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