International Conference on Fast Reactors and Related Fuel Cycles: Safe Technologies and Sustainable Scenarios (FR13)

Paris, France

March 4-7, 2013

Development of Advanced 9Cr Ferritic-

Martensitic Steels and Austenitic Stainless

Steels for Sodium-Cooled Fast Reactors

T.-L. Sham, L. Tan and Y. Yamamoto

Oak Ridge National Laboratory, USA

Outline

Motivation for advanced structural materials

U.S. effort on SFR materials downselection

Advanced 9Cr ferritic-martensitic (FM) steels

– Tensile properties

– Thermal aging resistance

– Creep resistance

Advanced austenitic stainless steels

– Weldability

– Tensile properties

– Thermal stability

– Creep resistance

Summary

2

Motivation for

Advanced Structural Materials

Ferritic-martensitic steels and

austenitic stainless steels are

primary structural materials of

SFRs.

Choosing the right materials can impact key requirements for advanced fast reactor development

– Economy: reduce capital costs through reduced commodities and simplifications

– Flexibility: higher material performance allows greater options to designers

– Safety: higher material performance promotes larger safety margins and more stable performance over longer lives

200 mm Dia.

593.3 C

32 MPa

3

U.S. Advanced Materials Development

for Sodium Fast Reactors

2008

Established Alloy

Development Priority List

2009-2012

Alloys Downselection

2013-2015

Intermediate Term

Testing to Confirm

Enhanced Properties

• Considered a large class of

structural materials for further

development

• Involved 5 U.S. national

laboratories and 5 U.S.

universities

• Considered experience from

Fusion, Gen IV, Space

Reactor, and development

activities in Fossil Energy

• Established alloy

development priority list: ─ Ferritic-Martensitic steels

• Grade 92 (NF616)

• Grade 92 with thermo-

mechanical treatment (TMT)

─ Austenitic stainless steels

• HT-UPS

• NF-709

• Established comprehensive

downselection metrics

• Considered tensile properties, creep,

creep-fatigue, toughness, weldability,

thermal aging, sodium compatibility,

mechanical and TMT processes

• Integrated R&D activities by DOE Labs Oak Ridge National Laboratory

Argonne National Laboratory

Idaho National Laboratory

• Materials considered include Optimized-Gr92, Ta/Ti/V-modified 9Cr, Gr92,

Gr91 (baseline material)

HT-UPS (Fe-14Cr-16Ni), Modified HT-UPS,

NF709 (Fe-22Cr-25Ni), 316H (baseline

material)

• Based on overall performance w/

comprehensive metrics (and accelerated

test data), Optimized-Gr92 with TMT and

NF709 were downselected for further

assessment

• Further optimize

mechanical and TMT

processes

• Procure larger heats

• Validate performance

gains

• Longer-term testing of

base metals and

weldments

• Irradiation campaign

planning

• Development of

roadmap for ASME

nuclear code cases

4

This Presentation

Focus on some aspects of downselection results

– Creep properties

– Resistance to thermal aging

– Weldability of austenitic stainless steels

5

Advanced 9Cr FM Steels:

Tensile Properties

Compared to commercial Gr92, the advanced 9Cr FM steels had about 30-

60% increase in yield strength with about 5-40% reduction in total

elongation.

– The reduced elongations are still greater than the minimum requirement of Gr92

according to the ASTM standard A335/A213.

Δσy = (σ – σG92)/σG92

Δεt = (ε – εG92)/εG92

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σG92 – yield strength of Gr92

εG92 – total elongation of Gr92

Room-temperature tensile tests indicate that the Ta/Ti/V-modified heats showed

aging-induced softening. However, the yield strength of the Ti-modified heat was

partly recovered with an increased total elongation after 1,000 h aging.

The optimized-Gr92 with TMT (Gr92-2b) showed aging-induced hardening with a

slight reduction in total elongation.

Advanced 9Cr FM Steels:

Thermal Aging Resistance at 600oC

7

100h

1,000h

100h

1,000h

100h

1,000h

100h

1,000h

Δσy = (σ – σG92)/σG92

Δεt = (ε – εG92)/εG92

σG92 – yield strength of Gr92

εG92 – total elongation of Gr92

Microstructures of the Aged

Optimized-Gr92 with TMT

The aging-induced softening was

primarily due to dislocation recovery.

– The dislocation density reduction would

reduce the strength by ~47% as

suggested by

The significantly increased ultrafine

precipitates after the aging at 600oC

for 1,000 h compensated the aging-

induced softening in the optimized-

Gr92 with TMT.

– The increased ultrafine particles would

increase the strength by ~70% as

suggested by the Orowan stress.

s r = 0.5MGb r f( )1 2

8

Advanced 9Cr FM Steels:

Creep Resistance at 600oC

Compared to Gr91, the advanced 9Cr FM steels had significant increases in

creep life, especially for the optimized-Gr92 with TMT and the Ti-modified

heats with up to about 700 times increase.

= L

/LG

r91

600oC

9

Alloy A HT-UPS

(Many small cracks at HAZ) (No cracks observed)

*Gas Tungsten Arc Welded, 11V, 180A, 8 inch/min, under Ar cover gas, applied 10% cold-roll prior to the welding.

1in

ch

1in

ch

Advanced Austenitic Alloys:

Weldability

Successful improvement of weldability of HT-UPS;

– Through careful control of alloying elements (both Alloys A and B).

– No defects on the surface and cross-sectional views.

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• No issues on

4t bend

specimens

• No visible

defects in

welds

Advanced Austenitic Alloys:

Weldability (cont’)

1” thickness plate weldments for further property evaluation

– Used double-V gas tungsten arc welding with the same filler material.

– Passed the standard inspection (4t bend test, ASTM E190).

– 10% cold-work (CW) applied to Alloy B prior to welding.

11

Double-V Welding 4t Bend Test

Alloy B

NF709

0

1

2

3

4

0.1 1 10 100 1000 10000

Aging time, h 0

YS, 10% CW

HT-UPS

Alloy A

Alloy B

NF709

0

1

2

3

4

0.1 1 10 100 1000 10000

YS

re

lati

ve

to

re

fere

nc

e n

on

-CW

HT

-U

PS

Aging time, h 0

YS, no CW

Alloy B NF709

HT-UPS

Advanced Austenitic Alloys:

Tensile Properties at 650oC

Evaluated high temperature tensile properties after aging at 650oC.

– Base alloys showed a good stability of the properties .

– 10% cold work (CW) improved the yield strength, YS (greater than a factor of 2X of HT-

UPS with no CW).

• Original HT-UPS and Alloy B showed less thermal stability than the others after >1,000h aging at

650oC.

12

0

20

40

60

80

100

650oC/200MPa

(no CW only)

(>100x)

0

100

200

300

400

500

Cre

ep

lif

e r

ela

tive t

o 3

16H

, x

10%CWno CW

(1x)

(1.6x)

700oC/200MPa

Advanced Austenitic Alloys:

Creep-rupture Properties

Strong Cold Work (CW) effect on improving creep-rupture life.

– HT-UPS showed the best properties among the candidates.

– NF709 exhibited the second best lives with/without CW.

– Alloy B showed better properties only when CW was applied.

13

No

rmali

zed

cre

ep

str

ain

Relative time (linear)

Alloy B, 700oC, 200MPa no CW

10%CW

10%CW + Cross-weld

Advanced Austenitic Alloys:

Creep-rupture Properties (cont’)

Welding did not degrade the CW effect on the creep-properties (Alloy B).

– Effect of microstructural changes after welding was negligible.

– Literature reported that NF709 also showed no degradation of creep-properties after

welding (only for no cold work, CW).

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Summary of Creep, Thermal Aging

and Weldability Aspects

The creep resistance of advanced 9Cr FM steels was greatly enhanced by

optimizing their compositions as well as by using TMT.

– Up to about 700 times increase in creep life, compared to Gr91, was achieved under

the accelerated test conditions at 600oC.

The increased density of ultrafine precipitates facilitated the increase in

strength and thermal aging resistance, leading to the improved creep

resistance.

Properties of four candidate austenitic alloys, HT-UPS, NF709, and two

modified HT-UPS alloy (designated Alloys A and B), have been evaluated

and compared with 316H.

– Alloys A and B showed successful improvement in weldability.

– Only a little difference in thermal stability of the alloys in solution annealed conditions.

10% cold work increased the yield strength of the alloys for more than 200% compared

to the HT-UPS without cold work.

– HT-UPS exhibited the best creep properties among the alloys with and without cold

work, and NF709 followed.

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Acknowledgments & Copyright

Notice

This work is sponsored by the U.S. Department of Energy, Office of Nuclear

Energy, Advanced Reactor Concepts (ARC) Program, under contract DE-

AC05-00OR22725 with UT-Battelle, LLC.

This PowerPoint presentation has been authored by UT-Battelle, LLC, under

Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The

United States Government retains and the publisher, by accepting this

power point presentation for publication, acknowledges that the United

States Government retains a non-exclusive, paid-up, irrevocable, world-wide

license to publish or reproduce the published form of this power point

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purposes.

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