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• Understand why heavy section parts cannot be heated completely to the
austenitizing temperature rapidly, and why the center of such parts
cannot be cooled rapidly.
• Explain why only the outermost portions of thick carbon steel parts can
be heat-treated to form martensite with full strength and hardness.
• Describe why bainite is not formed in large, continuously cooled carbon
steel parts.
• Explain the difference between hardness and hardenability.
Learning Objectives
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• Recognize how certain alloy additions increase the hardening depth
in heavy section parts.
• Understand how changes in process, including differences in heat-
treating equipment and times, can produce major variations in part
strength and hardness.
• Describe actions that can be taken to correct or avoid variations and
produce consistent properties and parts.
Learning Objectives
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• Small steel parts are easily heated and cooled in heat-treating.
• Heat-treating parts thicker than 1″ (25 mm) is different.
• Cooling entire volume at one time not possible
• Examples include truck drive axles and die blocks.
• Heavy sections have thinnest portion exceeding 1″ (25 mm).
• Alloy additions are necessary to heat-treat large parts.
• Alloy steels can obtain high strength throughout entire part.
Small Parts versus Heavy Section Parts
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• Large parts heated above A3 line cannot be quenched quickly.
• Not possible to cool part center below 1000°F (540°C) in one second
• Carbon steels must cool this fast to completely form martensite.
• Quenching removes thermal energy (heat) from parts.
• Rate of heat removal controlled by H and heat capacity of steel
• Surfaces cool rapidly, but centers of heavy parts cool slowly.
Quenching Heavy Sections
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• Figure shows how 4″ (100 mm) diameter
shaft responds to water quenching.
• Starts at 1600°F (870°C), then quenched at
212°F (100°C)
• After one second, based on ideal quench
• Surface near 212°F (100°C)
• Center temperature still 1600°F (870°C)
• Less than 0.22″ (5.6 mm) of outside will
transform into martensite.
Temperature Profiles for Heavy Sections
Goodheart-Willcox Publisher
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• Math to calculate heat transfer profiles is available.
• Cooling curves like Figure 12-2 developed for different shapes
• Even agitated brine solutions do not quench at ideal rate.
• In practice, depth of transformed layer thinner than shown
• Run trials with specific parts to assure actual results acceptable.
Calculated Heat Transfer Profiles
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• Rapid quenches develop high stresses in parts.
• From uneven metal shrinkage during cooling
• From expansion as martensite forms
• Milder quenchants with lower H coefficients prevent cracking.
• Large carbon steel parts will not harden completely through.
• Through hardness requires alloying elements to slow austenite
transformation.
Alloy Additions to Achieve Martensite
and High Strength
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• Carbon steels must be rapidly cooled in less than a second to
produce martensite.
• Compare to common alloy steels
• UNS G41400 (AISI 4140) contains chromium and molybdenum.
• Full martensite is produced in any quench up to two seconds.
Hardness Profiles in Heat-Treated Heavy
Section Alloys
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• UNS G43400 (AISI 4340) steel contains nickel, chromium, and
molybdenum.
• These additions greatly increase ability to harden steel.
• Full martensite produced at any cooling time up to 30 seconds
• Large parts made of this alloy are often furnace cooled.
• Avoids warping and cracking due to uneven transformation
• Produces bainite instead of martensite
• Welding requires pre- and postheating to avoid cracking.
Alloy Steels with Cr, Ni, and Mo
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• For large sections of steel, transformation diagram for constant,
slower cooling rate needed
• Continuous cooling transformation (CCT) diagram is used.
• Shows transformation structures and times
• Based on a part cooled continuously at fixed rate
Continuous Cooling Transformation (CCT)
Diagrams
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• Different cooling rates experienced from
surface to center of heavy sections
• Surface struck by water sprays will
quench rapidly.
• Cooling faster than line A forms
martensite.
CCT Diagram for UNS G43400 Steel
Goodheart-Willcox Publisher
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• Center portion may take a day to cool in still
air.
• Indicated by line D, it transforms to several
microstructures.
• Ferrite, pearlite, bainite, and possibly
martensite
• No portion transforms only to bainite.
• Metal not at bainite transformation temperature
long enough
CCT Diagram for UNS G43400 Steel
(cont.)
Goodheart-Willcox Publisher
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• Different shaped parts will not exactly match CCT curves.
• CCT curves help in process design.
• Heat-treating simulation software predicts transformations and
hardness at every point in parts.
• For volume production, some trial parts evaluated first
• Allows for adjustments to obtain needed final properties
• Technicians and operators must watch for any changes.
CCT Curves for Process Design
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• Martensite on outer ring supplies needed
strength.
• Subjected to torsion and bending stress
• Torsion from engine turning axle
• Bending from weight of tractor on wheels
• Both types of stress are maximized along outside
layer of axle.
• Ductile unhardened core absorbs shock better.
Tractor Axle Hardness Depth
Practical Metallurgy
Jay Warner
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• Maximum attainable hardness depends on steel’s carbon content.
• To obtain parts hardened deeply, alloy additions needed
• Alloys that can harden deep into part possess hardenability.
• Common measure of hardenability is depth of hardening.
• Deepest point reaching tempered hardness of 55 HRC
Hardenability
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• Standardized test to measure
depth of hardening is Jominy
end-quench test.
• Heat standard sample bar
(Jominy bar) above A3
temperature.
• Quench sample from one end.
• Quenched end is rapidly cooled.
• Held end is moderately cooled.
Jominy Bars to Measure Hardenability
Goodheart-Willcox Publisher
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• After cooling, Jominy bar is machined
flat on opposite sides.
• Rockwell C hardness is measured from
quenched end.
• Measurements are taken until hardness
drops below 20 HRC.
• This alloy does not have good
hardenability.
• All plain carbon steels have low
hardenability.
Hardness Profile for a UNS G10400 Steel
Goodheart-Willcox Publisher
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• Hardenability increases as more metallic elements are added.
• Chromium, nickel, and molybdenum delay pearlite transformation.
• More of Jominy bar transforms to martensite.
Composition Effects on Hardenability
Goodheart-Willcox Publisher
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• H steels are steel alloys with hardenability
specifications.
• SAE standard specifies hardenability
curves for each alloy.
• Chemical composition limits are relatively
broad.
• Steel sold to have specific Jominy
hardness profile
• SAE alloy number has “H” suffix.
• UNS numbers start with H.
H Steels—Controlled Hardenability
Goodheart-Willcox Publisher
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• Fresh martensite is usually tempered to
reduce brittleness.
• But tempering reduces hardness.
• Alloy additions that increase hardenability,
such as molybdenum, help steel retain
hardness during tempering.
• Alloy additions change required heat-treating
conditions.
• Specific treatments should be used for each
alloy.
Effects of Alloying on Heat-Treat
Performance
Goodheart-Willcox Publisher
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• Very hard martensite is necessary to
minimize abrasive wear.
• Chromium or nickel additions help retain
hardness after tempering.
• Nickel additions improve properties.
• Weldability and low-temperature notch
toughness
• Adding 0.0005%–0.003% boron significantly
increases hardenability.
Alloying Element Effects
Kaband/Shutterstock.com
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• High-performance specialty steel alloys find
many uses.
• Examples include high-temperature springs,
ultra high-strength fasteners, and engine
valves.
• Design engineers select alloys with desired
properties for specific applications.
• Work with production staff to develop suitable
heat-treatment processes
Working with Specialty Alloys
Ensuper/Shutterstock.com
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• Alloy steel scrap and chrome-plated scrap are major sources of
alloys for specialty steels.
• Recycling this scrap reduces costs.
• Less landfill and mining of raw materials
• Reduces dependence on foreign countries
• One problem is getting properly sorted alloy steel scrap.
• Postconsumer scrap is less desirable.
Alloy Scrap and Specialty Steels
Sustainable Metallurgy
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• Furnaces are built and maintained to provide temperature uniformity
throughout the furnace.
• Aerospace, medical, and automotive customers require stringent
calibration procedures.
• Temperature controllers, thermocouples, and uniformity checked
frequently
• Calibration procedures and records audited annually
Furnace Temperature Uniformity
AFC-Holcroft
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Heat-Treating Furnace and Temper
Furnace
AFC-Holcroft
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• Some alloys must be quenched as soon as parts are removed from
furnaces.
• Quench tank should be set up near furnace.
• Minimizes time that hot parts are exposed to air
• Hot parts decarburize in air.
• Decarburization lowers fatigue life of parts.
Proximity of Furnace to Quench
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• Agitating a coolant increases cooling rate dramatically.
• Impellers are often used.
• Placement must be maintained to ensure consistent cooling.
• Jet sprays are used.
• Alignment of spray nozzles is important.
• Operators must be alert for any changes to impellers or spray.
Agitating Coolant
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• Most high-performance parts are formed and machined before final
heat treatment.
• Cutting oil and dirt must be removed before heat treatment.
• Could react and change surface composition
• Could change furnace atmosphere
• Oxidizing atmosphere forms oxide scale and decarburizes surfaces.
Furnace Atmosphere Problems for
Specialty Alloys
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• Heat-treat furnaces for specialty alloys usually have atmosphere
controls.
• Decarburization and carburization must be avoided or carefully
controlled.
• Vacuum heat-treating may be used.
• Affects required cooling rates
• Finish of parts as clean and shiny as when they went in
Furnace Atmosphere for Specialty Alloys
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• Power transmission gears in wind turbines have
stringent requirements.
• Gears made of UNS G48200, G93100, and
other alloys
• Soaked above A3 temperature
• Furnace atmosphere includes cracked ammonia.
• Forms carbo-nitride layer on surfaces
• Reduces surface wear on gears
Alloy Power Gears for Wind Turbines (Part 1)
Practical Metallurgy
AFC-Holcroft
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• Parts are integrally quenched in oil.
• Quench tank is inside furnace with pumps and impellers.
• Eliminates surface decarburization from exposure to air
• After cooling, parts are cleaned of quench medium.
• Finally, taken to atmosphere-controlled tempering furnace
• Cycle depends on desired surface hardness and core toughness
• Typically one hour at 600°F (315°C)
Alloy Power Gears for Wind Turbines (Part 2)
Practical Metallurgy
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• Every step of processing specialty alloys requires care.
• Maintain tight tolerances and uniform performance
• Operators and technicians must follow work instructions.
• Also use notes based on prior experience for each part
• Tracing field failures to their cause can be a lengthy process.
• What caused change in microstructure?
Precise Processing for Consistent
Properties
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• In-process inspection frequently done on specialty alloys
• Nondestructive tests for small cracks are common.
• Magnetic inspection and ultrasonic inspection
• Destructive tests are done less often (more costly)
• Metallographic evaluation for microanalysis
• Tensile testing and impact testing
In-Process Inspection for Heat Treatment
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• Atmospheres in heat-treating furnaces might slightly decarburize
surface.
• Fatigue life of parts suffers.
• Fatigue tests are slow and expensive.
• In-process inspection combined with metallographic sampling can
detect decarburized surface.
Negative Impact of Variations in
Atmosphere
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• When steel parts soak above A3 temperature, grain growth can
occur.
• Time to begin pearlite transformation increases.
• ASTM represents grain size with unitless number.
• Inversely related to number of grains per square inch
• Low grain size number indicates large grains.
• High number indicates small grains (usually more desirable).
Grain Size and Hardenability
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• UNS G41400 steel properly soaked at 1600°F (870°C)
• Grain size of 7
• Pearlite nose at 45 seconds
• Same alloy soaked at 1810°F (990°C)—above A3 temperature
• Grain size of 2 to 3 (very large)
• Starts transforming in 150 seconds
• Time and temperature control important for desired results
Effect of Grain Size on Hardenability
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• Decarburization is a reduction of carbon content in metal.
• Occurs when medium- or high-carbon steel is soaked in oxidizing
atmosphere
• Carbon diffuses to surface and reacts, forming carbon monoxide.
Decarburization Composition Profile
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• If carbon drops below 0.022%, surface
metal transforms from austenite to
ferrite.
• Oxidation occurs once carbon is gone.
• Final surface is covered in black iron
oxide and skin of soft ferrite.
Decarburization Composition Profile
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