Effect of weld microstructure on brittle fracture initiation in the thermally-aged boiling water reactor pressure vessel head weld metal

Abstract

Effects of the weld microstructure and inclusions on brittle fracture initiation are investigated in a thermally aged ferritic high-nickel weld of a reactor pressure vessel head from a decommissioned nuclear power plant. As-welded and reheated regions mainly consist of acicular and polygonal ferrite, respectively. Fractographic examination of Charpy V-notch impact toughness specimens reveals large inclusions (0.5–2.5 µm) at the brittle fracture primary initiation sites. High impact energies were measured for the specimens in which brittle fracture was initiated from a small inclusion or an inclusion away from the V-notch. The density, geometry, and chemical composition of the primary initiation inclusions were investigated. A brittle fracture crack initiates as a microcrack either within the multiphase oxide inclusions or from the de-bonded interfaces between the uncracked inclusions and weld metal matrix. Primary fracture sites can be determined in all the specimens tested in the lower part of the transition curve at and below the 41-J reference impact toughness energy but not above the mentioned value because of the changes in the fracture mechanism and resulting changes in the fracture appearance.

Keywords

reactor pressure vessel / brittle fracture / weld microstructure / thermal aging

References

[1] Haušild P, Berdin C, Bompard P. Prediction of cleavage fracture for a low-alloy steel in the ductile-to-brittle transition temperature range. Mater. Sci. Eng. A, 2005, 391(1–2): 188.

[2] Cottrell AH. Theory of brittle facture in steel and similar metals. Trans. Metall. Soc. AIME, 1958, 212, 192

[3] P. Joly, L. Sun, P. Efsing, J.P. Massoud, F. Somville, R. Gerard, Y.H. An, and J. Bailey, Characterization of in-service thermal ageing effects in base materials and welds of the pressure vessel of a decommissioned PWR pressurizer, after 27 years of operation, [in] 19th International Conference on Environmental Degradation of Materials in Nuclear Power Systems-Water Reactors 2019, Boston, 2019, p. 392.

[4] Lindgren K, Boåsen M, Stiller K, Efsing P, Thuvander M. Evolution of precipitation in reactor pressure vessel steel welds under neutron irradiation. J. Nucl. Mater., 2017, 488, 222.

[5] Nikolaev YA, Nikolaeva AV, Kryukov AM, Levit VI, Korolyov YN. Radiation embrittlement and thermal annealing behavior of Cr−Ni−Mo reactor pressure vessel materials. Nucl. Mater., 1995, 226(1–2): 144.

[6] Odette GR, Nanstad RK. Predictive reactor pressure vessel steel irradiation embrittlement models: Issues and opportunities. JOM, 2009, 61(7): 17.

[7] Xing RS, Yu DJ, Xie GF, Yang ZH, Wang XX, Chen X. Effect of thermal ageing on mechanical properties of a bainitic forging steels for reactor pressure vessel. Mater. Sci. Eng. A, 2018, 720, 169.

[8] K. Wallin, M. Yamamoto, and U. Ehrnstén, Location of initiation sites in fracture toughness testing specimens: The effect of size and side grooves, [in] Proceedings of the ASME 2016 Pressure Vessels and Piping Conference, Volume 1B: Codes and Standards, Vancouver, 2016, art. No. V01BT01A011.

[9] Griffith AA. The phenomena of rupture and flow in solids. Philos. Trans. R. Soc. London, 1921, 221, 163.

[10] Pineau A, Benzerga AA, Pardoen T. Failure of metals I: Brittle and ductile fracture. Acta Mater., 2016, 107, 424.

[11] W. Weibull, A statistical theory of the strength of materials, [in] Generalstabens Litografiska Anstalts Förlag, Stockholm, 1939.

[12] Weibull W. A statistical distribution function of wide applicability. J. Appl. Mech., 1951, 18, 293.

[13] Beremin FM, Pineau A, Mudry F, Devaux JC, D’Escatha Y, Ledermann P. A local criterion for cleavage fracture of a nuclear pressure vessel steel. Metall. Trans. A, 1983, 14, 2277.

[14] Briant CL, Banerji SK. Intergranular failure in steel: The role of grain-boundary composition. Int. Met. Rev., 1978, 23(1): 164.

[15] Hertzberg RW, Vinci RP, Hertzberg JL. Deformation and Fracture Mechanics of Engineering Materials, 2013, 5th ed., Chichester, John Wiley & Sons, 299

[16] Landes JD, Shaffer DH. Paris P. Statistical characterization of fracture in the transition region. Fracture Mechanics, 1980, West Conshohocken, ASTM International, 368.

[17] K. Wallin, Fracture Toughness of Engineering Materials: Estimation and Application, FESI Publishing, 2011.

[18] Bowen P, Druce SG, Knott JF. Effects of microstructure on cleavage fracture in pressure vessel steel. Acta Metall., 1986, 34(6): 1121.

[19] H. Hein, J. Kobiela, M. Brumovsky, C. Huotilainen, J. Lydman, B. Marini, B. Radiguet, O. Startsev, M. Serrano Garcia, R. Hernandez Pascual, F. Roeder, and H.W. Viehrig, Addressing of specific uncertainties in determination of RPV fracture toughness in the SOTERIA project, [in] Fontevraud 9-Contribution of Materials Investigations and Operating Experience to Light Water NPPs’ Safety, Performance and Reliability, Avignon, 2018.

[20] Boåsen M. Modeling of Structural Integrity of Aged Low Alloy Steels Using Non-Local Mechanics, 2020, Stockholm, Sweden, KTH Royal Institute of Technology [Dissertation]

[21] Hippsley CA, Druce SG. The influence of phosphorus segregation to particle/matrix interfaces on ductile fracture in a high strength steel. Acta Metall., 1983, 31(11): 1861.

[22] Bhadeshia H, Honeycombe R. Steels: Microstructure and Properties, 2017, 4th ed., Cambridge, Elsevier Ltd.

[23] Evans GM. Effect of manganese on the microstructure and properties of all-weld-metal deposits. Welding Res. Suppl., 1980, 59(3): 67

[24] Shim JH, Oh YJ, Suh JY, Cho YW, Shim JD, Byun JS, Lee DN. Ferrite nucleation potency of non-metallic inclusions in medium carbon steels. Acta Mater., 2001, 49(12): 2115.

[25] Gurovich BA, Kulehsova EA, Shtrombakh YI, Zabusov OO, Krasikov EA. Intergranular and intragranular phosphorus segregation in russian pressure vessel steels due to neutron irradiation. J. Nucl. Mater., 2000, 279(2–3): 259.

[26] Sprouster DJ, Sinsheimer J, Dooryhee E, Ghose SK, Wells P, Stan T, Almirall N, Odette GR, Ecker LE. Structural characterization of nanoscale intermetallic precipitates in highly neutron irradiated reactor pressure vessel steels. Scripta Mater., 2016, 113, 18.

[27] Lindgren K, Boåsen M, Stiller K, Efsing P, Thuvander M. Cluster formation in in-service thermally aged pressurizer welds. J. Nucl. Mater., 2018, 504, 23.

[28] Byun JS, Shim JH, Cho YW, Lee DN. Non-metallic inclusion and intragranular nucleation of ferrite in Ti-killed C-Mn steel. Acta Mater., 2003, 51(6): 1593.

[29] Sarma DS, Karasev AV, Jönsson PG. On the role of non-metallic incluisons in the nucleation of acicular ferrite in steels. ISIJ Int., 2009, 49(7): 1063.

[30] Zerbst U, Ainsworth RA, Beier HT, Pisarski HT, Zhang ZL, Nikbin K, Nitschke-Pagel T, Münstermann S, Kucharczyk P, Klingbeil D. Review on fracture and crack propagation in weldments—A fracture mechanics perspective. Eng. Fract. Mech., 2014, 132, 200.

[31] M. Boåsen, K. Lindgren, J. Rouden, M. Öberg, J. Faleskog, M. Thuvander, and P. Efsing, Thermal ageing of low alloy steel weldments from a Swedish nuclear power plant—A study of mechanical properties, [in] Fontevraud 9-Contribution of Materials Investigations and Operating Experience to Light Water NPPs’ Safety, Performance and Reliability, Avignon, 2018.

[32] Miller MK, Powers KA, Nanstad RK, Efsing P. Atom probe tomography characterizations of high nickel, low copper surveillance RPV welds irradiated to high fluences. J. Nucl. Mater., 2013, 437(1–3): 107.

[33] Finnish Standards Association ISO 148-1. 2016: Metallic Materials—Charpy Pendulum Impact Test —Part 1: Test Method, Standard, 2016, West Conshohocken, Finnish Standards Association

[34] ASTM International ASTM E185-16. Standard Practice for Design of Surveillance Programs for Light-Water Moderated Nuclear Power Reactor Vessels, 2018, West Conshohocken, Standard, ASTM International

[35] Hytönen N. Effect of Microstructure on Brittle Fracture Initiation in a Reactor Pressure Vessel Weld Metal, 2019, Tampere, University of Tampere [Dissertation]

[36] Jones SJ, Bhadeshia HKDH. Competitive formation of inter- and intragranularly nucleated ferrite. Metall. Mater. Trans. A, 1997, 28(10): 2005.

[37] Anderson TL. Fracture Mechanics Fundamentals and Applications, 1995, 2nd ed., Boca Raton, CRC Press

[38] Kroon M, Faleskog J. Micromechanics of cleavage fracture initiation in ferritic steels by carbide cracking. J. Mech. Phys. Solids, 2005, 53(1): 171.

[39] Zhang LF, Radiguet B, Todeschini P, Domain C, Shen Y, Pareige P. Investigation of solute segregation behaviour using a correlative EBSD/TKD/APT methodology in a 16MND5 weld. J. Nucl. Mater., 2019, 523, 434.