PROGRESS IN THE DEVELOPMENT OF NANO-STRUCTURED STEELS

A.F.ROWCLIFFE (ORNL)

ARIES PROJECT MEETINGGERMANTOWN

MAY 21st-22nd 2013

Compared to the RAFS ( F82H,EUROFER) the NFAs have the potential to expand

operating temperature windows up to ~800C while accommodating much higher levels of displacement damage and helium concentrations

The microstructural nano-features provide strong barriers to dislocation movement conferring high tensile strength combined with good fracture resistance and outstanding high temperature creep resistance

The combined particle-matrix interfaces and nano-grain interfaces comprise a high surface area:volume ratio (~6 x 107 m2/m3) and provide a very effective trapping environment at all temperatures for helium atoms and for the recombination of point defects.

Nanostructured Ferritic Alloys (NFAs)

<Grain size> = 136 (+/- 14) nm

BF-TEM of 14YWT SM10 HeatNv = 1-7 x 1023 m-3 <r> = 1.8 (+/- 0.4) nm

HR-TEM

2

DEVELOPMENT OF 14YWT UTILIZED 1-2 KG BATCHES OF ALLOY POWDERS TO ESTABLISH CRITICAL RELATIONSHIPS BETWEEN PROCESSING

PARAMETERS AND MICROSTRUCTURES/PROPERTIES

HIP near net shape final product

Y2O3

NE-FUNDED INITIATIVE IN 2011TO DEVELOP A BEST-PRACTICE FABRICATION TECHNOLOGY FOR 14YWT

• OBJECTIVE: DEFINE A BEST PRACTICE PROCESSING ROUTE FOR CONSISTENT AND REPRODUCIBLE COMPOSITION AND MICROSTUCTURAL CONTROL, PREDICTABLE MECHANICAL PROPERTIES WITH POTENTIAL FOR SCALE-UP

• COLLABORATION: LED BY LANL ( Malloy) ORNL (Hoelzer) UCSB (Odette) UC-BERKELEY /UT ( Wirth) CRUCIBLE RESEARCH/ATI POWDER ( Stewart) SOUTH DAKOTA SCHOOL OF MINES ( West)

PROJECT HAS SUCESSFULLY DEMONSTRATED REPRODUCIBLE CONTROL OF COMPOSITION/IMPURITIES , MICROSTRUCTURES

AND MECHANCIAL PROPERTIES BASED ON 22KG BATCHES

• GAS ATOMIZATION UTILIZED FOR POWDER PREPARATION; [O] CONTROLLED VIA FeO ADDITIONS

• BALL MILLING ATMOSPHERE SUCCESSFULLY CONTROLLED FOR ADJUSTING [O] PICK-UP LEVELS AND MINIMIZING [C,N] CONCENTRATIONS

• CONSOLIDATION AND HEAT TREATING CONDITIONS ADJUSTED TO PRODUCE OPTIMAL MATERIAL ( 14YWT-PM2) EXHIBITING NEAR-ISOTROPIC MICROSTRUCTURE AND GOOD BALANCE BETWEEN STRENGTH AND DUCTILITY.

• SCALE-UP TO 60kg BATCHES IN PROGRESS

Tensile properties of 14YWT-PM2

• Intermediate yield/tensile strength to balance performance & formability

Comparing L and T orientations

“Best Practice” processing of 14YWT:

Tensile properties of FCRD-NFA1

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FCRD-NFA1L + T Orientations

25ºC200ºC400ºC500ºC600ºC700ºC800ºC25ºC200ºC400ºC500ºC600ºC700ºC800ºC

Str

ess

(M

Pa

)

Strain (%)

L

T 0

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FCRD-NFA1Tensile Properties

L and T Orientations

YS (MPa) L-OrientationUTS (MPa) L-OrientationYS (MPa) T-OrientationUTS (MPa) T-Orientation

Str

ess (

MP

a)

Strain (%)

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FCRD-NFA1Ductility Properties

L and T Orientations

UE (%) L-Orientation

TE (%) L-Orientation

UE (%) T-Orientation

TE (%) T-Orientation

Elo

ng

atio

n (

%)

Strain (%)

Trends: Strength is lower; ductility is higher

compared to previous heats Only minor effect of anisotropy

ODS 14YWT-PM2 heat:

Anisotropy problem reduced

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PM2

Kj(M

Pa²

m)

Temperature °C

LE

EP

DT Max load KJc

14YWT-PM2 (Cross rolled 50%) OW4 (HIPed)

DT – Ductile TearingEP – Elastic PlasticLE – Linear Elastic

Extrusion/cross rolled (14YWT-PM2) vs. HIP (OW4) showed:• Higher fracture toughness at room temperature (25ºC)• Very low fracture toughness transition temperature (FTTT)• No effect of orientation on FTTT, i.e. no anisotropy UCSB: FT tests

L-TT-L

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22000 24000 26000 28000 30000 32000 34000 36000

NFA 12YWTNFA MA9579Cr-WMoVNb SteelNFA 14YWT

LMP T(K)[25 + log10t(h)]

Str

ess

(MP

a)

900°C, 1104h

800°C, 14235h

800°C, 817h

650°C, 13000h

600°C, 17000h

650°C, 1080h650°C16h

650°C>72h

825°C, 82488h(in test)

800°C, 38555hMA957

800°C, 41472h(14YWT in test)

Creep performance of 12YWT, MA957 and 14YWT

Creep tests still in progress: MA957: test at 825ºC and 70 MPa started in Oct., 2003 (INERI) 14YWT-SM10: test at 800ºC and 100 MPa started in April, 2008 (INERI)

D.T.Holzer et al.6th GETMAT Tech Meet.KIT,Jan 2013

The Larson Miller Parameter

TEM MICROSTRUCTURE OF Fe-Cr-Si-Mn-C SUPER BAINITIC STRUCTURAL STEEL

• ALTERNATING PLATELETS OF AUSTENITE AND FERRITE PRODUCED BY CONTINUOUS –COOLING TRANSFORMATION FROM AUSTENITE

• INTERPHASE INTERFACIAL AREA PER UNIT VOLUME IS EXTREMELY HIGH AND COMPARABLE TO THAT OF NFA-14YWT

CHARACTERISTICS OF NANOSTRUCTURED BAINITIC STEEL

• TYPICAL COMPOSITION WT.% : Fe-1.5Si-1.9Mn-0.2Mo-1.0Cr-0.8C

• THE INTERFACIAL AREA/UNIT VOLUME Sv IS LARGE ENOUGH THAT THE GOVERNING LENGTH SCALE ( PRINCIPAL DETERMINANT OF STRENGTH , DUCTILITY) L= 2/ Sv IS OF THE ORDER 20-50nm

-YIELD STRENGTHS ~2000MPa

-TOTAL ELONG. ~5-25% - FRACTURE TOUGHNESS ~30-40MPa.m-1/2

• THE EFFECTIVE GRAIN SIZE IS THE PLATELET THICKNESS; THE AUSTENITE PLATELETS PROVIDE STRAIN HARDENING CAPACITY AND CRACK TIP BLUNTING AND IMPROVED TOUGHNESS

POTENTIAL OF BAINITIC STRUCTURAL STEELS FOR FUSION APPLICATIONS

• CURRENT BAINITIC STEELS SUCH AS THE 3Cr-WV REDUCED ACTIVATION STRUCTURAL STEELS ARE CHARACTERIZED BY PLATELET THICKNESSES IN THE 250-400nm RANGE AND LOWER STRENGTH ( 600-750MPa) AND HIGHER TOUGHNESS COMPARED TO THE NANOSTRUCTURED STEELS

• THE AUSTENITE TO BAINITE TRANSFORMATION IS PARTICULARLY AMENABLE TO CONTROL VIA COMPOSITION CHANGE( Si,Mn,C,Cr,W) AND BY EITHER CONTINUOUS COOLING OR ISOTHERMAL HEAT TREATMENTS

• THE EXTENSIVE KNOWLEDGE BASE ON BAINITE TRANSFORMATION COULD PROVIDE A FRAMEWORK FOR THE POSSIBLE DEVELOPMENT OF BULK NANOSTRUCTURED ALLOYS WITH FUSION-SPECIFIC ATTRIBUTES ( CORROSION RESISTANCE, WELDING/ JOINING,RADIATION DAMAGE TOLERANCE)

Some Key Results

Y phase separates in atomized powders requiring milling for mixing and NF formation

Improved milling impurity (N, O) control showed the critical importance of O balance in forming NF and achieving excellent balance of properties

Baseline large batch of ATI powders contained low O• Powder subsequently milled with FeO to optimize O level• Led to OW4 HIPed at 1150°C (UCSB) and PM2 extruded at

850°C (ORNL)

PM2 was the final small precursor to the larger “best practice” heat (FCRD-NFA1) tentatively selected in part due to fine and uniform grain structures and balanced intermediate strength

MECHANICAL BEHAVIOR OF Fe-Si-Mn –Cr NANOSTRUCTURED SUPER BAINITIC STEEL

• (a) STRENGTH INCREASES WITH VOLUME FRACTION OF FERRTIE AND WITH DECREASING PLATELET THICKNESS

• (b) DUCTILITY AND TOUGHNESS INCREASES WITH VOLUME FRACTION OF AUSTENITE PLATELETS

NFAs have outstanding strength and creep properties

M.A.Sokolov, D.T.Hoelzer, L.Tan (ORNL)

12YWT: First discovery of NC by 3DAP in 1999 (ORNL)

MA957: Patented by INCO (1978); NC discovered by 3-DAP in 2003 (ORNL)

14YWT: Developed at ORNL; initial project started in 2001

• MA957 and 12YWT are not commercially available- 12YWT was produced only

once as a small heat by Kobe Steel, Ltd.

- Production of MA957 was discontinued by INCO

Temperature, oC

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Yie

ld S

tren

gth

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Pa

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12YWT9Cr-2WVTa14YWT-SM614YWT-SM10PM2000MA95714YWT-SM7F82H

FRACTURE TOUGHNESS-STRENGTH PROPERTIES

• UNIQUE NANO-SCALE MICROSTRUCTURE OF THE LOW ACTIVATION NFA 14YWT CHALLENGES THE HIGH STRENGTH-LOW TOUGHNESS PARADIGM FOR STRUCTURAL ALLOYS

D.T.Hoelzer ORNL, unpublished work

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14YWT

7000 Series Al Alloy

Multi-phase Nb Alloy

Zr BMG

Super Bainite Steel

AF1410

Fra

ctu

re T

ou

ghn

ess /

T

(MP

am

1/2/K

)

max

(MPa)

Neutron irradiation of 14YWT and 14WT at 300oC, 580oC, and 670oC to ~1.5 dpa (HFIR Rabbit capsules)

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14YWT (Tirr

=300oC) 14WT (Tirr

=300oC)

14YWT (Tirr

=580oC) 14WT (Tirr

=580oC)

14YWT (Tirr

=670oC) 14WT (Tirr

=670oC)

Str

ess

[MP

a]

Temperature [C]

Irradiated Yield Stress

D.A. McClintock et al., JNM 386-388 (2009)

No DBTT shift was observed in 14YWT after irradiation at 300oC to ~ 1.5 dpa compared to 85oC

shift in Eurofer-ODS

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14YWT [KJc(1T)

] (unirradiated)

14YWT [KJIc

] (unirradiated)

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] (Tirr

= 300oC)

14YWT [KJIc

] (Tirr

= 300oC)Fra

ctur

e T

ough

ness

[M

Pa

m]

Temperature [C]

Irradiated 14YWT Fracture Toughness

D.A. McClintock, et al., JNM 392 (2009)

Eurofer-ODS

HELIUM MANAGEMENT IN ADVANCED STRUCTURAL ALLOYS

• TEM images of Eurofer 97 (TMS), (a, b), and MA957 (NFA) (c,d)• Neutron irradiated to 25dpa at 500C and simultaneously implanted with

~1450 appm He• G.R.Odette et al., Fusion Materials Semi-Annual Report, DOE/ER-0313/51

HELIUM MANAGEMENT IN ADVANCED STRUCTURAL ALLOYS

• High helium content (1450appm) promotes the formation of a large population of voids in neutron irradiated Eurofer 97

• Same quantity of helium is trapped by ~2nm clusters/particles in MA957 with no void formation

• Swelling rate theory extrapolations indicate that NFA will remain damage-tolerant to much higher levels of dpa and helium

• Extrapolations for the TMS indicate continuing growth of voids with eventual transition to a linear (with dose) swelling regime

NFAs WITH IMPROVED FRACTURE RESISTANCE

• NFAs based on 12-14% Cr tend to exhibit reduced grain boundary fracture resistance with increasing temperature > 400C

• New approaches to NFA fabrication involve controlled phase transformation during high temperature rolling to strengthen grain boundaries while retaining the nano-scale microstructure for high strength and radiation damage resistance

• New compositions are based on Fe-9Cr-2W-0.4Ti-0.2V- 0.3 Y2O3

• Initial results from a joint ORNL-KAERI project funded by Office of Nuclear Science and Technology show promise.

MICROSTRUCTURE OF NEW NFAs

T.S.Byun,D.T.Hoelzer (ORNL), J.H.Koon (KAERI); unpublished work26

9YWTV-PM2Fe-9Cr-2W-0.4Ti-0.2V-0.05C+0.3Y2O3

9YWTV-PM1Fe-9Cr-2W-0.4Ti-0.2V-0.12C-0.3Y2O3

PM1PM2

Strength & Ductility of 9Cr NFAs

• New base NFAs retain YS higher than 500 MPa at 700°C.• Improved ductility for both new NFAs compared to 14YWT

T.S.Byun, D.T.Hoelzer (ORNL), J.H.Koon (KAERI); unpublished work 27

28

Effect of Controlled Rolling 9YWTV-PM on High Temperature Fracture Resistance

T.S.Byun, D.T.Hoelzer ( ORNL ), J.H.Koon (KAERI); unpublished work

• Improvement of fracture toughness in controlled rolled 9YWTV-PM2 is significant.

• The 9YWTV-PM2 controlled rolled at 900°C for 50% reduction resulted in the best fracture toughness among NFAs, which is as high as those of non-ODS F/M steels.

• Potential avenue for NFAs based on 9Cr with improved high temperature fracture resistance

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NFA 12YWTNFA MA9579Cr-WMoVNb SteelNFA 14YWT

LMP T(K)[25 + log10t(h)]

Str

ess

(MP

a)

900°C, 1104h

800°C, 14235h

800°C, 817h

650°C, 13000h

600°C, 17000h

650°C, 1080h650°C

16h

650°C>72h

825°C, 57600h(in test)

800°C, 38555h

800°C, 16584h(in test)

Creep performance of NFAs

• Creep test on ruptured MA957 started in Oct., 2003 (INE

• Creep test on 14YWT-SM10 started in April, 2008

• Klueh et al, J. Nucl. Mat., (2005)• I-NERI FY01-04

-1.000

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CR

EE

P S

TR

AIN

(%

)

TIME (HOURS)

Ruptured after ~38,555 h