progress in the development of nano-structured steels a.f.rowcliffe (ornl) aries project meeting...
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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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0 5 10 15 20 25 30
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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0 100 200 300 400 500 600 700 800
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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40
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-200 -150 -100 -50 0 50
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
0 100 200 300 400 500 600 700 800 900
Yie
ld S
tren
gth
, M
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)
-200 -100 0 100 200 300 400 500 600 700
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14YWT (unirradiated) 14WT (unirradiated)
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)
14YWT [KJc(1T)
] (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