies.
• Workpiece is kept under compressive load to
avoid cracking.
• Rod rolled between threaded rollers or plates
creates threads.
Cold Forging
Tawansak/Shutterstock.com

• Three ways to join pieces together
• Metallurgical bond, glue, or mechanical fasteners
• Metallurgical bond joins metals at atomic level.
• Metallurgical joining includes welding, brazing, and soldering.
• Workpieces (materials to be joined) are called parent metals.
• Metal added to joint is called filler metal.
Joining Steel Parts

• Three requirements for metallurgical bond
• Heat to melt either parent or filler metal
• Disruption of surface oxide of all metals at joint region
• Protection from oxidation or contamination while parts are hot
• American Welding Society (AWS) offers standards and certifications for
welding.
• AWS definition of welding
• Process that melts part of parent metals so liquid forms a metallurgical bond
between pieces, with or without filler
Requirements for a Metallurgical Bond

• Brazing and soldering do not melt parent metal.
• Form metallurgical bond between parent metals with filler metal
• AWS defines brazing as occurring above 840°F (450°C).
• Soldering occurs below 840°F (450°C).
• In all joining processes, parent metal is heated.
• Changes in parent microstructure are common.
Brazing and Soldering

• Metals have surface oxide films that hinder metallurgical bonds.
• When oxide film is disrupted, metal atoms from both sides merge
together.
• Braze or solder fluxes help melted filler metal penetrate oxides.
• Metallurgical bond can form.
Metallurgical Bonds and Fluxes

• Metals to be joined must be compatible.
• If joint serves structural purpose, it must not become brittle.
• Nickel can be safely welded to steel rod.
• Liquid metals mix and form sound metallurgical bond.
• Some metals cannot be joined without becoming brittle.
• Aluminum is not metallurgically compatible with steel.
• Aluminum-to-steel welds immediately form brittle compounds.
Metallurgical Bonds and Compatibility

• Parent metals are melted in area to be joined.
• Flux or cover gas protects hot metal from oxidation.
• Flux or cover gas protects metal from atmospheric moisture.
• H2O is source of hydrogen and oxygen.
• Over 70 welding variations exist to produce combination of heat,
filler, and shielding.
Welding

• Arc welding is most common method
of joining metals.
• Shielded metal arc welding (SMAW)
uses flux-coated electrode.
• Flux melts and shields weld area.
• Electrode (a metal wire) is filler metal.
Arc Welding: SMAW

• Gas metal arc welding (GMAW)
uses cover gas but no flux.
• Gas may be inert (argon or helium).
• Gas mixtures may be chemically
reducing.
• Without flux, there is no slag to chip
off.
Arc Welding: GMAW

• Welders must wear protective clothing and equipment.
• Cotton and leather items are preferred to synthetics.
• Welding arc produces harmful intense light.
• UV radiation damages exposed skin and permanently damages eyes.
• Opaque face shields and dark-tinted glass view ports are required.
• Plastic curtains should surround welding areas to protect anyone
nearby from UV radiation.
Safe Arc Welding
Safety Note

• Gas tungsten arc welding (GTAW) uses cover gas (inert or
reducing).
• Uses nonconsumable tungsten electrode
• Separate filler metal may be used.
• GTAW gives welder great flexibility.
• GTAW requires more training.
Arc Welding: GTAW

• All arc welding methods melt some parent
metal.
• Microstructure near weld is affected.
• Cast microstructure occurs near center of weld.
• HAZ is between cast structure and unaffected
base metal.
• Includes partially melted parent metal
• Recrystallized microstructures found there
• Welds can become brittle there.
Heat-Affected Zone (HAZ)

• High-carbon steels and cast iron may become brittle.
• Welding these requires special procedures.
• Special filler alloys
• Preheating and postheating practices
• Welds may crack during cooling.
• They should be inspected before shipping.
• Nondestructive test methods are often used.
• Dye penetrant tests
• X-ray
• Ultrasonic inspection
Problems in Welding Steel
NDT Specialists, Inc.

• Resistance welding is used to
make small tube on tube mill.
• Thin sheet is roll-formed into
tube.
• Resistance welding joins two
edges using rotating electrodes.
• Tubes can be drawn and
annealed.
Welded Small Tube
aaltair/Shutterstock.com; Reprinted with Permission from Plymouth Tube Co. (www.plymouth.com

• Spot welding is resistance welding small
areas between parts.
• Resistance welding uses metals’
electrical resistance for heat.
• Copper electrodes press metal pieces
together, shielding from air.
Resistance Welding

• Electrical current melts weld joint.
• Amount of current and time must be controlled carefully.
• Electrodes must be clean and dressed.
• Electrodes may only last one shift.
Resistance Welding—Electrical Current

• Parts with cylindrical profile can be friction
welded.
• One part spins and presses against another part.
• Heat generated by friction melts both metals.
• Forcing parts together shields weld area from air.
• Drives out any liquid metal
• Some difficult-to-weld combinations can be
friction welded.
Friction Welding
Manufacturing Technology, Inc.

• Forge bonding (forge welding) is very old process.
• Swords were made by hammering hot blooms together.
• Used multiple iron blooms with different carbon content.
• Modern forge bonding involves hot-rolling different alloys to bond
them into single piece.
• Carbon steel can be sandwiched between slabs of stainless steel
and rolled.
• Finished sheet resists corrosion like stainless steel sheet.
Forge Bonding/Welding

• Brazing can make leak-tight joints quickly and
consistently.
• Valuable for producing fluid containers, such as
radiators
• Braze joints use filler alloy, since parent alloys
are not melted.
• Flux or controlled atmospheres are used.
Brazing
Handy & Harman/Lucas Milhaupt, Inc.

• When filler alloy melts, it wets parent metal.
• Capillary action pulls liquid into joint area.
• Property of liquid to flow and fill spaces due to surface tension
• Creates brazed joints
• Operator must control several things.
• Amount and area of heating
• Flux and filler additions
Brazing and Capillary Action

• Parts can be brazed in controlled-atmosphere furnace.
• No flux is needed.
• Atmosphere must be monitored frequently.
Brazing and Capillary Action (cont.)

• Soldering joins parts without high temperatures.
• Requires suitable flux
• Solder filler wets steel and fills joints like brazing.
• Makes leak-free joints in steel pipe
• Makes permanent electrical connections
• Less strong then other weld joints
Soldering

• Training is required to perform
processes consistently.
• Personnel must know what to
adjust and inspect.
• Welders
• Brazing technicians
• Furnace operators and
technicians
Skilled Metallurgical Bonding

• Gluing parts together with adhesives is an option for product
designers.
• Where two sheets overlap closely for 3/8″ (1 cm) or more, this
method works.
• Advantages include bonding without heat.
• Cleaning and preparation of parent surfaces is critical.
Adhesive Joining

• Sheets can be joined mechanically with bolts, rivets, or screws.
• Heat is not needed.
• Dissimilar materials can be joined.
• Holes may be needed and can allow leakage.
• Mechanical fastener locations are stress raisers.
Mechanical Joining with Bolts

• Steel sheets can be joined by clinching
two sheets together.
• Sheets can be clinched together in less
than a second.
• Joint does not puncture either sheet
and remains leak-free.
• Key disadvantage is visible button
formed on surface.
• Clinching is usually done in areas not
visible to user.
Mechanical Joining by Clinching

• Machining processes can make smooth, precise surface or final
shape.
• Machining, cutting, grinding, and polishing remove metal from
workpiece.
• All involve pressing sharp tool or abrasive against workpiece.
Cutting, Grinding, and Polishing

• Tougher, stronger materials are more difficult to cut.
• ASTM International has machinability ratings for metals.
• Alloy UNS G11120 has 100% machinability rating.
• Metals with high machinability rating do not wear out cutting tools
quickly.
• They show little galling.
• Galling is wear caused by two surfaces rubbing and sticking together.
Machinability of Steel Alloys

• Steels with sulfur or lead have improved
machinability.
• Alloys called free-machining steels
• Iron sulfide and lead globules cause chips to
break easily.
• Small machining chips produced instead of
long curls
• Limitations on use
• Should not hot-work high-sulfur steels
• Should not weld these steels
Free-Machining Steels
Iakiv Pekarskyi/Shutterstock.com

• Grinding wheels do not cut well when clogged with soft metal.
• Grinding wheels for sharpening steel tool bits should never be used
to grind soft metals.
• Aluminum or mild steel embed in a wheel’s surface.
• Prevents wheel from grinding hardened steel tools
• Wheel must be dressed before being used again.
Care of Grinding Wheels
Practical Metallurgy

• Parts polished to smooth, glossy finish with fine polishing grit.
• Polishing powder must not become contaminated.
• With coarser polishing powder
• With small stray metal particles (fines)
• Lapping and burnishing are different polishing procedures.
Polishing

• Steel rusts in typical outdoor air.
• It forms iron oxide (Fe2O3).
• Iron oxide eventually flakes off and forms pits or holes.
• Three ways to reduce corrosion in steel
• Protect with coating
• Connect electrically to more electronegative metal
• Process to be less susceptible
Controlling Corrosion

• Metal coatings can be passive or protective.
• Passive coatings protect steel only by shielding it from air.
• Protective coatings actively protect when steel becomes exposed.
Controlling Corrosion with Coatings

• Tinplate (tin-coated steel) is passive protection.
• If scratched, exposed steel can corrode.
• Nearly 1/10 of all steel produced in US is used for tinplate.
• Most ends up in food packaging.
Tinplate Coatings

• Zinc-coated (galvanized) steel has active
protection.
• Zinc is more reactive and corrodes before steel.
• Galvanizing is usually done at end of production.
• Example: Galvanizing after joining steel parts
into structure
Zinc Coating: Galvanizing
Jay Warner

• Steel is electroplated with corrosion-
resistant chromium.
• Provides shiny surface on steel parts
• Chrome plate for outdoor applications
uses layer of copper and nickel under
chrome.
• Improves adherence
Chrome Plate
streetphotog66/Shutterstock.com

• Years ago, fasteners were commonly plated with cadmium.
• Cadmium metal is toxic to humans.
• Cadmium is now classified as carcinogenic by health agencies.
• Cadmium-plated parts are effectively forbidden by EU and US.
Cadmium Plate
Safety Note

• Food containers can react with food stored in them.
• Need inner coating that does not react with food
• Various organic coatings used, depending on food
• Example: Nonacidic beans use different coating than acidic pineapple.
Organic Coatings

• Galvanic corrosion occurs when dissimilar metals are in contact.
• Corrosion of more reactive metal protects second metal.
• Called sacrificial corrosion
Protecting Steel by Sacrificial Corrosion
Joe Mabel

• Magnesium commonly used to protect steel parts
• Can occur in weldments due to composition variation
• Filler metal should be selected to avoid this problem.
Protecting Steel by Sacrificial Corrosion
(cont.)

• Process variables can change corrosion resistance of parts.
• Through composition changes
• Through microstructure changes in selected areas
• Welded stainless steel is an example.
• Chromium carbides can form during cooling.
• Chromium content is lowered next to grain boundaries.
• Intergranular corrosion can occur.
Protecting Steel through Process
Variables

• Processes discussed in this chapter create by-products (waste).
• Waste may be personally or environmentally hazardous.
• Cold-rolling and forming lubricants
• Smoke and dust from welding and brazing
• Fluxes for brazing
• Solvents for paint or organic coatings with VOCs
• Operations work to reduce negative impact.
Considering the Impact
Sustainable Metallurgy

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