Thermal ageing of nickel-base Alloy 690 TT

Transcription

1 SAFIR The Finnish Research Programme on Nuclear Power Plant Safety RG5 Structural Integrity: THELMA (Thermal Ageing of Materials) one topic in the project: Thermal ageing of nickel-base Alloy 690 TT Roman Mouginot 1, Teemu Sarikka 1, Mykola Ivanchenko 3, Unto Tapper 3, Mikko Heikkilä 2, Ulla Ehrnstén 3, Young Suk Kim 4, Sung Soo Kim 4 & Hannu Hänninen 1 1 Aalto University School of Engineering 2 University of Helsinki 3 VTT Technical Research Centre of Finland LTD 4 Korea Atomic Energy Research Institute SAFIR2018 INTERIM SEMINAR AND WEBINAR

2 Background: Why study thermal ageing? Nuclear power safety: - Strong emphasis for long design time (60 years) and to improve plant operation and maintenance. - Critical when extending the planned service life of nuclear power plants. - Wide range of potential materials ageing issues to consider in the license renewal. Structural integrity of components: - Components are designed to resist failure under every condition of operation, but material degradation from thermal ageing/irradiation/corrosion is unavoidable. - The risk of catastrophic brittle fracture due to embrittlement of materials or stresscorrosion cracking (SCC) is the main issue when considering structural materials. Thermal ageing (the degradation of a material over time caused by changes at the atomic level due to temperature). - Reduction of mechanical properties such as ductility. - Increase the susceptibility to SCC and other failure mechanisms. - Testing with accelerated ageing (higher temperature ± longer time) until materials from NPPs are available.

3 Background: Why study thermal ageing? Nuclear power safety: - Strong emphasis for long design time (60 years) and to improve plant operation and maintenance. - Critical when extending the planned service life of nuclear power plants. - Wide range of potential materials ageing issues to consider in the license renewal. Structural integrity of components: - Components are designed to resist failure under every condition of operation, but material degradation from thermal ageing/irradiation/corrosion is unavoidable. - The risk of catastrophic brittle fracture due to embrittlement of materials or stresscorrosion cracking (SCC) is the main issue when considering structural materials. 40 years 60 years High temperature (325 C) Embrittlement Thermal ageing (the degradation of a material over time caused by changes at the atomic level due to temperature). - Reduction of mechanical properties such as ductility. - Increase the susceptibility to SCC and other failure mechanisms. - Testing with accelerated ageing (higher temperature ± longer time) until materials from NPPs are available.

4 Background: Why study thermal ageing? Nuclear power safety: - Strong emphasis for long design time (60 years) and to improve plant operation and maintenance. - Critical when extending the planned service life of nuclear power plants. - Wide range of potential materials ageing issues to consider in the license renewal. Structural integrity of components: - Components are designed to resist failure under every condition of operation, but material degradation from thermal ageing/irradiation/corrosion is unavoidable. - The risk of catastrophic brittle fracture due to embrittlement of materials or stresscorrosion cracking (SCC) is the main issue when considering structural materials. Thermal ageing (the degradation of a material over time caused by changes at the atomic level due to temperature). - Reduction of mechanical properties such as ductility. - Increase the susceptibility to SCC and other failure mechanisms. - Testing with accelerated ageing (higher temperature ± longer time) until materials from NPPs are available. 40 years 60 years High temperature (325 C) Embrittlement Simulation and characterization of long-term behavior of materials.

5 Background: Why Alloy 690 TT? Primary structural materials used in PWRs: - Low-alloy steels (LAS): Pressure vessels - Stainless steels (SS): Piping, cladding - Ni-base alloys (Ni-Cr-Fe): Penetrations, steam generator, welds Ni-base alloys: - High performance materials to replace stainless steels for critical applications. - Good corrosion resistance and toughness at high temperatures. - Suffer from a special form of intergranular stress corrosion cracking in the primary circuit: primary water stress corrosion cracking (PWSCC). Alloy 690 TT (Ni-30Cr-10Fe): - Extremely high PWSCC resistance. - Used in modern PWR designs. - Not totally immune to PWSCC. - Concern for long-term operation (>60 years) at temperatures higher than 300 C.

6 Background: Issues of Alloy 690 TT Alloy 690 TT (Ni-30Cr-10Fe): - Replaced Alloy 600 since the 90 s. - Thermally treated (TT) condition with intergranular (IG) carbides. - Used in RPV head penetrations and steam generator tubing. - PWSCC resistance is critical. Inhomogeneous microstrucure in Alloy 690 plate IG carbide precipitation before and after thermal ageing Factors affecting the PWSCC resistance: - High levels of cold work (20 % CW). - Inhomogeneous microstructure. - Carbide precipitation. Thermal ageing of Alloy 690 TT: - Increase of PWSCC, but the mechanisms are unclear. - Promotion of IG carbide precipitation. - Trigger an atomic ordering. Atomic ordering

7 Background: Issues of Alloy 690 TT Alloy 690 TT (Ni-30Cr-10Fe): - Replaced Alloy 600 since the 90 s. - Thermally treated (TT) condition with intergranular (IG) carbides. - Used in RPV head penetrations and steam generator tubing. - PWSCC resistance is critical. Factors affecting the PWSCC resistance: - High levels of cold work (20 % CW). - Inhomogeneous microstructure. - Carbide precipitation. Thermal ageing of Alloy 690 TT: - Increase of PWSCC, but the mechanisms are unclear. - Promotion of IG carbide precipitation. - Trigger an atomic ordering. Inhomogeneous microstrucure in Alloy 690 plate IG carbide precipitation before and after thermal ageing Atomic ordering Strain localization Protective film rupture Hydrogen embrittlement

8 Background: Issues of Alloy 690 TT Alloy 690 TT (Ni-30Cr-10Fe): - Replaced Alloy 600 since the 90 s. - Thermally treated (TT) condition with intergranular (IG) carbides. - Used in RPV head penetrations and steam generator tubing. - PWSCC resistance is critical. Factors affecting the PWSCC resistance: - High levels of cold work (20 % CW). - Inhomogeneous microstructure. - Carbide precipitation. Thermal ageing of Alloy 690 TT: - Increase of PWSCC, but the mechanisms are unclear. - Promotion of IG carbide precipitation. - Trigger an atomic ordering. Inhomogeneous microstrucure in Alloy 690 plate IG carbide precipitation before and after thermal ageing Atomic ordering Strain localization Protective film rupture Hydrogen embrittlement Thermal ageing IG carbide precipitation Ordering reaction

9 Background: Issues of Alloy 690 TT Alloy 690 TT (Ni-30Cr-10Fe): - Replaced Alloy 600 since the 90 s. - Thermally treated (TT) condition with intergranular (IG) carbides. - Used in RPV head penetrations and steam generator tubing. - PWSCC resistance is critical. Factors affecting the PWSCC resistance: - High levels of cold work (20 % CW). - Inhomogeneous microstructure. - Carbide precipitation. Thermal ageing of Alloy 690 TT: - Increase of PWSCC, but the mechanisms are unclear. - Promotion of IG carbide precipitation. - Trigger an atomic ordering. Inhomogeneous microstrucure in Alloy 690 plate IG carbide precipitation before and after thermal ageing Atomic ordering Strain localization Protective film rupture Hydrogen embrittlement What link? Thermal ageing IG carbide precipitation Ordering reaction

10 Background: Issues of Alloy 690 TT Alloy 690 TT (Ni-30Cr-10Fe): - Replaced Alloy 600 since the 90 s. - Thermally treated (TT) condition with intergranular (IG) carbides. - Used in RPV head penetrations and steam generator tubing. - PWSCC resistance is critical. Factors affecting the PWSCC resistance: - High levels of cold work (20 % CW). - Inhomogeneous microstructure. - Carbide precipitation. Thermal ageing of Alloy 690 TT: - Increase of PWSCC, but the mechanisms are unclear. - Promotion of IG carbide precipitation. - Trigger an atomic ordering. Inhomogeneous microstrucure in Alloy 690 plate IG carbide precipitation before and after thermal ageing Atomic ordering Strain localization Protective film rupture Hydrogen embrittlement What link? Thermal ageing IG carbide precipitation Ordering reaction

11 Short-range ordering of Alloy 690 TT Tc = 550 C at 0 wt.% Fe Ordering reactions: - Stronger attraction between non-similar atoms (Ni and Cr). - Re-arrangement where neighbouring atoms are different. - Formation of the Ni 2 Cr phase. - Stronger attraction = smaller distance between atoms. Short-range or long-range ordering: - Short-range order (SRO), when only nuclei of Ni 2 Cr phase. - Long-range order (LRO), when the phase extends over large domains. - The reaction occurs below a critical temperature, depending mostly on the Fe content in the Ni-Cr-Fe system. Temperature ( C) Alloy is Disordered Alloy Can Develop Short Range Order Alloy Can Develop Long Range Order Tc ± 420 C at 9.5 wt.% Fe Times >10,000 hours required to show LRO below ~350 C Consequences in Alloy 690: - Lattice contraction. - Hardess increase. - Reduction of mechanical properties. Iron Content (at.%) in Approximately Ni 2 Cr

12 Short-range ordering of Alloy 690 TT Tc = 550 C at 0 wt.% Fe Ordering reactions: - Stronger attraction between non-similar atoms (Ni and Cr). - Re-arrangement where neighbouring atoms are different. - Formation of the Ni 2 Cr phase. - Stronger attraction = smaller distance between atoms. Short-range or long-range ordering: - Short-range order (SRO), when only nuclei of Ni 2 Cr phase. - Long-range order (LRO), when the phase extends over large domains. - The reaction occurs below a critical temperature, depending mostly on the Fe content in the Ni-Cr-Fe system. Consequences in Alloy 690: - Lattice contraction. - Hardess increase. - Reduction of mechanical properties. Temperature ( C) Alloy is Disordered Alloy Can Develop Short Range Order Alloy Can Develop Long Range Order Tc ± 420 C at 9.5 wt.% Fe Times >10,000 hours required to show LRO below ~350 C Iron Content (at.%) in Approximately Ni 2 Cr Issue SRO is difficult to identify Lattice contraction in the range of 0.03 % Limited hardness variations

13 Materials and experimental methods Alloy 690 Chemical composition Element Ni Cr Fe Mn Si P Cu C S wt. % 62,6 28,7 9,18 0,21 0,12 0,08 0, ,00037 <0,00001 Condition SA+WQ SA+ WQ + 17 h at 700 C 6 conditions Ageing temperature ( C) / / Code SA TT TT 350 TT 420 TT 475 TT 550 SA: WQ: TT: solution anneal at 1100 C for 1 h water quench thermal treatment at 700 C for 17 h Methods Microstructures SRO levels Strain localization Scanning electron microscopy (SEM) Electron backscatter diffraction (EBSD) Energy-dispersive X-ray spectroscopy (EDS) Transmission electron microscopy (TEM) Microhardness (9.8 N) Nanoindentation (1.5 mn) X-ray diffraction (XRD) Atomic force microscopy (AFM) High resolution EBSD

14 Materials and experimental methods Alloy 690 Chemical composition Element Ni Cr Fe Mn Si P Cu C S wt. % 62,6 28,7 9,18 0,21 0,12 0,08 0, ,00037 <0,00001 Tc ± 420 C 6 conditions Condition SA+WQ SA+ WQ + 17 h at 700 C Ageing temperature ( C) / / Code SA TT TT 350 TT 420 TT 475 TT 550 SA: WQ: TT: solution anneal at 1100 C for 1 h water quench thermal treatment at 700 C for 17 h Methods Microstructures SRO levels Strain localization Scanning electron microscopy (SEM) Electron backscatter diffraction (EBSD) Energy-dispersive X-ray spectroscopy (EDS) Transmission electron microscopy (TEM) Microhardness (9.8 N) Nanoindentation (1.5 mn) X-ray diffraction (XRD) Atomic force microscopy (AFM) High resolution EBSD

15 Materials and experimental methods Alloy 690 Chemical composition Element Ni Cr Fe Mn Si P Cu C S wt. % 62,6 28,7 9,18 0,21 0,12 0,08 0, ,00037 <0,00001 Tc ± 420 C 6 conditions Condition SA+WQ SA+ WQ + 17 h at 700 C Ageing temperature ( C) / / Code SA TT TT 350 TT 420 TT 475 TT 550 SA: WQ: TT: solution anneal at 1100 C for 1 h water quench thermal treatment at 700 C for 17 h Methods Microstructures SRO levels Strain localization Scanning electron microscopy (SEM) Electron backscatter diffraction (EBSD) Energy-dispersive X-ray spectroscopy (EDS) Transmission electron microscopy (TEM) Microhardness (9.8 N) Nanoindentation (1.5 mn) X-ray diffraction (XRD) Atomic force microscopy (AFM) High resolution EBSD

16 Alloy 690 TT microstructure SEM with ArgusTM FSE/BSE detector Austenitic (fcc) matrix IG carbides Golden TiN particles Twin boundaries Optical microscopy

17 Evolution of grain size and IG carbide precipiation SA TT TT aged at 420 C TT aged at 550 C =200 µm; Euler; Step=1 µm; Grid747x559 =200 µm; Euler; Step=1 µm; Grid747x560 =200 µm; Euler; Step=1 µm; Grid747x560 =200 µm; Euler; Step=1 µm; Grid747x560 SA TT TT aged at 420 C TT aged 550 C

18 Nature of IG carbide precipitation TT aged at 420 C for h TT aged at 420 C TT aged at 550 C for h TT aged at 550 C TT aged at 550 C TT aged at 550 C TT aged at 550 C Cr 7 C Cr 23 C 6 -Cr Identification with TEM: Cr-rich M23C6 carbides Distribution with EBSD: GBs, migrated GBs, twin boundaries Twin boundaries: carbide plates

19 Study of the SRO levels Microhardness (9.8 N load) - No influence of ordering. - Clear influence of grain size and IG precipitation Hardness (HV 1 ) SA TT TT 420 TT Grain size (µm) >100 px/grains 15 SA TT TT 420 TT 550

20 Study of the SRO levels 170 Hardness (HV 1 ) Microhardness (9.8 N load) - No influence of ordering. - Clear influence of grain size and IG precipitation Need to discriminate between SRO and other factors SA TT TT 420 TT Grain size (µm) >100 px/grains 15 SA TT TT 420 TT 550

21 Study of the SRO levels 170 Hardness (HV 1 ) SA TT TT 420 TT Microhardness (9.8 N load) - No influence of ordering. - Clear influence of grain size and IG precipitation Need to discriminate between SRO and other factors Nanoindentation (1.5 mn) Small load/good spatial resolution HVIT Grain size (µm) >100 px/grains Grain2 Grain3 Grain 4 20 Grain1 15 SA TT TT 420 TT m

22 Study of the SRO levels 170 Hardness (HV 1 ) SA TT TT 420 TT Microhardness (9.8 N load) - No influence of ordering. - Clear influence of grain size and IG precipitation Need to discriminate between SRO and other factors Nanoindentation (1.5 mn) Small load/good spatial resolution HVIT Grain size (µm) >100 px/grains Grain2 Grain3 Grain 4 20 Grain1 15 SA TT TT 420 TT m

23 Study of the SRO levels Hardness (HV IT ) SA TT TT 350 TT 420 TT 475 TT 550 3,5785 Nanoindentation: - Hardness increase upon thermal ageing. - Highest hardness at 420 C. - Disordering reaction at higher temperatures. XRD: - Lattice contraction upon thermal ageing. - Contraction in the range of % (SRO). - Strongest contraction at 420 C. - No lattice contraction at 550 C (disordering). Lattice paramter (Å) 3,578 3,5775 3,577 3,5765 SA TT TT 350 TT 420 TT 475 TT 550

24 Study of the SRO levels Hardness (HV IT ) 600 Alloy 690 TT with 20% CW SA TT TT 350 TT 420 TT 475 TT 550 3,5785 Nanoindentation: - Hardness increase upon thermal ageing. - Highest hardness at 420 C. - Disordering reaction at higher temperatures. XRD: - Lattice contraction upon thermal ageing. - Contraction in the range of % (SRO). - Strongest contraction at 420 C. - No lattice contraction at 550 C (disordering). Lattice paramter (Å) 3,578 3,5775 3,577 Highest hardness/lattice contraction at 420 C (9.8 wt.% Fe). Levels similar to that of 20 % CW. 3,5765 SA TT TT 350 TT 420 TT 475 TT 550

25 Effect of SRO on strain localization Phase TT aged at 420 C Phase TT TT Height profile 420 C TT aged at 420 C 475 C 550 C TT aged at 475 C TT aged at 550 C HR EBSD: - No effect of SRO on strain levels. - No effect of SRO on strain localization. - Effect of IG carbide precipitation on strain localization. AFM: - No effect of SRO on GB hardness.

26 Conclusions Thermal ageing of Alloy 690 TT was studied, with a special focus on the formation of SRO and IG carbide precipitation and their link with strain localization and effect on PWSCC resistance. IG carbide precipitation of Cr-rich IG M 23 C 6 carbides increased during thermal ageing with the ageing temperature. The formation of SRO was suggested upon ageing at 350 C and more clearly upon ageing at 420 C, while the effects of a disordering reaction were seen at 475 and 550 C. Thermal ageing at 420 C increased the hardness of Alloy 690 TT to levels higher than that of Alloy 690 TT with 20% CW. Heat treatment of Alloy 690 after solution annealing promoted ordering in relation to a lower carbon content after IG carbide precipitation, favoring the nucleation of SRO. No link found between SRO and strain buildup or IG strain localization. IG carbide precipitation increased IG strain localization. The combination of IG strain localization and SRO is deemed detrimental to the PWSCC resistance over longer ageing times. All results are extracted from R. Mouginot s doctoral thesis, including more Alloy 690 conditions and the study of welds.

27 Further work in THELMA Thank you for your attention! - Study of the link between thermal ageing and hydrogen embrittlement of Alloy Atome probe tomography (APT) at Chalmers University, Sweden, to observe SRO nucleation directly. List of publications - Mouginot, R., Sarikka, T., Heikkilä, M., Ehrnstén, U., Kim, Y.S., Kim, S.S. & Hänninen, H., Effect of heat treatment and cold work on the ordering of Alloy 690 at 475 C. In 17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems Water Reactors, Ottawa, Canada, P. - Mouginot, R., Sarikka, T., Ehrnstén, U., Kim, Y.S., Kim, S.S. & Hänninen, H., Short-range ordering of Alloy 690 aged for h at 420, 475 and 550 C. In Baltica X International Conference on Life Management and Maintenance for Power Plants, Helsinki-Stockholm- Helsinki, 7-9 June P. - Mouginot, R., Sarikka, T., Heikkilä, M., Ivanchenko, M., Ehrnstén, U., Kim, Y.S., Kim, S.S. & Hänninen, H., Thermal ageing of Alloy 690 between 350 and 550 C. Journal of Nuclear Materials, 485, pp Mouginot, R., Sarikka, T., Heikkilä, M., Ivanchenko, M., Ehrnstén, U., Kim, Y.S., Kim, S.S. & Hänninen, H., Development of SRO and IG carbide precipitation upon thermal ageing of Alloy 690 TT. In 18th International Conference on Environmental Degradation of Materials in Nuclear Power Systems Water Reactors, August 13-17, 2017, Portland, Oregon, USA. (Submitted) - Mouginot, R., Sarikka, T., Heikkilä, M., Ivanchenko, M., Ehrnstén, U., Kim, Y.S., Kim, S.S. & Hänninen, H., Advanced characterization of Alloy 690 TT upon thermal ageing. Journal of Nuclear Materials. (To be submitted)