ALD-Dynatech Furnaces Pte, Ltd.Janusz Kowalewski Managing Director and CEO

Amanabad, December 2014

Minimizing and Controlling Distortion in Vacuum Furnaces

Organizational Chart

Agenda

• 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 isothermal quench

• Provide useful information

General Causes of Distortion

• 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.

Furnace

Furnace

Type of Distortion

SIZE DISTORTION SHAPE DISTORTION

Total size distortion is equal to the sum of the distortions arising during the heating and cooling . Changes in dimensions are due to structural transformation and are characterized by material shrinkage or expansion.

Internal stresses are createdby a lack of uniformity intemperature during phase transformations.

Distortion is a general term describing all types of dimensional changes. There are two types of distortion: size distortion and shape distortion.

Definition

Heat treatment distortions

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:

Residual stresses (are induced before heat treatment by casting, forging, machining etc.)

Thermal stresses (temperature gradient while heating and quenching)

Transformation stresses (transformation from ferrite to austenite 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.

Soft Heated to Quenched to Austenitize Martensite

Shape Change in Heat Treatment

Size Change in Heat Treatment

Before Hardening After Hardening

Volume Changes During Heating & Cooling

Size

Size

TemperatureTemperature

200 400 600 800 1000 800 600 400 200 200 400 600 800 1000 800 600 400 200 ooCC

392 752 1112 1472 1832 1472 1112 752 392 392 752 1112 1472 1832 1472 1112 752 392 ooFF

Expan

ding

Expan

ding

Contracting

Contracting

AACC11

AACC33

MMSS

MMFF

Temperature / Size Correlation

201220121832183216521652147214721292129211121112

932932752752572572392392212212

1100110010001000

900900800800700700600600500500400400300300200200100100

ooFF ooCC

Tem

pera

ture

Tem

pera

ture

1 2 3 4 5 6 7 8 9 101 2 3 4 5 6 7 8 9 10Time (Hours)Time (Hours)

SurfaceSurfaceTemp.Temp.

SurfaceSurfaceTemp.Temp.

CoreCoreTemp.Temp.

CoreCoreTemp.Temp.

MMFFMMSS

EECC

EE

CCEE

CC

EE

EE

CC

CC

CC

EE

E - ExpandingE - ExpandingC - ContractingC - Contracting

Metallurgical Reactions at Various Temperature Ranges Metallurgical Reactions at Various Temperature Ranges and Related Physical Changes in Steeland Related Physical Changes in Steel

Stage Temperature range

Metallurgical Reaction Expansion/ Contraction

1 0-200°C32-392°F

Precipitation of έ-carbide

Contraction

2 200-300°C392-572°F

Decomposition of retained austenite

Expansion

3 230-350°C446-662°F

Є-carbide decompose to cementite

Contraction

4 350-700°C662-1292°F

Precipitation of alloy carbides

Expansion

Source: Carsten JensenSource: Carsten Jensen

Bubble Boiling

Film Boiling

Convection

t = 10 s

750°C700°C

700°C600°C500°C400°C300°C

200°C

Temperature distribution

t = 10 s

Heat transfer coefficient

5000 10000 15000 20000

Öloil Wasser

water

[W/m K]2

ref.: Stick, Tensi, HTM 50, 1995

Heat Transfer and Temperature distribution at liquid Quenching

Heat transfer coefficient

1000 2000 3000 4000 [W/m K]2

Temperature distribution

750°C

650°C

550°C

450°C

350°C

250°C

Gas direction

Only convection

Heat Transfer and Temperaturedistribution at High Pressure Gas Quenching

Source: C.C. TennenhouseSource: C.C. Tennenhouse

Temperature difference at which thermal stresses Temperature difference at which thermal stresses equal the yield point of various materials.equal the yield point of various materials.

300

250

200

150

100

50

400 800 1200 1600 2000

Temperature, oC

Tem

pera

ture

Diff

eren

ce, o F

200 400 600 800 1000 1200

Temperature, oF

160

140

120

100

80

60

40

20

Tem

pera

ture

Diff

eren

ce, o C

Thermal stressesbelow yield pointunder curve

Inconel 718 (Hardened)

Plasticdeformation occurs abovecurve

Hastelloy X1010 Steel

304 StainlessInconel 600

Haynes

No. 25

Alloy

Thermal expansion curves for several common materials.Thermal expansion curves for several common materials.

Source: NASASource: NASA

.018

.016

.014

.012

.010

.008

.006

.004

.002

100 200 300 400 500 600 700 800 900 1000 1100Temperature, oC

Tota

l Exp

ansi

on, 2

1 o C

to T

emp.

, mm

/mm

(7

0 o F

to T

emp.

, In/

In)

400 800 1200 1600 2000Temperature, oF

Graphite

Tungsten Carbide

Titanium -6A1-4V

6061

Alu

min

um

OFHC C

oppe

r

302 S

tainle

ss10

18 S

teel

Recrystallization annealing.Claim: DistortionCause: Wrong jigging

Example of Distortion case by fixturing

Predictable Size Change

• Distortion behavior is significantly influenced by the design of the components.

Study by C.M. Bergstrom • Material variability accounts for over 50% of distortion problems.

Study by Bell Helicopter and IIT Research Institute.

Uniformity of Cooling

• Gas flow pattern and uniformity of flow

• Control of cooling speed

• Load position and fixtures design

• Pressure and furnace design

Cooling Speed Parameters

• Pressure

• Gas velocity - design, furnace size, blower, water system, ratio between load and hot zone surface

• Gas type

Cooling speed

Δt = (V/A p c)s (1/α) ln [(T1 –Tg) / (T2 – Tg)

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

.31 λ.69

20

MaterialMaterial FurnaceFurnace

- 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 GasCooling Gas

MetallurgyMetallurgy

ProductionProduction

Cost Cost

21

Factors causing distortion during heat treatment process

Speed and uniformity of heating

Speed and uniformity of cooling

Fixtures, baskets and load configuration

22

Increase Uniformity of Heating

Convection HeatingCylindrical Hot ZoneWide Bend Heating ElementsInsulationWorking Thermocouple Location and Control

23

CONVECTIONCONVECTIONCONVECTIONCONVECTION

CONVECTIONCONVECTION

COST MIN. DISTORTIONMIN. DISTORTION

From ambient temperature to 1400°F

24

0

20

40

60

80

100

120

140

160

Out-of roundness Out-of-flatness

CHANGE

μm

Influence of heating method on changes in shape and dimension

CONVECTION RADIATION Source: Altena

25

Uniformity of Cooling

Gas flow pattern and uniformity of flowControl of cooling speedLoad position and fixtures designPressure and furnace design

26

Cooling Speed Parameters

PressureGas velocity - design, furnace size, blower, water

system, ratio between load and hot zone surfaceGas type Cooling speed Δt = (V/A p c)s (1/α) ln [(T1 –Tg) / (T2 – Tg) Heat exchange coefficient

α=c w.7 p .7 ŋ-.39 cp .31 λ.69

27

Vacuum Furnace Schematic

Hot Zone Heat Exchanger

Quench Motor

Quench Fan

Charge/ Load

28

External CoolingExternal Cooling

HeatHeatExchangerExchanger

CoolingCoolingBlowerBlower

RadiationRadiationShieldsShields

Isolation ValveIsolation Valve

External fanExternal fan

External heat exchangerExternal heat exchanger

29

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

CHANGE

μm

30

Load Configuration

Space between parts / load density

Grouping of similar partsGas flow restrictionPart placement Parts hanging and type of

fixturing

31

Distortion Control – continue

Use convection heating from ambient to 1400°FUse isothermal quench processH-13 hold at 1200°F and 1560°F to allow for equalization of temperature (ΔT 100°F at 1200°F and

ΔT 80°F at 1560°F) and use isothermal quench.Stack or hang long parts verticallyUse the “right” pressure to minimize distortionGroup or tie together similar parts

32

Distortion Control

Control temperature uniformity during the phase transformation.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 loading – dummy parts, shields, low gage fixtures, baskets and grid made from low

expansion material. (Graphite or CFC material)

Low distortion heat treatment of transmission components

Quench Cell design- uniform gas flow pattern

Fixture design- Optimized mech. support of components and optimized gas flow pattern in the load

Optimized LPC & HPGQ process -application of convective heating

-application of Dynamic / Reversing Quenching and choose Helium as quench-gas

Stable manufact. chain before heat treat

- Low level of residual stress in components before heat treatment

Summary

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THANK YOUJanusz Kowalewski

janusz@dynatechfurnaces.com