Control of Distortion and Residual Stresses

8/11/2019 Control of Distortion and Residual Stresses

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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 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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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 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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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

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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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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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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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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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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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!

Date post:	03-Jun-2018
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Control of Distortion and Residual Stresses

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