O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 1 Tungsten Armored Ferritic Steel...

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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY

Tungsten Armored Ferritic Steel

Glenn Romanoski & Lance Snead

June 2004

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Phase I : Fabrication Process and RepairTungsten Armored Low Activation Ferritic Steel

Objective: select and optimize methods for bonding tungsten to a Low Activation Ferritic Steel and assess the integrity of these coatings under IFE relevant thermal fatigue conditions.

Approach: - Evaluate methods for applying tungsten coatings to F82H steel substrates.

Fabricate and study adherence and thermal stability. Is this material combination viable? FY-04 Milestone.

- Given W thickness (100μm to 250µ nominal) and thermal boundary conditions, assess the stability and fatigue performance of the underlying LAF. FY-04 Milestone.

- Screen coupon coatings using thermal fatigue facility. Select candidate monolithic armor system or move to “engineered structure.” FY-05 Milestone.

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IFE Relevant Performance Assessment is Critical to Armor Design and Material Selection

The goal of materials performance assessment is to be relevant if not equivalent.

The goal of materials selection and design is to meet or exceed the operational and durability requirements of the IFE first wall.

How close is our most relevant thermal fatigue test conditions to IFE equivalent conditions?

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IR Thermal Fatigue Facility

0

5

10

15

20

25

-200 0 200 400 600 800 1000Time (ms)

Hea

t flu

x (M

W/m

2 )

• Facility has been used for interfacial fatigue of W/LAF

• Previously 20 MW/m2 (time average), 20 msec pulse, 10 Hz, 10 cm2

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Facility Improvements : IR Thermal Fatigue

• Now capable of 100 MW/m2 (time average), 2 msec pulse, 10 Hz, 5 cm2

• Phase 1 goal 1000 MW/m2 (time average), 0.1 msec pulse, 10 cm2

10

100

1000

104

0.001 0.01 0.1 1 10 100

Hea

t L

oad

(M

W/m

2 )

Time (milliseconds)

IFE

~104 MW/m2

~ 10 sec

IR upgrade

~102 MW/m2

~ 2 msec

IR Thermal FatigueFacility~20 MW/m2 ~ 20 msec

> 0.1 MJ/m2

~ 0.4 MJ/m2

~ 0.2 MJ/m2

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Simulating HAPL Interface Stresses

The IR heat load cannot duplicate tungsten surface stresses, but it can duplicate the interface stresses

If the tungsten layer is sufficiently thick, preserving the time-averaged heating is sufficient

The key metric is the stress distribution

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Comparison of Stress Distribution

7.2 MW/m2 20 ms pulse

-200

-100

0

100

200

0 0.5 1 1.5 2 2.5 3 3.5

HAPL baseline

Infrared heating

Str

ess

(MP

a)

depth (mm)

Armor interface

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F82H Ferritic Steel with 100µm of Tungsten Armoris the Reference Material/Design Solution

Infrared fusion of tungsten powder

Diffusion bonding of tungsten foil

Vacuum plasma spraying powder

Alternative approaches, e.g., CVD

Processing Method Method of Screening

Thermal stability of the interface

will be assessed under cyclic and

isothermal conditions.

Thermal fatigue performance of the

tungsten armor and substrate will

be assessed with the IR plasma arc

lamp.

Interfacial Strength will be

measured using flexural tests.

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Preliminary Thermal Cycling Test Results Illustrate the Issues Pertinent to Material Selection, Processing and Testing.

Processing Method

Initial tests showed promise -----Coatings adhered after 1000 cycles.

Tensile cracks developed in the substrate due to CTE mismatch and phase changes. Diffusion bonding below the phase transformation temperature will be tried.

Dissolution of carbides in the steel at the interface indicated that the temperature probably exceeded 900ºC. Diffusion of tungsten into the steel could generate brittle phases such as FeW and Fe2W.

Thermal management of the substrate and incident heat flux is critical to a meaningful thermal fatigue test.

Diffusion bonded tungsten foil

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Thermal conductivity of W and F82H defines the interface temperature for fixed geometry and thermal boundary conditions.

Thermal Conducti vi ty versus Temperature

1

10

100

1000

10000

1 10 100 1000 10000

Temperature (K)

k(W/

mK) Tungsten

F82H Steel

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Microstructural stability of F82H will limit the interface temperature to about 800C

Coarsening of carbides in the F82H steel above 800C and dissolution around 900C will degrade mechanical properties.

The alpha – gamma - alpha phase transformation and CTE mismatch will impart strains at the interface.

A critical thickness of tungsten may be required to dissipate the heat pulses to maintain the interface in an acceptable temperature regime. Will that be adequate?

A thermal model of our experiment and appropriate instrumentation of specimens is the key to running meaningful experiments.

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Alternative Design/Material Approaches

Given the selection of a ferritic steel as the substrate material, the interface temperature is limited to 800ºC. What thickness of W coating and back face heat flux is required to maintain a stable interface? Is 600ºC an appropriate far field temperature?

Given the selection of tungsten as the armor material, are there other substrate materials with higher temperature capability?

Group VA alloys (V, Nb or Va) could serve as an intermediate layer having higher temperature capability. Sensitivity to oxygen would be a consideration.

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04 Milestone : Go/No Go on tungsten armor. Is tungsten clad F82H steel a viable material option?

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Where are we ???

• Vacuum plasma sprayed W on F82H is the principal material candidate. A number of material conditions are ready for testing. Additional material conditions exploring the extremes of the W/F82H material option will be produced, e.g., thick tungsten coatings.

• Achieving thermal similitude between our thermal cycling test and the IFE condition is critical. Well modeled and instrumented tests are our goal.

• Long-term stability of interface is required. Is the temperature limits imposed by the current choice of materials and/or design too limiting for the thermal boundary conditions of the IFE first wall?

Do these temperature limits result in desired system efficiency?

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Development of Armor

Fabrication process and repair

He management Mech. & thermal fatigue testing

“Engineered Structures”Ablation

Underlying Structurebonding (especially ODS)

high cycle fatiguecreep rupture

Armor/Structure Thermomechanicsdesign and armor thickness

detailed structural analysiis

thermal fatigue and FCG

Structure/Coolant Interface

corrosion/mass transfer/coating

2003 2004 2005 2006 2007

Development of W/LAF : Phase 1 Effort and Milestones

!!

!

!

} !

scoping optimization scaling

!

!scoping & modeling optimization

! !

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Comparing Data from Different Exposure Experiments It is customary to correlate surface effects with fluence, but exposure

times and deposition depths vary across the different experiments This implies different stresses for the same fluence

If roughening is a thermomechanical phenomenon, then we must compare thermomechanical results across the experiments (peak surface temperature, peak surface temperature gradient, peak stress, plastic strain range, stress intensity factor, etc.)

We intend to use peak surface temperature, stress intensity factor, and plastic strain range for our comparisons

If the correlation across the experiments is consistent, then we will use this as the primary design criterion

It is important that we are able to deduce the damage mechanism as a result of this process

pulse

reft

TT1

Fixed fluence; surface heat

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RHEPP Simulations (p, N+, N++)

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500

1000

1500

2000

2500

3000

3500

4000

0 0.5 1 1.5 2 2.5

Fluence (J/cm2)

Ma

x S

urf

ac

e T

em

p (

C)

0

0.5

1

1.5

2

2.5

3

3.5

0 0.5 1 1.5 2 2.5

Fluence (J/cm2)

Su

rfac

e S

trai

nra

ng

e (%

)

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Next Step: Fracture Comparisons

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0 200 400 600 800 1000 1200

25 um crack50 um crack75 um crack100 um crack150 um crack

Sre

ss I

nten

sity

(M

Pa

-m1

/2)

Crack spacing (m)

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 1 Tungsten Armored Ferritic Steel...

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