WL 112 Ch11 ch11 presentation

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Metallurgy
Fundamentals
Ferrous and Nonferrous

Heat-Treating Steels for
Strength, Toughness, and
Ductility
Chapter 11

Copyright Goodheart-Willcox Co., Inc. May not be posted to a publicly accessible website.
• Understand the methods for heating and cooling steel in heat
treatment.
• Explain the primary difference in the microstructure of steel cooled
at a moderate rate and at a rapid rate.
• Describe the effect of quenching on macroscopic (human-scale)
properties.
Learning Objectives

• Explain the properties of steel that are most enhanced by the rapid
cooling of steel from above 1400°F (760°C).
• Describe the four stages of cooling that happen when plunging hot
metal into water.
• Explain why brine quenching is the most severe, fastest cooling
possible.
• Understand why some alloy steels are strengthened at slower
cooling rates.
Learning Objectives

• Explain the difference in properties obtained if, instead of quenching
in water, the part is cooled in forced air.
• Understand the microstructural and human-scale benefits of
tempering martensitic steel.
• Understand the potential benefits of martempering and
austempering on properties.
Learning Objectives

• Steel can be heat-treated for strength, toughness, and ductility.
• Heat to high temperature to make austenite, then cool
• Cool at moderate rate or rapidly
• Each cooling rate produces radically different microstructures.
• Rapid cooling produces martensite with much higher strength and
hardness.
Introduction

• Carbon content determines maximum
hardness obtainable in quenched steel.
• Hardness of martensite increases as % carbon
increases.
• Small parts heat and cool uniformly.
• Small parts are this chapter’s focus.
• Parts less than 3/8″ (10 mm) and up to
1′ (0.3 m) long
Composition and Size Matter
Goodheart-Willcox Publisher

• Heat treatment refers to heating and cooling to produce desired
microstructure.
• Typical steel heat treatments
• Steel heated above A1 temperature, producing austenite
• Then cooled along specific temperature path
• This produces desired microstructure.
Heat Treatment

• Involves heating steel parts over lower
critical temperature
• Lower critical is A1 line, approximately
1340°F (727°C).
Heat Treatment—Austenitizing
Temperature

• Hypoeutectoid steels maximize strength when heated above upper
critical temperature.
• Upper critical is A3 line, which varies with carbon content.
• Steels heated to between A1 and A3 partly transform to austenite.
Heat Treatment—Austenitizing
Temperature (cont.)

• Elements change transformation curves slightly.
• Chromium, nickel, manganese, and others
• A1, A3, and Acm transformation temperatures are reported for each
alloy.
• This means different instructions for processing different alloys
• Incorrect heat-treating process may not produce desired results.
Heat Treatment—Effects of Alloying
Elements Besides Carbon

• Heating parts is not instantaneous.
• Small parts loaded in basket heat unevenly.
• Center parts reach temperature more slowly.
• Soak time at temperature is needed to dissolve Fe3C particles.
• Can be minutes or hours
• Depends on part size, size of load, and furnace type
• Center parts should be fully soaked.
Time Needed to Austenitize Steel Parts

• Parts may be cooled slowly, rapidly,
or with delays during cooling.
• Usually, in practice, steel parts are
not processed exactly as expected
by IT diagram.
• IT diagram still shows nearly same
microstructures as production
parts.
Cooling Austenite

• Pearlite nose shifts toward less time for low-carbon
steels.
• Makes heat treatment more difficult
• Steels over 0.8% carbon difficult to cool rapidly
without cracking
• Most commercial heat-treated carbon steels are
0.2%–0.8% carbon.
• No hardening occurs at less than 0.2% carbon.
• No increase in hardening occurs over 0.8% carbon.
Very Low- and Very High-Carbon Steels

Different Heat Treatment Paths

• Heat treatment furnaces are heated with
gas, oil, or electricity.
• Oil-fired furnaces less common since
1970s
• Most heat-treat furnaces run on natural
gas (CH4).
• Clean-burning gas requires less
maintenance
• Some furnaces are heated by electric
resistance heating elements.
Types of Furnaces
Steelwind Industries, Inc, Oak Creek, WI

• Most fuel-fired furnaces today are heated
indirectly.
• Combustion products pass through radiant
tubes.
• Tubes radiate heat to working volume of
furnace.
• Some furnaces inject controlled amounts
of gases like methane.
• Reacts with steel and modifies surface
composition of workpieces
Controlled Atmosphere
ThermTech

• Vacuum furnaces are carefully sealed.
• Pumps remove most air from furnace.
• Pressures as low as 2 millionths of an
atmosphere are used.
• Workpieces in vacuum furnaces do not
develop oxide.
Vacuum Furnace Processing
Solar Atmospheres

• Some furnaces vent combustion products directly into furnace.
• Directly onto heated workpieces
• Concern is to balance efficiency of heating with removing carbon
from workpiece surfaces.
Combustion Products Vented into
Furnace

• Furnaces can have reducing or oxidizing atmospheres.
• Reducing atmospheres prevent surface oxidation.
• Also called endothermic atmosphere
• Remove oxygen and oxidizing gases and use reducing gases
• Oxidizing atmospheres oxidize steel workpieces.
• Also called exothermic atmosphere
• Neutral atmospheres do not affect surface.
Furnace Atmosphere Affects Surface
Composition

• Oxidizing atmospheres have low CO/CO2 ratio.
• Oxygen reacts with carbon near surface of
steel.
• Carbon is removed, forming layer of low-carbon
ferrite.
• This is called decarburizing.
• Has lower surface hardness
• Has black oxide scale on surface
Decarburizing
ThermTech

• CO/CO2 ratio is carefully monitored.
• Adding methane to reducing atmospheres alters
surface.
• Carbon will diffuse into steel parts.
• Increases carbon content near surface
• Reducing atmospheres burn when mixed with air.
• Furnaces use positive pressure, good door seals,
and burn-off tubes.
Preventing Decarburization
ThermTech

• CO/CO2 ratio can be held at a balance point.
• Creates a neutral atmosphere
• Neutral atmosphere will not increase or decrease carbon in steel.
Neutral Furnace Atmosphere

• Stationary furnaces (batch furnaces) do
not move parts during heating cycle.
• Parts put into baskets and moved by
forklift.
• Parts or baskets placed on carts and
wheeled into furnace.
• Door seals maintained to minimize
energy consumption
• Produces desired atmospheric conditions
Stationary Furnaces
ThermTech

• Continuous furnaces move parts on chain-link belts or rollers.
• Parts slowly move through long furnace.
• Multiple zones can heat up, soak, and cool down.
• Zones are separated by interior curtains.
• Hot zone may be elevated.
• Minimizes loss of heated atmosphere
• Cool down done in air or by dumping parts into quench tank
Continuous Furnaces

• Induction heating can heat selected areas of parts.
• Electric current is induced in workpiece.
• Eddy currents quickly heat parts to temperature.
• Very close control of heating and short time are possible.
• Liquid salt baths can precisely heat or cool parts.
• Gas torches are used for small production runs.
• Uniformity and consistency are difficult.
Additional Methods for Heating

• Moderate or slow cooling transforms steel into pearlite.
• Many cooling paths used to do this
• Furnace cooling
• Still air
• Forced air
• Water spray mist (cools slightly faster)
• Run-of-the-mill stock
• Steel taken off hot-roll line and cooled using method chosen by mill
• Shipped in this condition
Moderate Cooling Rate

• Term applies to processes where metal is heated to specific temperature
then cooled at moderate rate.
• Cooling is usually in air.
• Each type of annealing is slightly different.
• Process annealing
• Spheroidizing annealing
• Full annealing
• Blue annealing
• Many others
Annealing

• Heats to just below A1 austenite transformation
temperature
• Dislocation tangles are quickly removed.
• Ferrite recrystallizes.
• Ductility is largely restored.
• Results in less distortion due to lower
temperature
Process Annealing

• Heats high-carbon steel just below A1
temperature
• Soaks for 2 to 10 hours
• Pearlite platelets break up.
• Form small spheres of cementite in ferrite matrix
• Done in sealed or controlled-atmosphere
stationary furnaces.
• Prevents oxidation during long soak times
Spheroidizing Anneal

• Furnace is set so load never exceeds A1 temperature.
• Critical applications require quarterly calibration.
• Plain carbon steel runs between 1240°F and 1275°F (670°C and
690°C).
• No part of load reaches A1 temperature.
• Resulting steel is much more ductile and formable than other
processed steel.
Temperature Control for Spheroidizing

• Heats steel above upper critical (A3)
temperature
• Fully austenitic
• Cools at a moderate rate
• Stationary-furnace and continuous
annealing of strip may be full anneals.
• Uniform elongation and forming are
expected.
Full Annealing

• Used to make large steel parts with a
consistent pearlite microstructure
• Involves heating steel parts into austenite
region, then removing to let them cool in still
air
• Parts are soaked at 1600°F (870°C).
• Replaces cast and hot-worked
microstructures
• Large parts can be cooled on shop floor.
Normalizing

• Forced air cools parts faster than still
air.
• Parts should be spread out.
• Air blast hits them uniformly.
• Variations in air temperature have
little effect on final properties.
Forced Air
Iron Castings Society

• Sometimes very slow cooling is specified.
• Parts left in furnace and cooled few degrees per hour
• Produces very coarse pearlite
• Very good ductility
• No cooling stresses
• Oxidation will occur unless proper atmosphere is used.
Furnace Cooling

• Cooling of parts done two ways in vacuum chamber
• Radiating heat to chamber walls
• Filling chamber with nitrogen after heating cycle
• With no oxygen, there is no oxidation.
• Parts come out as clean as they went in.
Cooling in a Vacuum

• Rapid cooling means cooling to form martensite.
• Cooling from A1 temperature to under Ms temperature
• Avoids formation of pearlite
• Usually means dropping workpiece into liquid quenchants
• Quenchants are near room temperature.
Thermal Processing—Rapid Cooling

• Quenching liquid used is called quenching medium.
• Water
• Water mixed with salt
• Water mixed with polymer
• Oil
• Melted salt or metal (intermediate temperature)
Quenching Media and Techniques

• Water is most common quenching medium.
• Effective
• Inexpensive
• Nonflammable
Water Quenching

• Hot metal turns into steam, forming steam blanket (stage A).
• Steam covers hot metal, dramatically reducing cooling rate (stage B).
• As steel surface cools, steam blanket collapses (stage C).
• Water touches metal surface and boils (stage D).
• Steel temperature drops rapidly.
Four Stages of Water Quenching

• Steam blanket can be disrupted, which speeds up quenching.
• Water jet sprays
• Agitated water
• Dropping small parts into water
• Lessens time at stage B
• Parts cool much faster.
Speeding Up Quench Rate

• Disadvantages include only two cooling
rates.
• Still water
• Agitated water
• Large parts cool slowly in still water.
• Steam blanket effectively shuts off cooling
for tens of seconds.
Summary of Water Quenching

• Most heat-treating operations filter and reuse water in quench tanks.
• Only makeup water is needed.
• Contaminated water is not put into wastewater stream.
• Scale removed from quench tank is recycled.
Water Quenching
Sustainable Metallurgy

• Bath is water with a few percent salt.
• Salt is deposited on hot steel as steam forms.
• Bits of salt spall (chip off), disrupting steam blanket.
• This sharply increases cooling rate.
• Agitated brine solution is fastest quench.
• Equipment in work area corrodes faster.
Brine Quenching

• Some alloy elements delay start of pearlite
transformation.
• These include chromium and manganese.
• Still attain maximum martensite, strength,
and hardness
• Slower cooling with oil
• Less cracking
• Less residual stress
• Most alloy steels are oil quenched.
Oil Quenching
Pavel Nesvadba/Shutterstock.com

• Degraded (spent) quench oils must be disposed of carefully.
• Some operations burn it for building or process heating.
• Spent quench oils can be recycled.
• They can be cleaned and regenerated with fresh additives.
• This reduces atmospheric contamination from burnt oil.
Quenching Oil

• Alloy steel may suffer severe cracks if water quenched.
• Rejection rates will jump as a result.
Importance of Proper Quenching Medium
Practical Metallurgy

• For some alloys, water is too fast and oil is too slow.
• Water-soluble polymer (like polyalkylene glycol) added to water
produces adjustable quench rate.
• Polymers are hydrocarbons.
• Thin film forms on part surfaces as water boils.
• Reduces cooling rate slightly
• Adjust cooling rate by amount of polymer added
• Process control must be tighter than for water.
Polymer Quenching

• Salt baths used for cooling to intermediate temperatures use nitrate
or chloride salts.
• Cooling is faster than air.
• Bath temperature is adjustable.
• Salt is very stable and does not burn.
• Liquid metal can also be used.
Salt Baths and Metal Baths

• Operators must be aware of salt bath hazards.
• Liquid salt can splash and burn operator’s skin.
• Some high-temperature salts react with low-temperature salts.
• Explosions can occur.
• Splashed water will probably cause steam explosion.
• Fume hoods must remove all metal fumes.
Hazards with Salt Baths and Metal Baths
Safety Note

• Mixture of particulate solid material and
fluid
• Hot air pumped into tank of fine sand
• Behaves like quicksand
• Baskets of parts easily settle into sand.
• Parts reach temperature faster than in air
furnaces.
• Removed parts are dry and salt-free.
Fluidized Bed
MHI, Inc.

• Rapid cooling methods can be described in
terms of cooling or heat transfer rate.
• Called heat transfer coefficient (H coefficient)
• Two extremes of cooling rates
• Still air: H = 0.01
• Agitated brine: H = 5.0
H Coefficients: Comparing Cooling Rates

• Factors in addition to quench medium and amount of agitation
• Temperature of quench fluid
• How cooling fluid moves around or through workpieces
• Amount of hot steel quenched at one time
• Must control minor changes to achieve consistent results
• Reduced cooling rate can produce less martensite.
• Less strength and hardness
Factors Influencing Cooling Rate

• If parts do not reach full austenitizing
temperature
• Not all cementite dissolves into austenite.
• Final part strength is reduced.
Effects of Process Variations
ThermTech

• Size of baskets for loading parts
• Must soak long enough for center parts to be properly heated
• Outside parts are hot longer, which can cause grain growth.
• Increases hardness and cracking potential
• Area of parts that cool first may warp.
Effects of Process Variations (cont.)

• Long, straight parts can easily warp.
• Angle of parts entering quench tank
affects warping.
• Special baskets to hold parts help
prevent this.
• Many parts have little allowance for
process changes.
• Technicians and operators must be
alert to any changes.
Minimizing Warping and Other Problems
Goodheart-Willcox Publisher; ThermTech

• As-quenched martensite (fresh martensite)
• High strength and hardness
• Zero ductility and low impact strength
Tempering Martensite

• Toughness obtained by tempering
• Heating one hour between 300°F (150°C) and 1100°F (590°C)
• Internal stress reduced as ductility and toughness increase
• Above 900°F (480°C), yield and tensile strength drop.
Tempering Martensite (cont.)

• As-quenched hardness of alloy steels matches plain carbon steels.
• Tempering is affected by alloying elements.
• Alloys can keep strength of tempered martensite higher.
• Covered in future chapters
Tempering Alloy Steels

• Air furnaces perform most martensite tempering.
• Parts should be evenly spaced to heat equally.
• Must prevent decarburization if:
• Tempering time over one hour
• Tempering temperature over 800°F (430°C)
• Time is less important.
• A little too long is OK.
• Times much longer cause additional softening.
Tempering Equipment and Procedures

Heat-Treating Procedures for Plain
Carbon Steel

Heat-Treating Procedures for Chromium
Alloy Steel

• Two interrupted quenching
processes used for steels
• Either produce bainite or
martensite
• Both produce much less internal
stress than rapid cooling.
Interrupted Quenching

• Reduces warping and cracking due to uneven cooling
• Steel cooled rapidly to below transformation nose
• Held just above Ms temperature
• Air cooled, then tempered like any quenched steel
• UNS G10900 steel: quench + temper vs. martemper + temper
• Tempered to same hardness
• Martempered steel has impact strength doubled.
Martempering

• Produces bainite
• Very fine flakes of cementite in ferrite
• Strength nearly as high as martensite, with better ductility
• Process is more complex (using liquid salt bath).
• Steps after heating
• Cooled rapidly to below transformation nose
• Held between nose and Ms temperature
• Air cooled after allowing time for bainite to form
Austempering

• Steel cutting tools are heat-treated to
tempered martensite.
• High hardness helps retain sharp
cutting edge.
• Blades are used in many applications.
• Shears for cutting annealed or mild
steel
• Professional chef and butcher knives
Applications of Heat-Treated Steels:
Blades
Iroquois Ironworker, Inc.; Dienes USA

• Tooling used to roll form parts from sheet
must resist wear.
• Heat treatment produces required high
hardness.
• Professional-grade hand tool bits are
heat-treated for strength and toughness.
• Darkened surface is black oxide
treatment.
• Shiny bits are usually not heat-treated.
Forming Rolls and Drive Bits
James P. Riser; Chapman MFG

• Small compression springs must have high
fatigue life.
• May be used millions of times in critical-to-
safety applications
• Usually formed while annealed
• Heat-treated to produce high yield strength
Compression Springs
Optimum Spring Corp.

• Hardened steels sacrifice some ductility and impact strength for
increased strength.
• Additional drawbacks exist.
• Martensitic carbon steel has low ductility at low temperatures.
• Heat-treated steels subject to impact loads in winter.
• Use alloy steels for good impact strength at low temperatures.
• Many believe this was partial cause of Titanic disaster.
Drawbacks of Tempered Martensitic
Steels

• Welding hardened steel causes loss of strength.
• Some parent metal in HAZ heats above A1 temperature.
• Heat-affected zone softening and loss of strength occurs.
• This must be considered in welded parts.
• Design for lower strength at weld joints.
• If possible, weld first then heat-treat.
Welding Tempered Martensitic Steels

• Heat treatment helps the environment.
• Allows use of less metal and ensures parts last longer
• Improved properties offset environmental costs.
• Assuming proper disposal or recycling of waste materials
• Changes made to reduce waste will improve on this balance.
The Impact of Heat Treatment

WL 112 Ch11 ch11 presentation

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