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9Cr ODS Material Development

T.S. Byun

Oak Ridge National Laboratory

Materials Science and Technology Division

Presented in Webinar on July 31, 2013

DOE-NE Materials Crosscut

Coordination Meeting - 2013

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Acknowledgement & Contents

Low toughness issue in high strength NFAs

(Advanced ODS Steels)

Proposed approach for toughening Fe-9Cr NFAs

Integration of process technologies for high

toughness NFAs

Microstructural characteristics & stability of

developed materials

High temperature mechanical properties

Summary & further studies

• DOE/FC R&D Core Materials (led by S.A. Maloy of LANL)

• I-NERI Collaboration (US-Korea) for Developing 9Cr NFA with High

Toughness/Third (last) Yr

• T.S. Byun & D.T. Hoelzer (ORNL)/J.H. Yoon (KAERI)

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Tensile fracture at 900°C (14YWT-SM10).

Microcracks propagate along grain

boundaries without significant

deformation: Low energy GB

decohesion results in low fracture

toughness.

High Temp. Fracture Process & Toughness

0

50

100

150

200

250

0 200 400 600 800

Temperature, °C

KJQ

, M

Pa

√m

14YWT (SM10)

HT-9 Steel

Application to reactor core components requires high toughness for both

irradiation performance & manufacturing process:

• Low DBTT shift; resistance to radiation-induced embrittlement

• High irradiation & thermal creep strength; high swelling resistance

• Deformability & cracking resistance for tube drawing and forming processes

Brittle fracture in linear elastic region, semi-

brittle fracture with small inelastic strain, or

low load crack propagation

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Approach for Toughening NFAs:

Modification of Grain Boundary

Designed to enhance diffusion

bonding at grain boundaries:

I. Partial phase transformation

(intercritical annealing)

II. Partial phase transformation

along with plastic deformation

(controlled rolling)

Erase/modify weak grain &

aggregate boundaries for higher

bonding.

Diffusion is greatly enhanced

with partial phase

transformation, and further

enhanced by plastic deformation.

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Characterization

Process Technology:

Production of Base Materials (NFA 9YWTVs)

Two alloy power heats (8 kg each) have

been produced by gas atomization

process at ATI Powder Metals:

Fe-9Cr-2W-0.4Ti-0.2V-0.12C+0.3Y2O3 &

Fe-9Cr-2W-0.4Ti-0.2V-0.05C+0.3Y2O3

Extrusion below 850°C &

Cut into 4 inch long coupons

Ball milling for

40 hours in Zoz

CM08 machine

(6 loads)/

Canned &

degassed (6

cans, 920g

each)

• Post-Extrusion TMT

Optimization

• Micro & High T. Mechanical

Characterization

• Feedbacks for new processing

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Base Materials:

NFA 9YWTV-PM1 & PM2

9YWTV-PM2

Fe-9Cr-2W-0.4Ti-0.2V-0.05C+0.3Y2O3

9YWTV-PM1

Fe-9Cr-2W-0.4Ti-0.2V-0.12C-0.3Y2O3

Chromium (ferrite stabilizer)

equivalent of NFAs:

- 14YWT (17%)

- 9YWTV-PM1 (10%)

- 9YWTV-PM2 (11.7%)

*α to γ transformation needs

Cr-eq < ~12%

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Base Materials:

Strength & Ductility of 9YWTVs & 14YWT

• New base NFAs retain YS higher than 500 MPa at 700°C.

• Improved ductility is measured for both new NFAs.

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Process Development & Optimization:

Isothermal Annealing (IA)

Mini tensile (SS3) and fracture

(TPB & DCT) specimens

Servohydraulic testing system (MTS 858 table top

model) equipped with high vacuum (10-7 torr), high

temperature (1000°C) furnace (Oxy-Gon, custom

model, Ta heating elements)

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• The as-extruded coupons were hot-rolled in controlled conditions: at

900 – 1000ºC for 20 or 50% total thickness reduction.

Process Development & Optimization:

Controlled Rolling (CR)

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Production of 9Cr NFAs

• 1st Production: 9YWTV-

PM1 & PM2

• About 6 kg total

• Consumed for basic

property examination and

major mechanical

properties

• 2nd Production: 9YWTV-

PM2 only

• About 3 kg total

• Consumed for major

mechanical property tests

• Further studies

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Annealing

Time/Temp.

30 or 60 min 200 min 20 hr

830 °C O O

850 °C O O O

875 °C O O

900 °C O O

950 °C O O

975 °C O

Rolling

Strain/Temp.

20% 50% Remark (Bend Bar)

900 °C O O L-T/T-L

925 °C O L-T

950 °C O L-T

975 °C O L-T

1000 °C O L-T

Isothermal (Intercritical) Annealing (IA)

Controlled Rolling (CR) with IA

9YWTV PM1: Fe(bal.)-9Cr-2W-0.4Ti-0.2V-0.12C

9YWTV PM2: Fe(bal.)-9Cr-2W-0.4Ti-0.2V-0.05C

Process Development & Optimization:

TMT Matrix

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Relatively equiaxial in A; higher aspect ratio in B, RD//110 grains dominant

B. 975ºC, 50% rolling

5㎛

A. 900ºC, 20% rolling

ND

ND

RD

RD 111

001 101

EBSD Images of 9Cr NFAs (PM2) after CR

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Grains after 900C rolling are slightly larger and have are more equiaxial.

Grains grow from 100 – 300 nm in as-extruded condition to 300 – 900 nm after CR.

Grain Size of 9Cr NFAs (PM2) after CR

(Equivalent Circular Diameter)

1 2 30

2

4

6

8

10

1 2 30

2

4

6

8

10

Grain size [㎛]

Fra

cti

on

[%

]

1 2 30

5

10

15

20

1 2 30

5

10

15

20

Fra

cti

on

[%

]

Aspect ratio

900C 20%R 975C 50%R

Grain size [㎛]

Aspect ratio

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High Temp. Fracture Test Data

Effect of IA in 9YWTV-PM1

Small change of strength is observed after annealing at 975°C.

No evidence of fracture toughness improvement is found in annealed

9YWTV-PM1 except for the 975°C annealed specimen tested at 500°C.

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High Temp. Fracture Test Data

Effect of IA in 9YWTV-PM2

Fracture toughness is improved in 9YWTV-PM2 by intercritical annealing,

but further improvement is desirable especially at high temperatures.

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High Temp. Fracture Test Data

Effect of CR in 9YWTV-PM1

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High Temp. Fracture Test Data

Effect of CR in 9YWTV-PM2

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0 2 4 6 8 10 12 14 16

0

400

800

1200

1600

2000TENSION (R.T.)

9YWTV-PM1 (High C)

9YWTV-PM2 (Low C)

PM1-975C-50%

PM2-975C-50%

PM2-900C-50%

PM2-1000C-50%

PM1-1000C-50%PM1-900C-50%

En

g. S

tre

ss (

MP

a)

Eng. Strain (%)

PM1-900C-20%

PM2-900C-20%

Uniaxial Tensile Curves

Effect of CR in 9YWTV-PM1 & PM2

0 4 8 12 16 20 24 28

0

200

400

600

800

1000

1200 TENSION (500oC)

9YWTV-PM1 (High C)

9YWTV-PM2 (Low C)

PM2-975-50%

PM1-1000-50%

PM2-900-20%

PM2-900-50%

PM2-1000-50%

PM1-975-50%

PM1-900-20%

En

g. S

tre

ss (

MP

a)

Eng. Strain (%)

PM1-900-50%

Strength decreased with

the rolling temperature and

reduction of thickness.

High temperature strength

was reduced by CR (by

~200 MPa).

RT

500°C

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High Temp. Crack Resistance Test

Effect of CR in 9YWTV-PM2

Improvement of fracture

toughness in controlled

rolled 9YWTV-PM2 is

significant.

The 9YWTV-PM2 controlled

rolled at 900°C resulted in the

best fracture toughness

among NFAs, which is as

high as those of non-ODS

F/M steels.

TMT condition is selected for

detailed characterization and

further studies: 900°C 50%R.

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9YWT-PM2 900C 50%R tested at 700°C 9YWT-PM2 900C 50%R tested at 22°C

9YWT-PM2 after extrusion tested at 22°C 9YWT-PM2 after extrusion tested at 700°C

Effect of CR on Fracture Mechanism in 9YWTV-PM2

A change in high T fracture mechanism from boundary

decohesion to formation of flake-like shear tongues.

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Volume fractions of FCC phase (red) and BCC phase (blue) in 9YWTV-PM1

(left) and in 9YWTV-PM2 (right) after heat-treatment at 1000oC

Issue in Process Development:

Microstructural Characterization by In-Situ X-Ray

• The volume fraction of FCC in 9YWTV-PM1 gradually increases with time up

to about 65%, while that in 9YWTV-PM2 appears to saturate at about 5%.

• This sluggish phase transformation in PM2 might be due to the early

depletion of carbon content in ferrite, which is a strong austenite former.

• Detection accuracy is low with these nanostructured materials.

Equilibrium Volume Fraction:

100%

Equilibrium Volume Fraction:

50%

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1) Very high strength can be achieved in NFAs at the expense of fracture

toughness and ductility. Low energy decohesion at boundaries causes

poor fracture resistance at high temperatures.

2) Isothermal annealing & controlled hot-rolling were used to strengthen

the powder-metallurgy produced weak boundaries by enhanced

diffusion bonding.

3) Improvement of fracture toughness in controlled rolled 9YWTV-PM2 was

significant: fracture toughness was as high as those of non-ODS F/M

steels.

4) In particular, the 9YWTV-PM2 controlled-rolled at 900°C resulted in the

best fracture toughness among NFAs (> 150 MPa√m over RT - 700°C).

High toughness (engineering grade) NFA has been developed.

5) Detailed characterization & further studies on selected materials need to

be done: (a) delayed phase transformation, (b) stability of nanoclusters,

(c) grain growth, (d) irradiation experiments of the base 9Cr NFAs and

high toughness NFAs, and (e) testing in thin-walled tube form.

Summary & Further Studies