control of distortion and residual stresses
TRANSCRIPT
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Control of Distortion and
Residual Stresses
D. S. MacKenzie, PhDHoughton Internat ional, Val ley Forge PA
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Hardenability Concepts Related to Steel
Steels requires rapid cooling Transformation of Austenite to
Martensite
Achieved by fast cooling to avoidthe formation of upper
transformation products (Bainite
and Pearlite)
Critical Cooling rate All austenite transforms to
Martensite
Just misses knee of CCT curve
Dependant on steel chemistry
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Hardenability Concepts Related to Steel
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Hardenability Concepts Related to Steel
Hardenability is the ability tothrough-harden
Is not the ability to get hard
Governed by chemistry
Alloying additions Moves knee to the right
Allows more time for the
transformation to Martensite to
occur
Allows slower quenching
Increasing carbon content Moves knee slightly to the right
Increases Ms Temperature
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Hardenability Concepts Related to Steel
Lack of through hardening can beover come by:
Increasing steel hardenability
Increasing quenching speed
Increasing quenching speed Center of part (or specified
location) exceeds critical cooling
rate
Achieved by changing to a faster
oil
If polymer quenchant, reducing
concentration
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Hardenability Concepts Related to Steel
Hardenability measured by: Jominy End Quench Test
Chemistry
Jominy End Quench Test Cylindrical bar quenched at one
end
Infinite number of cooling rates
Hardness is measured as afunction of distance from
quenched end
Well accepted test
Relates cooling rate tomicrostructure and hardness.
ASTM/DIN/SAE/JS Standard
High repeatability and easy to
perform
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Mechanism of Quenching (cont.)
Vapor Phase Nucleate
Boiling Phase
Convection
Phase
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Mechanism of Quenching (cont.)
0
2 0 0
4 0 0
6 0 0
8 0 0
1 0 0 0
1 2 0 0
1 4 0 0
1 6 0 0
0 1 5 3 0 4 5 6 0
C O O L IN G R A T E (O
F /S E C .)
C O O L IN G T IM E ( S E C .)
TEMPE
RATURE
(oF)
0 9 0 1 8 0 2 7 0 3 6 0
Cooling Curve Analysis
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Mechanism of Quenching (cont.)
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Mechanism of Quenching
1. Moment of immersion vapor film around probe
2. After 5 seconds boiling commences at corners
3. After 10 seconds boiling front moves along the probe
4. After 15 seconds showing vapor. Boiling and convection phases
5. After 30 seconds convection phase
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Mechanism of Polymer Quenchants
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Quenching Performance
GM Quenchometer ASTM D 3520
Measures quenching speed
Provides a qualitative measure ofcomparing quench oils
Not usually used for monitoring
used quench oils
Cooling Curve ASTM D 6200
Most useful test for monitoring
in-service quench oils Determines effect of viscosity
and contamination
Provides picture of entire cooling
sequence
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What is distortion?
Distortion is the unexpected or inconsistent change in size orshape caused by variation in manufacturing process
conditions.
Caused by residual stresses
From all manufacturing processes not just quenching
Thoughts on Distortion A leading cause of quality problems, scrap, and rework is the shape
changes caused by heat treatment DISTORTION.
As long as parts have been heat treated, DISTORTION has been a
concern. As greater dimensional accuracy is required for components,
DISTORTION becomes even more of a problem.
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THIS IS NOT WHAT YOU WANT TO SEE!
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Distortion and Corrective Action
Distortion and Cracking is the result of large residual stresses Causes:
Differential Cooling
Transformational Stresses
Martensitic transformation in steel (1-3%)
No volumetric phase transformation in aluminum
The slowest quench rate that achieves desired properties willminimize residual stresses
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MaterialAlloyQuench Sensitivity
Prior Condition and Microstructure
Alloy Segregation
Decarburization
Transformation Induced Stresses
QuenchingQuench Temperature
Quench Agitation
Type of Quenchant
ContaminationRacking
Load Density
Part to Part Interactions
Handling during Quench
Temperature at Withdrawal
DesignAlloy Procurement
Alloy SelectionPart Geometry
Prior to Heat-TreatMachining Stresses
Cold work
Prior Condition and Microstructure
Grinding StressesShot and Grit blasting
Plating for Decarburization Control
Heat-TreatFurnace TemperaturePreheat
Heat-up rate
Temperature Uniformity
Non-uniform Heating
Racking
Load Density
Part to Part InteractionsCarburizing
Atmosphere Control
Post-QuenchPart HandlingDelay before Tempering
Washing Temperature
Wash Velocity
Uneven Cooling
Refrigeration
TemperingFurnace Temperature
Preheat
Heat-up rateTemperature Uniformity
Non-uniform Heating
Racking
Load Density
Part to Part Interactions
Fixturing
FinishingMachining
Grinding
PicklingShot and Grit Blasting
Straightening
Plating
Baking after Painting
Stress Relief
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Causes of Distortion and Corrective Action
Alloy Distortion and tendency toward
cracking decreases as Martensite
transformation temperature
increases Poor correlation between
Austenitic grain size and quench
cracking
Quench Sensitivity Cracking and distortion increase
as Carbon Equivalent is
increased
Alloy is Crack Sensitive if Ceq> 0.52
Alloy Segregation Can cause localized hardness and
microstructure gradients
Increases tendency toward
cracking
Decarburization Surface is depleted of carbon.
Low tensile strength, increased
tendency toward cracking
Transformation Induced Stresses Transformation from Austenite to
Martensite causes volumechange.
Increases in volume increase
tendency toward distortion and
cracking
( ) ( )CVVVV
Vaac 21.264.410068.1100 ++=
101055
NiCrMoMnCCeq ++++=
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Material, Design and Procurement
Alloy Procurement Specify surface condition
(minimal decarburization and
scale) Wide variation in allowable
chemistries within grade
Results in variations in
properties and distortion
Specification of alloy chemistry
(H Grades)
Specification of hardenability
(Hardness at specific J
positions on Jominy EndQuench)
Alloy Selection Some steel grades prone to
macrosegregation of chromium
(banding) or gross segregation ofmanganese (AISI 1340, AISI
1536, AISI 4140H and AISI
4340)
Quench Sensitivity
Cracking and distortion increase
as Carbon Equivalent is
increased
Alloy is Crack Sensitive if
Ceq > 0.52 Increases in amount of retained
austenite also increase tendency
toward cracking and distortion
Transformation Temperature
(Ms)
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Design - Component Configuration
Large Section Sizes Fast quench rates required to
achieve desired properties at
center
Sharp Radii Differential cooling
Depending on racking may cause
distortion or cracking Mismatched Section Sizes
Thin cools faster than thick
Large section constrained
Fat section transforms, placing
thin section in tension, resulting
in cracking
Part Geometry Long parts with small cross
sections or thin parts with large
surface area
Asymmetrical shapes with sharp
transitions between thick and thin
sections
The presence of hole, deep
keyways and grooves
Large and generous radii
Contributes to non-uniform
heating and cooling
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Prior to Heat Treatment
Machining Stresses Cold work Prior Condition and
Microstructure Local segregation of carbon, or
alloying elements
Some steel grades prone to
macrosegregation of chromium(banding) or gross segregation of
manganese (AISI 1340, AISI
1536, AISI 4140H and AISI
4340)
Presence of scale or
decarburization
Grain Size Increased grain size increases
hardenability
Smaller grain size decreases
hardenability
Mixture of grain sizes causes
differential hardenability
Decarburization Decarburization causes low
transformation stresses at surface
Normal transformation stresses at
interior Prior Residual Stresses
Warpage during heat-up
Relief of stresses at different
times and rates
Use of a preheat minimizes
distortion on heat-up
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Prior to Heat Treat
Grinding Stresses Localized stresses
If abusive, localized martensitic
microstructures
Surface tensile stress, withsubsurface compresive
component
Shot and Grit blasting Localized surface stresses
Very shallow, and compressive
surface stresses
Subsurface tensile stresses
Plating for DecarburizationControl
Changes in surface condition
Changes in emmissivity
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Heat Treatment
Furnace Temperature Increases in furnace/part
temperature increases distortion
May cause non-uniform grain
growth or overall grain growth Increases hardenability
May cause variations in local
hardenability
May reverse normalization, andincrease local segregation or
banding
Preheat Behaves as stress-relief
Relieves prior stresses
Grinding
Machining
Allows uniform growth
Heat-Up Rate Behaves similar to preheat
Rapid Heating causes stress-relief
in thin sections, but not in thick
sections
Local temperature gradients,
causing unequal volumetric
growth
Temperature Uniformity Part experiences uniform
temperature
Uniform microstructure Uniform volumetric growth
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Causes of Distortion and Corrective Action
Racking Shield parts from heat source
Unsupported parts may sag
Allows uniform flow ofatmosphere through workload
Load Density High load density shield parts
from heat source
Can cause excessive heat, and
contributes to non-uniform
heating
Part to Part Interactions Parts are soft at elevated
temperature
Prone to bending and surfacedamage from other parts or rack
Carburizing Carbon gradients present
Changes local carbon equivalent Can increase tendency for
cracking or distortion
Different volumetric distortion
due to transformation stresses
Different Martensite start
temperatures between surface
and core
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Furnace Conditions
Atmosphere Control High Carbon Potential
Sooting, resulting in oil contamination
Work is dirty, resulting in enhanced nucleate boiling
Soot trapping, resulting in non-uniform heat transfer
Low Carbon Potential
Decarburization Low properties
Differential transformation stresses
Temperature High temperature causes increased differential cooling
Potential for grain growth, or mixed grain size
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Causes of Distortion and Corrective Action
Quench Temperature Reduces temperature gradients by providing slower quench
Results in lower distortion
Quench Agitation Provides uniform heat transfer across all surfaces of the parts
Provides adequate flow through workload
Non-uniform agitation causes temperature gradients within workload
and part
May not be uniform
Rolling on surface is poor guide to agitation
Must be measured or modeled
Type of Quenchant Must be suited to application
Quench rate must be fast enough to achieve desired properties, butslow enough to minimize distortion
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Racking and Fixturing (cont.)
Mechanical Damage Material at temperature is soft
Lower mechanical properties
Must spread-out loading andsupport
Part to Part Interactions Parts are soft as enter quench
Parts can shield quenchant fromother parts, creating localized
hot spots
Handling during Quench Mechanical handling of parts can
contribute to distortion
Jerky motions, causing parts to
hit each other, or the sides of the
basket
Gripping of parts
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Causes of Distortion and Corrective Action
Contamination of Quenchant Organic Contamination
(Hydraulic fluids)
Increase the cooling rate during
vapor phase Increases cooling rate during
convection
Increases tendency toward
cracking and distortion
Oxidation
Increases viscosity
Slows quench rate, properties
may not be achieved
Drag-out increased
Staining of parts increased
Water
Changes cooling curve
Causes spotty work, and
promotes cracking and distortion
Can be safety hazard if water
content exceeds 0.1%
Soot
Increases Ledenfrost
temperature and decreases vaporphase stability
Increases maximum cooling rate,
and decreases temperature of
maximum cooling
Increases the cooling rate during
convection stage
Increases rate of oxidation and
can cause staining of parts
Indicates a strong maintenanceprogram is necessary
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Causes of Distortion and Corrective Action
Load Density Uniform quenchant flow
throughout workload
Minimize local temperaturegradients from part proximity
Temperature at Withdrawal Withdrawal temperature as a
function of Martensite start (Ms)temperature
Higher temperatures tend to
reduce cracking and distortion
May reduce oil quenchant drag-
out because of higher surface
temperatures
May increase oil oxidation
because high surface area and
oxygen availability
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Causes of Distortion and Corrective Action
Delay before Tempering Increases tendency toward
cracking
Amount of time available isdependant on section thickness,
amount of retained Austenite and
alloy
Washing Impact Velocity
Effects effectiveness of cleaning,
and removal of soil
Can shock parts, and cause
cracking
Distortion and cracking
generally a function of the
amount of retained Austenite
present
Refrigeration Minimizes amount of retained
Austenite present
Improves hardness andpercentage of Martensite present
Reduces residual stresses from
presence of retained Austenite
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Conclusions
Primary driving factors Design for manufacture
Distortion and residual stress control
Environmental concerns
Just Remember Things Change!