Bonding and Long Term Stability of Tungsten-Armored Ferritic Steel

1
Bonding and Long Term Stability of
Tungsten-Armored Ferritic Steel

• Glenn Romanoski, Lance Snead,
• Adrian Sabau, Joseph Kelly,
• Steve Zinkle
• HAPL Workshop
• University of Rochesters Laboratory for Laser
  Energetics
• November 8 and 9, 2005

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Development of W/LAF Phase 1 Effort and
Milestones

Development of Armor Fabrication process and
repair He management Mech. thermal fatigue
testing Engineered
Structures Ablation Underlying
Structure bonding (especially ODS) high cycle
fatigue creep rupture Armor/Structure
Thermomechanics design and armor
thickness detailed structural analysis thermal
fatigue and FCG Structure/Coolant
Interface corrosion/mass transfer/coating
2003
2004
2005
2006
2007
scoping
optimization
scaling
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scoping modeling
optimization
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Phase I MilestonesTungsten Armored Low
Activation Ferritic Steel

• Objective select and optimize methods for
  bonding tungsten to LAF steel and assess the
  integrity of these coatings under HAPL-relevant
  thermal fatigue conditions.
• Phase 1 Endpoint Perform scaling studies and
  carry out prototype thermal fatigue at IFE
  relevant conditions
• gt 105 cycles, lt100ms pulse width
• gt 103 MW/m2 (during pulse)
• gt 10 cm2 sample face

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Fabrication and CharacterizationTungsten Armored
Low Activation Ferritic Steel

• Objective Evaluate methods for bonding tungsten
  to F82H Low Activation Ferritic (LAF) Steel and
  assess the integrity of these coatings under IFE
  relevant thermal fatigue conditions.
• Where we are A number of options for
  fabricating W/LAF were studied. Plasma sprayed
  tungsten clad LAF has been selected as the
  primary candidate armor. Initial testing are
  still very positive
• - good uniformity and adhesion
• - thermal fatigue properties were promising
• Recent Work With a viable material the next
  questions involve
• long-term stability
• - low cycle fatigue (crossing
  brittle-to-ductile transition)
• - high cycle fatigue (thermal and mechanical
  loading)
• - long-term thermal (microstructural)
  stability

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Isothermal aging experiments are continuing to
characterize the thermochemical stability of the
Tungsten/F82H coating system.
Invar Screws
Assembled Diffusion Couple
Plasma Sprayed Specimen
Evacuated and Sealed Quartz Tube
F82H
Tungsten Foil

• Tungsten/F82H diffusion couples are being aged at
• 550ºC to 900ºC for times ranging from 100h
  to 4,000h.
• Post aging analysis of samples includes
• - Phase identification by XRD
• - Microchemistry profiles by Electron
  Microprobe and Auger
• - Phase equilibrium analysis by Thermocalc
  and JMat Pro

F82H
Diffusion Couple
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Fe-W binary phase diagram indicates the potential
formation of two intermetallic phases, Fe2W and
FeW, in the temperature regime of interest.
These phases were not identified by XRD
???
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Isothermal Aging of W-F82H Samples Iron-Tungsten
Carbides Formed at Interface
Temp. Aging Time Aging Time Aging Time Aging Time
Temp. 100 h (Completed) 1000 h (Completed) 2000 h (Completed) 3000 h (Completed) 4000 h (Dec. 05)
550ºC - - underway -
600ºC No Reaction No Reaction Fe6W6C underway
650ºC - - Fe6W6C -
700ºC No Reaction No Reaction Fe6W6C Fe2W2C Fe3W3C Fe6W6C
750ºC - - Fe3W3C Fe6W6C -
800ºC Fe3W3C Fe3W3C Fe6W6C Temperature Discontinued Temperature Discontinued Temperature Discontinued
900ºC Fe3W3C Fe6W6C Fe3W3C Fe6W6C Temperature Discontinued Temperature Discontinued Temperature Discontinued
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The small volume of Fe-W carbides formed at the
interface of assembled diffusion couples may be
attributed to limited availability of free carbon
available for reaction.

• The majority of the 0.1 wt. carbon in alloy F82H
  Steel is bound to matrix carbides.
• Plasma sprayed tungsten typically has carbon
  impurities around 50 ppm.
• Debris ions from the fuel represent a potentially
  unlimited supply of free carbon.

Tungsten Foil
Reaction Product
F82H Steel
2000h _at_ 650C
W-F82H 2000h _at_650C
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Iron-Tungsten Carbides can be seen in this
isothermal section (_at_ 1000ºC) of the Fe-C-W phase
diagram. Thermodynamic equilibria are less well
characterized at lower temperatures.
Atomic Percent Tungsten
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Carbon from fuel debris ions will likely react
with the tungsten surface to form tungsten
carbides. Surplus carbon will diffuse to
interface and potentially react.
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Auger Electron Spectroscopy define carbon
concentration profiles in the substrate,
interface and coating.
O
C
Tungsten
Interface
F82H Steel
10.0kX
20.0keV
1.0 µm
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IR Thermal Fatigue Capability200 MW/m2 upgrade
completed
Heat Source Max Energy _at_ Min Pulse Width (MJ/m2 ) Min Pulse Width (ms) Max Heat Flux (MW/m2 ) Frequency (Hz)
IFE f(radius) 0.1 0.01 104 5 to 10
1x10 cm Plasma Lamp Standard Power Supply 0.7 20 35 10
1x10 cm Plasma Lamp Capacitor Power Supply 0.4 2 200 tbd
10x30 cm Plasma Lamp Standard Power Supply 0.1 20 5 10
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The IR thermal fatigue test has been
re-configured to achieve steeper temperature
gradients and to manage excess incident radiation.

• Steady state heat flux experiments have
  demonstrated a constant temperature profile in
  the chill block.
• The bottom end temperature is pegged near the
  inlet water temperature.
• Steady state experiments are used to calibrate
  thermal fatigue tests

Chill Block Height 2.2 cm
Steel Column Copper Base
Water Cooled Base
Active Shielding of Excess Radiation
Specimen
Water Cooled Copper Shield
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Low Cycle Fatigue experiments are underway.
Encapsulated, tungsten-clad F82H steel samples
are furnace cycled 100ºC to 650ºC to 100ºCat a
rate of 4 cycles/day. No cracking or spalling
has been observed at 200 cycles.
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Current Status
Vacuum plasma sprayed W on F82H Steel remains
the principal material candidate. All
preliminary testing suggest that this coating
system will be tenacious and tolerant to thermal
cycling. Long-term stability of the
interface is required. Isothermal aging
experiments will continue to assess the
thermochemical stability of the interface.
Iron-Tungsten Carbides are forming. Diffusion
couples are revealing kinetics. A combination of
Furnace Annealing Infrared Fatigue testing will
indicated if intermetallic formation is
problematic. --gtTests that incorporate excess
carbon to simulate a debris ion source are
needed. An experimental program combining
C-ion implantation, annealing, and Infrared
Fatigue testing is proposed.
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Current Status-continued
The thermal fatigue test has been reconfigured
to result in a steeper and better controlled
temperature gradient across the W-F82H interface.
Software issues for continuous operation delayed
the start of high cycle thermal fatigue tests.
Fatigue tests with durations of 1 million cycles
will begin soon. Low cycle fatigue tests will
continue with encapsulated samples (100ºC to
650ºC to 100ºC) at a 4 cycle/day frequency. To
date, no problems found.
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2000C
Non-Implanted Region 1 micron grains
Single step annealed for 2 sec. 10 micron grains
1000 step annealed for 33 min. 100 micron grains
Size of TEM specimen
Size of TEM specimen
20mm
2?m .
20mm
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New thermal fatigue configuration enables rapid
equilibration of a steady state heat flux during
preheat and thermal cycling.
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A decrease in the chill block height resulted in
greater calculated temperature swing, DT, at the
interface. Continuous cycling test will be
performed this month.
Chill Block 4.4 cm height
Chill Block 2.2 cm height
Temperature C
Temperature C
DT 100ºC
DT 55ºC
TI Interface Temperature