met ht conference-ald dynatech 18-11

8/10/2019 Met Ht Conference-Ald Dynatech 18-11

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Minimizing and Controlling

Distortion in Vacuum Furnaces

ALD-Dynatech Furnaces Pte, Ltd.

Janusz Kowalewski

Managing Director and CEOAhmedabad, December 2014


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Products and People

MonoTherm ModulThermVacuum Oil

Quench


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Define distortion Identify factors influencing distortion during heat

treatment

Process of selecting a vacuum furnaces to minimize

distortion Demonstrate new furnace design to minimize distortion

Validate importance of convection heating and isothermalquench

Provide useful information

Agenda


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Fast and non-uniform heating and cooling

Stresses during the heating cycle

Residual stresses

Phase transformation

Dissimilar metals

Part design

Material accounts for over 50% of variability. Study by Bell

Helicopter and IIT Research Institute.

General causes of Distortion


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SIZE DISTORTION SHAPE DISTORTION

Total size distortion is equal

to the sum of the

distortions arising duringthe heating and cooling .

Changes in dimensions are

due to structural

transformation and are

characterized by material

shrinkage or expansion.

Internal stresses are created

by a lack of uniformity in

temperature duringphase transformations.

Types of Distortion


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Distortion is a general term describing all types of

dimensional changes. There are two types of distortion: sizedistortion and shape distortion.

Definition


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Heat treatment distortions occur if:

Stress in the Material > Yield stress of the Material.

Yield stress decreases dramatically with increasing temperature

of the material.

There are 3 different types of stress:

1. Residual stresses (are induced before heat treatment by

casting, forging, machining etc.)

2. Thermal stresses (temperature gradient while heating and

quenching)

Heat Treatment Distortions


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Heat Treatment Distortions- Contd..

3. Transformation stresses (transformation from ferrite toaustenite during heating and transformation from austenite to

martensite / bainite during quenching)

These stresses add up to the total stress in the component.

They depend on part-geometry, steel-grade, casting, forging,

machining etc. and they depend on the heat treatment. If the

total stress in the component exceeds the yield stress we get

plastic deformation. This means we get distortion of the

component.

Si Ch i H t T t t


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Soft Heated to Quenched to

Austenitize Martensite

Shape Change in Heat Treatment

Size Change in Heat Treatment

Before Hardenin After Hardenin


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Size

Temperature

200 400 600 800 1000 800 600 400 200 o

392 752 1112 1472 1832 1472 1112 752 392 oF

AC1

AC3

MS

MF

Volume during Heating & Cooling


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2012

1832

1652

1472

12921112

932

752

572

392212

1100

1000

900

800

700600

500

400

300

200100

oF oC

Tempera

ture

1 2 3 4 5 6 7 8 9 10Time (Hours)

SurfaceTemp.

SurfaceTemp.

CoreTemp.

Core

Temp.

MFMS

E - Expanding

C - Contracting

Temp/ Size correlation


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Stage Temperature

range

Metallurgical Reaction Expansion/

Contraction

1 0-200C

32-392F

Precipitation of -carbide Contraction

2 200-300C

392-572F

Decomposition of

retained austenite

Expansion

3 230-350C

446-662F

-carbide decompose to

cementite

Contraction

4 350-700C

662-1292F

Precipitation of alloy

carbides

Expansion

Source: Carsten Jense

Metallurgical Reactions at Various

Temperature Ranges and Related Physical Changes in Steel


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Bubble Boiling

Film Boiling

Convection

t = 10 s

750C

700C

700C600C

500C400C300C

200C

Temperature distribution

t = 10 s

Heat transfer coefficient

5000 10000 15000 20000

loil Wasser

water

[W/m K]2

ref.: Stick, Tensi, HTM 50, 1995

Heat Transfer and Temperature

distribution at liquid Quenching


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Heat transfer coefficient

1000 2000 3000 4000 [W/m K]2

Temperature distribution

750C

650C

550C

450C

350C

250C

Gas direction

Only convection

Heat Transfer and Temperature

distribution at High Pressure Gas Quenching


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Source: C.C. Tennenhouse

300

250

200

150

100

50

400 800 1200 1600 2000

Temperature, oC

Tem

peratureDiffe

rence,

oF

200 400 600 800 1000 1200

Temperature,o

F

160

140

120

100

80

60

40

20

Te

mperatureDifference,

oC

Thermal stressesbelow yield pointunder curve

Plasticdeformationoccurs abovecurve

Temp diff at which thermal

stresses equal the yield point


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Source: NASA

.018

.016

.014

.012

.010

.008

.006

.004

.002100 200 300 400 500 600 700 800 900 1000 1100

Temperature, oC

TotalExpansion,

21oC

toTemp.,mm/m

m

(70oFto

Temp.,

In/In)

400 800 1200 1600 2000Temperature, oF

Thermal Expansion Curves


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Recrystallization

annealing.

Claim: Distortion

Cause: Wrong jigging

Example of Distortion case by fixturing


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Distortion behavior is significantly influenced by thedesign of the components.

-Study by C.M. Bergstrom

Material variability accounts for over 50% of distortionproblems.

-Study by Bell Helicopter and IIT Research Institute.

Predictable size change


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Gas flow pattern and uniformity of flow

Control of cooling speed

Load position and fixtures design

Pressure and furnace design

Uniformity of cooling


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Pressure Gas velocity - design, furnace size, blower, water system,

ratio between load and hot zone surface

Gas type

Cooling speedt = (V/A p c)s (1/) ln *(T1Tg) / (T2Tg)

Heat exchange coefficient

=c w.7p .7 -.39 cp.31 .69

Cooling speed parameters


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- shape- weight- material

- production

- specifications

- horizontal- vertical- internal- external- hot zone- heating

elements

-gas type (Argon, Nitrogen, Helium)

-gas mixture (Nitrogen / Helium / Hydrogen)

-gas flow and pressure ( velocity , direction)

Cooling Gas

Metallurgy

Production

Cost

Material Furnace


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Speed and uniformity of heating

Speed and uniformity of cooling

Fixtures, baskets and load configuration

Factors causing Distortions during

heat treatment process


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Convection Heating

Cylindrical Hot Zone

Wide Bend Heating Elements

Insulation

Working Thermocouple Location and Control

Increase uniformity of cooling


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CONVECTION

CONVECTIONCONVECTION

COSTMIN. DISTORTION

From ambient temperature to 1400F

MIN. DISTORTION


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0

20

40

60

80

100

120

140

160

Out-of roundness Out-of-flatness

CH

A

N

G

E

m

CONVECTION RADIATION Source: Altena

Influence of heating method on changes

in shape and dimension


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Gas flow pattern and uniformity of flow

Control of cooling speed

Load position and fixtures design

Pressure and furnace design

Uniformity of cooling


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Pressure

Gas velocity - design, furnace size, blower, water system,

ratio between load and hot zone surface

Gas type

Cooling speed

t = (V/A p c)s (1/) ln *(T1Tg) / (T2Tg)

Heat exchange coefficient=c w.7p .7 -.39 cp

.31 .69

Cooling speed parameters


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Hot ZoneHeat

Exchanger

Quench

Motor

Quench Fan

Charge/ Load

Vacuum Furnace schematic


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HeatExchanger

CoolingBlower

RadiationShields

Isolation Valve

External fan

External heat exchanger

External cooling

fl f l d


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0

50

100

150

Out-of-roundness Out-of-flatness

Influence of cooling gas pressure and

loading on changes in shape and

dimension (Source: Study by Altna, Stola and Klima)

10 Bar / Horizontal 15 Bar / Vertical15 Bar / Horizontal

CHA

NGE

m


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Control temperature uniformity during the phasetransformation.

Heat up the parts uniformly up to to stress reliving

temperature within +/- 80F until the stress relief

temperature is reached. Use properly designed fixtures with tolerance for

thermal expansion. (Graphite best/Inconnel good)

Use smart loadingdummy parts, shields, low gage

fixtures, baskets and grid made from low expansionmaterial. (Graphite or CFC material)

Distortion control


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Use convection heating from ambient to 1400F Use isothermal quench process

H-13 hold at 1200F and 1560F to allow for

equalization of temperature (T 100F at 1200F and

T 80F at 1560F) and use isothermal quench. Stack or hang long parts vertically

Use the rightpressure to minimize distortion

Group or tie together similar parts

Distortion control- Contd..


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Low distortion heattreatment of transmission

components

Quench Cell design

- uniform gas flow pattern

Fixture design- Optimized mech. support of

components and optimized gas flowpattern in the load

Optimized LPC & HPGQ process-application of convective heating-application of Dynamic / Reversing

Quenching and choose Helium as quench-gas

Stable manufact. chainbefore heat treat

- Low level of residual stress incomponents before heat

treatment

Summary


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THANK YOUJanusz [email protected]

met ht conference-ald dynatech 18-11

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