Regular and ODS Ferritic Steel as Structural Materials for Power Plant HHFC s

Transcription

1 Regular and ODS Ferritic Steel as Structural Materials for Power Plant HHFC s D.T. Hoelzer Contributions on 9Cr Steels: J.T. Busby, and R.L. Klueh Development of 14YWT Nanostructured Ferritic Alloy: M.K. Miller, J. Bentley, C.L. Fu, M.A. Sokolov, D.A. McClintock (ORNL) G.R.Odette (UCSB), TN International HHFC Workshop on Readiness to Proceed from Near Term Fusion Systems to Power Plants UCSD, La Jolla, CA December 10-12, 2008

2 Sound materials science and engineering are essential to a successful reactor program DESIGN OF OF SUBSYSTEMS AND INTEGRATION INTO POWER PLANT CONCEPT DEVELOPMENT COMPONENT AND SUBSYSTEM MANUFACTURE Performance Goals/Requirements Material Properties Properties for Engineering Design Materials Production & Joining Joining Technology POWER STATION CONSTRUCTION MATERIALS SCIENCE AND ENGINEERING Specification of Operating Conditions/Limits (e.g. corrosion) OPERATION

3 New structural materials will need to be developed to meet the severe operating environments of advanced nuclear energy systems Fission (GEN-IV) Fusion Coolant H 2 O(SC), He, Pb, PbBi, Na H 2 O, He, Li, PbLi, FLiBe Particle Energy < 1-2 MeV < 14 MeV Temperatures ºC ºC Max Displacement Damage dpa ~ 200 dpa He/dpa ~0.1 appm/dpa 10 appm/dpa Stresses Moderate, nearly constant Moderate, nearly constant A key strategy for designing these new materials: The primary method currently envisioned to provide high strength and fracture toughness as well as appropriate radiation resistance in high temperature metallic components involves a high density of nanoscale second phase features to serve as point defect recombination centers for helium trapping. Workshop on Advanced Computational Materials Science: Application to Fusion and Generation IV Fission Reactors, Washington D.C. (2004)

4 Impact of improved structural materials on reactor performance Higher strength for ~150 MPa ~ ABR ~ 150ºC constant temperature: Reduced commodities Greater safety margins Longer lifetimes Higher temperature for constant stress: Higher plant performance (?) Reduced commodities Greater safety margins in accident scenarios Combinations of above: Greater flexibility

5 Higher performance can more than offset increased cost of materials Higher performance can come at a premium for raw material costs When raw material costs are normalized for improved performance advanced materials provide the best bargain Alloys for further development under GNEP

6 The development of advanced structural material will not be free, but is affordable. The US National Clad and Duct Materials Development program was extremely successful in improving materials for fast reactors. This development program ran for ~12 years with an overall R&D cost of $200M (1980). In 2006 dollars, this would be ~$44M/year. Most estimates for developing and qualifying an advanced alloy for reactor service are on the order of $30-50M (spread over years). This cost could be recovered in the first commercial reactor built.

7 A science-based approach for alloy improvement may allow for faster development Commercial alloy development is a slow process Traditional experimental metallurgy can take decades for small improvements in performance Experimental alloy development is often very expensive due to the large number of trial heats required for optimization Using computational thermodynamics and kinetic models, alloy composition and heat treatment can be optimized before alloys are melted Alloy compositions and treatments can also be custom tailored for specific applications Making small modifications to approved alloys may result in improved performance without having to generate a new code qualification case

8 Alloy development efforts must focus on a variety of issues Many factors must be assessed during alloy development Irradiation resistance Mechanical performance Creep performance Corrosion performance Thermal properties Joining Fatigue Fracture toughness Alloy development must be integrated with code qualification and licensing needs By utilizing a science-based approach and making adjustments to existing alloys, the risk for having poor performance in other factors is reduced

9 Evolution of steels for power generation industry

10 7-12% Cr Steels Have Tempered Martensite Microstructure Steels are used in normalized-and-tempered condition Precipitates: M 23 C 6 small amounts MX Sandvik HT9

11 Fracture Behavior Varies for Different Steels HT9 vs. 9Cr-2WVTa 9Cr-2WVTa superior to HT9(12Cr-1MoVW) Mod 9Cr-1Mo superior to HT9 9Cr-2WVTa superior to mod 9Cr-1Mo

12 Commercial steels for higher operating temperatures Commercial steels for higher temperatures NF616: Fe-9Cr-1.8W-0.5Mo-0.06N-0.004B-0.1C HCM12A: Fe-12Cr-1.8W-1Cu-0.5Mo-0.07N-0.1C Advantages of NF616 and HCM12A: 130% increase in 10 5 h rupture strength at 600ºC over HT9, 40% over modified 9Cr-1Mo Maximum use temperature of 620ºC compared to 565ºC for HT9, 593ºC for modified 9Cr-1Mo Irradiation effects not determined for steels

13 Performance improvement of generation 3 steel over generation 2 steels

14 Improved performance with advanced structural materials Historical evolution of materials performance X2 strength 316SS HT-9 NF years HT-UPS HTUPS X2 strength NF616 Selected alloys offer superior performance over traditional materials These materials must be qualified for reactor service Irradiation damage, and embrittlement issues are initial needs

15 Advanced steels: NF616 with TMT processing Special TMT applied to commercially available steels can dramatically improve performance Strength and ductility comparable to high-strength experimental ODS steel (12YWT) TMT of modified 9Cr-1Mo produced steel with order-of-magnitude increase in rupture life Ridge This TMTLaboratory processing can also be modified for use on NF616 Oak National U T -B A T T E L L E

16 Nanostructured Ferritic Alloys (NFA) represent a new class of dispersion strengthened steels NFA are derived from oxide (Y 2 O 3 ) dispersion strengthened (ODS) ferritic alloys but contain: A dispersion of nano-size atom clusters that provide significant (high-temperature) strength and creep resistance Fine-scale microstructural features that may improve the radiation damage resistance of ferritic alloys, including the suppression of He bubble formation on grain boundaries that cause the embrittlement of structural materials in fusion reactors Potential to extend the operating limit of steels from T 550ºC to T 800 C for application in advanced energy systems

17 Oxide-dispersion strengthened (ODS) ferritic alloys Plansee PM % PS - L UTS - L 12 Stress (MPa) % P.S. - T UTS - T Elongation (%) 200 UE - L UE - T Temperature (ºC) The particle dispersions in typical ODS steels usually consist of a relatively high number density of Y-oxide (Y 2 O 3, YAG, YTiO 3, etc.) particles, but are often nonuniformly distributed and have a broad size distribution Nevertheless, the oxide dispersion improves the (high-temperature) strength and thermal creep properties of ODS steels over conventional steels

18 Nanoclusters were discovered in the 12YWT ferritic alloy by 3-DAP at ORNL in 1999 This was a mechanically alloyed Fe - 12%Cr - 3%W - 0.4%Ti %Y 2 O 3 ferritic alloy that was produced in a cooperative effort between Kobe Steel, Nagoya University and ORNL in the late 1990 s T. Okuda (Kobe), K. Miyahara and I.S. Kim (Nagoya), and P.J. Maziasz and R.L. Klueh (ORNL) NC were discovered by atom probe tomography (APT) at ORNL (D.J. Larson and M.K. Miller) Characteristics Radius: 2.0 ± 0.8 nm Number density: 1.4 x m -3 Composition: 8.1 ± 5.2 %Y 42.1 ± 5.6 %Ti 44.4 ± 8.2 %O

19 Similar nanoclusters are also present in INCO MA957 As-processed rg = 1.2 +/- 0.4 nm ~2 x 1024 m-3 Annealed 1300ºC/1h rg = 1.8 +/- 0.7 nm ~7 x 1023 m-3 Element (at. %) As received X 1 h at 1300 C X PF Cr Mo Ti Y O Al Mn PF Compositions ( X ) and partitioning factors (PF) of the particles in the as-received and annealed conditions. The balance is Fe. U T -B A T T E L L E

20 The dispersion of nanoclusters is responsible for the superior high-temperature strength of NFA Stress (MPa) However, the 12YWT and MA957 are not commercially available: - 12YWT was produced only once as a small heat by Kobe Steel, Ltd. - Production of MA957 was discontinued early by INCO 0 * * * Tested at CEA ( 10-3 s -1 ) * Tested at ORNL ( ε = 10-3 s -1 ). ε =. Hamilton et al., PNNL-13168, 2000 ( ε = 4.1 x 10-4 s -1 ) YS - 12YWT UTS - 12YWT YS - MA957* UTS - MA % PS - PM2000 UTS - PM2000 YS - 9Cr-2WVTa* UTS - 9Cr-2WVTa Temperature (ºC). * * NFA Plansee ODS alloy Tempered Martensitic 9Cr Steel

21 12YWT and MA957 have excellent creep properties Stress (MPa) C 16h 650 C >72h 650 C, 1080h 600 C, 17000h 650 C, 13000h 800 C, 817h NFA 12YWT NFA MA957 9Cr-WMoVNb Steel 800 C, 14235h 900 C, 1104h 50 I-NERI FY C, 38555h 825 C, h Klueh et al, JNM 2005 (failed recently) (in test) Tested in air at 800ºC and 100 MPa CREEP STRAIN (%) TIME (HOURS) Ruptured after ~38,555 h LMP T(K)[25 + log10t(h)] The extensometer creep strain prior to rupture was 0.361%, which corresponded to a displacement of ~0.003 in. over 38,555 h The minimum creep rate that was measured was ~1.2 x s -1 (dε/dt)

22 Comparison of steady state creep rates between NFA MA957 and 12YWT vs 9Cr1Mo steel same strain rate same stress ORNL Report, ORNL-6303, J. R. Distefano, et al. B. Wilshire and T. D. Lieu, Mater. Sci. & Eng. A 386 (2004) 81. D. K. Mukhopadhyay, F. H. Froes and D. S. Gelles, J. Nucl. Mater (1998) ORNL/CEA I-NERI (FY07-10)

23 The nanoclusters are extremely stable at 800ºC As-received After 38,555 h at 800ºC 3.6 nm Fe M jump ratio Fe M jump ratio EFTEM reveals effectively no change in the size of the nanoclusters after 38,555 h (~4.4 years) at 800ºC and 100 MPa *Obtained from CEA, Saclay in 2003 in the previous INERI F ORNL/CEA I-NERI (FY07-10), Fusion Materials

24 Nanoclusters may also provide radiation tolerance The nanoclusters may function as trapping centers for neutron irradiation induced vacancies and self-interstitials and transmutation products such as He (fusion) The new theoretical model elucidating the critical role of vacancies on the structure and stability of nanoclusters may enhance the sink strength Catalyze the recombination of vacancies and self-interstitial atoms (SIA) Trap He and nucleate nano-size bubbles He bubble Grain boundary NC NF SIA recombines trapped vacancy Mitigating the detrimental changes in dimension and mechanical properties

25 Results of In-situ Neutron + He Implantation of MA957 Simultaneous neutron and He implantation HFIR: 9 dpa and up to 380 ppm He at 500ºC L.K. Mansur and W.A. Coghlan, ASTM STP 1046, 1989 Lift-Out FIB Specimen Ti-, Y-, and O-rich nanoclusters are stable during irradiation He trapping - If cavities exist, they are too small (<~2 nm) to detect reliably using standard through focus imaging J. Bentley et al., Microsc. Microanal., V13(Suppl 2), 2007, CD1072 T. Yamamoto et al., JNM, , 2007

26 G.R. Odette, P. Miao, T. Yamamoto (UCSB) and D. Edwards, and R. Kurtz (PNNL)

27 The 14YWT NFA (Nanostructured Ferritic Alloy) Unfortunately, both the 12YWT and MA957 ferritic alloys are not commercially available 12YWT was produced once as a small heat by Kobe Steel, Ltd. Production of MA957 was discontinued early by INCO This led to the research and development of 14YWT NFA beginning in 2001 at ORNL (and later with UCSB) Processing conditions were identified for forming stable oxygenrich nanoclusters The high temperature strength and low temperature fracture toughness of 14YWT are exceptional First-principles calculations based on local-density-functional approximation identified vacancy as a new atomic entity in promoting bonding for cluster formation Pseudopotential method Full-potential linearized augmented plane wave method

28 NFA are produced by Mechanical Alloying Powder processing method that allows for any desired combination of matrix composition and dispersoid, such as oxides, carbides, borides, etc. (Fe-14Cr-3W-0.4Ti) Y 2 O 3 Time consuming process with many processing variables, which can lead to microstructural non-uniformities and batch-to-batch variations HIP near net shape final product

29 Composition and processing conditions Steps to produce 14YWT (ORNL) Fe-alloyed powder produced by Ar gas atomization (Crucible Research, 2001) Composition: Fe - 14%Cr - 3%W - 0.4%Ti Powder sieved to -100/+325 mesh, or 45 to 150 µm diameter size distribution The Fe alloyed powder is blended with Y 2 O 3 powder (Nanophase Technology Corp) 17 to 31 nm particle size (aggregates) 1 to 5 µm size agglomerates form Blended powders are ball milled in water-cooled Zoz CM01 attritor mill 40 h 10:1 ball/powder ratio Ar atmosphere Ball-milled powders packed in steel can, vacuum degassed and extruded at 850 C with a 4:1 or 7:1 Reduction in Area (RA)

30 The ball milled powders are consolidated under pressure at elevated temperatures, i.e. hot extrusion 1.2 kg heat hot extruded through a rectangular-shaped die at 850ºC ~21 in Annealed at 1000ºC/1 h Hot rolled at 850ºC in multiple passes Specimens machined from small plates Specimens annealed at 1000ºC/1 h This heat contains the smallest grain size compared to the previous heats of 14YWT (14 total) The VHN is the highest: 546 +/- 15.6

31 The consolidation step results in the formation of a high number density of nanoclusters and nano-size grains BF TEM of SM10 Heat Local Electrode Atom Probe (LEAP) Grain size = 136 (+/- 14) nm GAR = ~ 1.2 Very high interfacial surface area (S v ) for trapping point defects S V(NC) = ~5.03 x 10 7 m 2 /m 3 S V(Grain Size) = ~1.47 x 10 7 m 2 /m 3 S v = 0.08 x 10 6 m 2 /m 3 for 25 µm grain size O Ti Y Fe Cr N v = 1-7 x m -3 <r> = /- 0.4 nm

32 The grain boundaries are decorated with nanoclusters EFTEM Fe M Jump Ratio Image Local Electrode Atom Probe (LEAP) Grain boundaries The nano-size grains present in 14YWT are attributed to formation of NC on the grain boundaries U T -B A T T E L L E

33 Stress-strain curves for ODS 14WT and NFA 14YWT Stress (MPa) SM4 (NC) SM1 (ODS - TiO 2 ) ºC ºC ºC 600ºC ºC ºC ºC Crosshead Displacment (mm/mm) Stress (MPa) ºC 364ºC 500ºC 600ºC 650ºC 700ºC 800ºC Crosshead Displacment (mm/mm) Temperature (ºC) σ ys σ UTS WT-SM1 UE σ ys TE YWT-SM4 σ UTS σ ys and σ UTS in MPa and UE and TE in (%) UE TE

34 Comparison of the high-temperature strength of three nanostructured ferritic alloys * * *. Tested at ORNL ( ε = 10-3 s -1 ). Hamilton et al., PNNL-13168, 2000 ( ε = 4.1 x 10-4 s -1 ) 1400 Stress (MPa) YS - 12YWT UTS - 12YWT YS - 14YWT-SM4 UTS - 14YWT-SM4 YS - MA957 * UTS - MA957 * * Temperature (ºC)

35 14YWT shows surprisingly good fracture toughness 14YWT alloy has much better upper shelf toughness and very low transition temperature than 12YWT The fracture DBTT is shifted from ~75 ºC for 12YWT to -150 ºC for 14YWT, or more than 200ºC 1-T FRACTURE TOUGHNESS, MPam 1/ YWT T o <-150 o C 12YWT T o =102 o C K from J Ic TEMPERATURE, o C L-T Orientation Pre-cracked: crack length to width (a/w) ratio of 0.5 Tested using the unloading compliance method (ASTM ) K Jc for brittle cleavage calculated from critical J-integral at fracture, adjusted to 1-T reference specimen K Jc(1T) K JIc for ductile deformation behavior calculated from critical J-integral at onset of stable crack growth

36 The fracture toughness of 14YWT was unaffected by neutron irradiation at low dose and temperature No DBTT shift was observed after irradiation at 300ºC to ~ 1.5 dpa Fracture Toughness [MPa m K Jc(1T) (unirr.) K J1c (unirr.) K Jc(1T) (T irr. = 300ºC) K J1c (T irr. = 300ºC) Temperature (ºC) D.A. McClintock et al., ICFRM-13, to be published

37 Summary Dispersion strengthening is an effective method for improving the high-temperature strength and creep properties of 9Cr steels and ODS 12-14Cr ferritic alloys However, both approaches are in the research and development phase and will require further development and testing, and in some cases, significant advances Dispersion strengthened 9Cr steels are near term solutions for structural materials, but require further development: Thermal aging Mechanical properties evaluation Fracture Toughness and Impact testing Creep and Fatigue testing Joining issues, which will be similar to ODS ferritic alloy

38 Summary Nanostructured Ferritic Alloys, such as 14YWT containing nanoclusters, have excellent high temperature creep properties and show the greatest potential for achieving radiation tolerance, but are still far term solutions Although 14YWT has outstanding combination of strength and fracture resistance, it has currently only been produced in numerous small heats Creep testing is currently in progress Very little fabrication experience other than hot rolling Joining is a significant problem Problems due to mechanical alloying will limit scale-up efforts A major issue with development of NFA and ODS ferritic alloys is that important industrial companies, such as Special Metals and Plansee, have exited the ODS manufacturing business

39 Problems with the Mechanical Alloying approach Complete redistribution and forced dissolution of Y 2 O 3 uniformly in the Fe alloyed powder is difficult to achieve due a variety of reasons such as: Wear and tear of ball mill components lowers the milling intensity Milling time Initial particle size range of the Fe alloyed powder The formation of the nanocluster is sensitive to composition (Ti, Y, and O) and the thermal history during consolidation, i.e. time, temperature, and heating rate Impurity pick-up during ball milling and powder handling may be both beneficial (O) or unfavorable (C, N) Controlling the O levels is difficult. Thus, will nanoclusters form with very high O levels? FeO ball milling experiment