3. Copyright Goodheart-Willcox Co., Inc. May not be posted to a publicly accessible website.
• Understand how the Fe-C phase diagram describes the phases
present in iron-carbon alloys.
• Understand how different cooling paths in steel produce different
microstructures.
• Define by examples the difference between “phase” and
“microstructure.”
Learning Objectives
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• Explain why UNS G10200 (Fe with 0.2% C), G10800 (Fe with
0.8% C), and G10950 (Fe with 0.95% C) steels develop different
microstructures with the same moderate cooling rate.
• Understand the difference between moderate cooling and rapid
cooling in terms of the isothermal transformation diagram.
• Discuss the different microstructures developed in carbon steel by
slow cooling and very fast cooling to room temperature.
Learning Objectives
5. Copyright Goodheart-Willcox Co., Inc. May not be posted to a publicly accessible website.
• Describe the major properties resulting from the microstructures
developed by moderate, interrupted, and rapid cooling of UNS
G10800 steel from 1500°F (816°C) to room temperature.
• Understand why tempering improves the toughness of martensitic
steel.
• Understand how a spheroidizing anneal changes pearlite
microstructure, and why this microstructure is easily formable.
Learning Objectives
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• Fe-C phase diagram (graph) includes alloy composition,
temperature, and phases for steels.
• Three factors influence microstructure of any steel.
• Composition, upper hold temperature, and cooling rate
• Influence of cooling rate requires diagram that includes time.
• Isothermal transformation (IT) diagrams show results of cooling rates.
• Show different microstructures based on cooling rates
• Different microstructures result in different properties.
Understanding Steel Phases
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• Designers specify properties needed.
• Engineers select process to achieve microstructure and properties.
• Technicians and operators process metal through correct cycles.
• Keeping process conditions within production tolerances
• Understanding relationships between composition, process,
microstructure, and properties
• Recognizing undesired process changes
• Making needed adjustments
Obtaining Specified Properties in
Product
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• Antifreeze mixed into water is a solution.
• Solution will freeze or not, depending on its composition and
temperature.
• If cold enough, combination freezes into mixture of solid ice and
water-antifreeze solution.
Solutions and Mixtures—Water and
Antifreeze
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• Glycol added to water lowers freezing temperature.
• Table on antifreeze container shows how much antifreeze to add.
Water-Antifreeze Solution
Goodheart-Willcox Publisher
10. Copyright Goodheart-Willcox Co., Inc. May not be posted to a publicly accessible website.
• Phase diagrams show results of concentration and temperature.
• For single phase
• For combination of two phases
• Boundaries between phases are seen by changes in properties.
• Such boundaries are called phase boundaries.
• Red line in pictured graph is phase boundary.
Phase Diagrams
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• People moving from Texas to North
Dakota should consider effects of
temperature and composition on car
cooling system.
• Water-glycol phase diagram shows how
to increase percent glycol in cooling
system for cold North Dakota winters.
Antifreeze—From Texas to North Dakota
Practical Metallurgy
Goodheart-Willcox Publisher
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• Pure iron transforms to several phases when
heated from room temperature to its boiling point.
• Five possible phases of iron
• Solid, three different types
• Liquid
• Gas (usually not shown for metals)
• With one variable, temperature, transformations
can be shown on single axis.
Iron-Carbon Phase Diagram
Goodheart-Willcox Publisher
13. Copyright Goodheart-Willcox Co., Inc. May not be posted to a publicly accessible website.
• Adding carbon changes transformation temperatures.
• Composition and temperature for phases strongly affect formation of
different microstructures.
• Microstructure strongly affects final properties.
• Y-axis shows temperature, x-axis shows composition.
• Composition from pure iron (0% C) to cementite (Fe3C, 6.67% C),
using weight percent.
Iron-Cementite Phase Diagram
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Iron-Cementite Phase Diagram (cont.)
Goodheart-Willcox Publisher
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• Liquid iron-carbon alloys form cast iron or
steel upon cooling.
• Division indicated by vertical line at 2.14%
carbon.
• Form cast iron with 2.14% to 6.67% carbon.
• Made of ferrite and large cementite particles
• Form steel with 0.022% to 2.14% carbon.
• Made of ferrite and fine cementite
Phase Diagram Regions Important to
Processing
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• Most steel alloys contain less than 1.0% carbon.
• Processing is done up to 2200°F (1200°C).
• Three transformation lines, A1, A3, and Acm, are important.
• Below A1 line, 1341°F (727°C), steel is ferrite and cementite.
• Above A1 line, steel is austenite (partially or completely).
• Above A3 line, steel is 100% austenite.
Steel Portion: Less Than 2.14% Carbon
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Steel Portion: Less Than 2.14% Carbon
Phase Diagrams
Goodheart-Willcox Publisher
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• Composition and temperature where
three lines touch is called eutectoid
point.
• Occurs at 1341°F (727°C)—called
austenitizing temperature
• Occurs at 0.77% carbon
Eutectoid Point
Goodheart-Willcox Publisher
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• Heated to 2200°F (1200°C) for forging
(point 1)
• Steel is 100% austenite.
• Below 1550°F (840°C), crosses A3 phase
boundary line (point 2)
• Steel becomes two phases, ferrite and
austenite.
• Cooling further crosses A1 phase boundary,
entering another two-phase region (point 3)
• Steel now becomes ferrite plus cementite.
UNS G10200 Steel (AISI 1020, 0.2% C)
Goodheart-Willcox Publisher
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• Cooled from 1800°F (980°C), it passes below
A3 temperature at about 1500°F (820°C).
• Some austenite transforms into ferrite.
• Ferrite cannot hold 0.2% carbon in solution.
• Carbon goes into remaining austenite.
• At 1341°F (727°C), A1 temperature, all
remaining austenite transforms to ferrite and
cementite.
UNS G10200 Steel (AISI 1020, 0.2% C)
(cont.)
Goodheart-Willcox Publisher
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• At room temperature, G10200 is ferrite and
islands of pearlite.
• Called hypoeutectoid alloy
• These are steels with less carbon than eutectoid
(0.77% C).
• Applications are based on properties.
• Good formability, machinability, and weldability
• Strength and wear resistance are low.
UNS G10200 Steel/Hypoeutectoid Alloys
ASM International
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• UNS G10800 steel (0.8% C) at point 4 is
all austenite.
UNS G10800 Steel
Goodheart-Willcox Publisher
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• UNS G10800 steel cooled to point 5 is all
pearlite.
• Used for railroad rails to withstand wear and
have high toughness
• “Grade 900” has 0.8% C and is completely
pearlite.
• Must be carefully welded and should not cool too
fast after welding
• Also used for high-strength bridge cables
UNS G10800 Steel (cont.)
ASM International
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• When cooled from point 6 to below Acm
line, cementite particles form along
austenite grain boundaries.
• When cooled to point 8, remaining
austenite forms pearlite.
• Called hypereutectoid alloy
• Carbon composition is greater than
eutectoid 0.77% C.
UNS G10950 Steel/Hypereutectoid Alloy
Goodheart-Willcox Publisher
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• Slow cooling forms large grains with
wide cementite layers.
• Coarser cementite reduces elongation
and ductility.
• Slightly faster cooling produces thinner
cementite layers.
• Better ductility
• Improved hardness and wear resistance
UNS G10950 Steel/Hypereutectoid Alloy
(cont.)
ASM International
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• Skid plates on snowplows and
earthmoving equipment.
• High abrasion resistance needed
• Rod mills use long rods to grind ore as
they tumble.
• Rods need wear resistance to grind
rock-size minerals to powder.
UNS G10950 Steel Applications
Goodheart-Willcox Publisher
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• Many terms used to describe steels based on
carbon content
• Common usage defines three general
categories of carbon steel.
• Low carbon
• Medium carbon
• High carbon
• Categories are useful in practice.
• Each responds differently to cold work and
heat treatment.
Low-, Medium-, and High-Carbon Steels
Goodheart-Willcox Publisher
28. Copyright Goodheart-Willcox Co., Inc. May not be posted to a publicly accessible website.
• Phase diagrams apply only to slow to moderate cooling rates.
• Differences in cooling can change transformations.
• Pearlite lamellar spacing
• Ferrite grain size
• Other phases can form.
• Mechanical properties can drastically change.
Effect of Cooling Rate on Structure and
Properties
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• Soaking means holding workpiece at constant temperature until
desired structural changes take place.
• Quenching means thrusting workpiece into liquid bath to rapidly cool
it to bath temperature.
• Liquids commonly used include water, salt, or oil.
• Salt bath is melted salt that can be heated to range of temperatures.
Slow to Moderate Cooling Methods
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• Small sample of UNS G10800 steel
quenched from 1500°F (820°C) into salt
bath reaches bath temperature in under
one second.
• If quenched from 1500°F (820°C) to
1300°F (700°C) and soaked an hour before
air cooling, it becomes coarse pearlite.
• If quenched from 1500°F (820°C) to
1000°F (540°C), it forms fine pearlite after
air cooling.
Cooling to Form Pearlite
Buehler Ltd.
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• UNS G10800 railroad rails are much larger than small sample.
• After hot-rolling, water is splashed on rail head to cool it to 1000°F
(540°C) quickly.
• Rail is not dropped into water bath.
• This produces finest pearlite possible.
Cooling Railroad Rails
Buehler Ltd.
32. Copyright Goodheart-Willcox Co., Inc. May not be posted to a publicly accessible website.
• Pearlite colonies form by growing ferrite
and cementite platelets together.
• When cooled to 1300°F (700°C) for an
hour, carbon can diffuse further.
• Produces pearlite with larger lamellar
spacing
• When cooled to 1000°F (540°C), carbon
cannot diffuse as far.
• Pearlite has much narrower bands.
Mechanism and Effects of Pearlite
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• Increasing tensile strength by 29 ksi (200 MPa) doubles wear
resistance of railroad rails, doubling service life.
• Rails made in late 19th century not as strong as today
• Less pearlite (carbon content was below 0.80%)
• Lamellar spacing larger because hot-rolled rails air cooled
Tougher Rails
Practical Metallurgy
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• Great care needed to water spray rail
heads correctly
• Maximum strength at head
• Maximum toughness at web and foot
Tougher Rails
Practical Metallurgy (cont.)
Goodheart-Willcox Publisher
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• Soaking samples at different temperatures and times reveals how
austenite transforms.
• Time before transformation starts depends on salt bath temperature.
• Isothermal transformation (IT) diagrams are graphic representations
of these transformations.
• For specific steel alloys based on hold temperature and time
Isothermal Transformation (IT) Diagram
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• Shortest time to begin transformation is
called pearlite nose.
• For UNS G10800 steel, nose is at about
1000°F (540°C) and one second.
• When cooled this fast, it forms finest
possible pearlite.
Isothermal Transformation (IT) Diagram
of G10800 Steel
Goodheart-Willcox Publisher
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• A part “cooled rapidly” is cooled to soak
temperature before any transformation
from austenite to pearlite begins.
• Cooling curves A and B produce coarse
and fine pearlite, respectively.
• Cooling curves C and D do not produce
pearlite.
• Cooling curve C forms bainite.
More Rapid Cooling of G10800 Steel
Goodheart-Willcox Publisher
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• Bainite steel microstructure has smaller platelets than pearlite.
• Bainite structure has higher yield and tensile strength than same
alloy with pearlite structure.
• Bainite structure has lower elongation and less formability.
• Processing requires more care than simply cooling.
• Parts cooled quickly from above A1 temperature to between 600°F
and 900°F (320°C and 480°C) in less than one second
• Parts then held for an hour before cooling to room temperature
Bainite
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Bainite in UNS G10800 Steel
ASM International
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• Steel cooled below 500°F (260°C) very
fast will “miss the nose” of IT diagram.
• Radically different transformation
occurs.
• No time for carbon atoms to diffuse
• Austenite undergoes diffusionless
transformation.
• Iron-carbon alloy forms small,
needlelike particles called martensite.
Very Rapid Cooling—Martensite
ASM International
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• Martensite consists of iron in body-centered tetragonal (bct) unit
cells with carbon in some unit cells.
• Carbon forces iron to become body-centered tetragonal (bct).
• Bct structure is like bcc structure but longer in one direction.
• Higher % carbon increases amount of distortion.
• Martensite is hardest and strongest transformation product.
Martensite and BCT Unit Cells
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Martensite and BCT Unit Cells Illustration
Goodheart-Willcox Publisher
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• Martensite forms when austenite cools to martensite start (Ms)
temperature.
• Martensite continues to form as steel sample cools to martensite
finish (Mf) temperature.
• Austenite remaining at room temperature is called retained
austenite.
Martensite Transformation Temperatures
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• Yield strength of martensite sample very high
• For UNS G10800 steel, almost three times higher than if pearlitic
• Most steel applications also need toughness.
• High ductility, elongation, and impact strength
• High strength means low toughness.
• As-quenched martensite has almost no ductility.
• When yield point of tensile specimen reached, sample fails.
• It qualifies as “brittle.”
Properties of Martensite
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• Applications are few for fully martensitic steel.
• Lack of ductility
• Lack of toughness
• It resists wear very well.
• Finishing is done by grinding, not cutting.
• Martensitic steel cannot take shock loading unless tempered.
Applications of Martensite
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• Tempering is process of reheating quenched steel to increase
ductility and relieve stress.
• Reheated to between 300°F (150°C) and 1200°F (650°C)
• Then soaked for about one hour
• Body-centered tetragonal (bct) martensite decomposes into two
phases.
• Body-centered cubic (bcc) ferrite
• Tiny spherical cementite particles
Tempering Martensite
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• Sharp needle structure is replaced with small, rounded particles.
• Reduces stress-riser effect and relieves stresses
• Tempered part will not shatter if struck with hammer.
Tempering Martensite (cont.)
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• Fine cementite particles give tempered
martensite very high yield strength and
greater ductility.
• Amount of microstructure change controlled
mostly by tempering temperature (time less
important)
• Higher temperature reduces hardness more.
• Yield strength stays high until temperature
reaches 1000°F (540°C).
Properties of Tempered Steel
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• Heat treatment is relatively easy to control.
• Heat, quench, then reheat to temper
• To produce bainite requires more complex steps.
• Parts must be quenched and held at intermediate temperature.
• Bainite specified only when its properties superior for application.
• This would justify use of specialized equipment.
Processing Advantages of Tempered
Martensite
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• Tempered martensitic G10800 steel makes excellent cutting tools
for woodworking.
• Automotive structural parts use tempered martensite.
• Components that help protect people inside cars from crashes
• Springs and clips
Applications of Tempered Martensite
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• Spheroidizing is process of heating
steel to near A1 temperature and
slowly cooling.
• Produces globular or spheroidal form
of iron carbide microstructure.
• Ductility of pearlite can be greatly
enhanced by spheroidizing.
Spheroidizing Pearlite
ASM International
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• Some cold-forming processes require high
formability.
• Spheroidized pearlitic steel often used for
deep-drawing applications
• Deep-drawing is forming sheet into deep well
or cylinder shapes by pulling (drawing) into die
cavity.
• Spheroidized pearlite also used for drawn wire
Applications of Spheroidized Pearlite:
Cold Forming
Jay Warner
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• Most tool steel is supplied to toolmakers in spheroidized condition.
• Machinability is improved.
• Heat-treatment response is improved.
Applications of Spheroidized Pearlite:
Machining
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• Wide range of strength and ductility
possible
• This results from different
microstructures developed.
Typical Mechanical Properties Based on
Processing to Different Microstructures
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