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Characterization of Reactor Pressure Vessel Steel by ABI Testing
Available online at www.sciencedirect.com
ScienceDirect Procedia Materials Science 12 (2016) 7 – 11
6th New Methods of Damage and Failure Analysis of Structural Parts [MDFA]
Characterization of Reactor Pressure Vessel Steel by ABI Testing Petr Haušilda,*, Jan Siegla, Aleš Maternaa, Miloš Kytkaa,b, Radim KopĜivaa,b a
Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Department of Materials, Trojanova 13, 120 00 Praha 2, Czech Republic b
ÚJV ěež, a. s., Hlavní 130, 250 68 Husinec – ěež, Czech Republic
Abstract Microstructure of the base metal, the multilayer welding seam and the two-layer cladding was characterized through the thickness of the WWER 440 reactor pressure vessel wall. Mechanical properties were determined by performing a series of instrumented indentations across the weld at room temperature. The results were treated by so-called automated ball indentation technique. Mechanical properties obtained by instrumented indentation from the local stress-strain behavior were compared with minimum values required by the standard. © 2016 Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license © 2014The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the VŠB - Technical University of Ostrava, Faculty of Metallurgy and Selection and peer-review under responsibility of the VŠB - Technical University of Ostrava,Faculty of Metallurgy and Materials Engineering. Materials Engineering
Keywords: instrumented indentation, automated ball indentation, reactor pressure vessel steel
1. Introduction The reactor pressure vessel is one of the key safety components in the complex safety assessments of nuclear power plants. The reactor pressure vessel cannot be replaced (from both technical and economical reasons) so it often becomes the component determining the operational safety of the nuclear power plants. WWER 440 nuclear reactor pressure vessel is fabricated by welding of thick walled ring-type components. The pressure vessel wall is composed of the base metal, the multilayer welding seam and the two-layer cladding (Fig. 1).
* Corresponding author. Tel.: +420-224-358-514; fax: +420-224-358-523. E-mail address: [email protected]
2211-8128 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the VŠB - Technical University of Ostrava,Faculty of Metallurgy and Materials Engineering doi:10.1016/j.mspro.2016.03.002
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Petr Haušild et al. / Procedia Materials Science 12 (2016) 7 – 11
Nomenclature
ırepr İrepr P D dp hp E1, E2
representative stress representative strain load ball diameter indentation (plastic) diameter (taking into account material pile-up) indentation (plastic) depth Young’s modulus of specimen and indenter, respectively
Mechanical properties can present a gradient through the wall thickness, which can hardly be assessed by conventional testing such as standard tensile or Charpy tests. Other techniques using e.g. miniature sample size or indentation are therefore necessary to be employed. In the present paper, the mechanical properties are determined by performing a series of instrumented indentations i.e., so-called automated ball indentation technique. 2. Material and experimental details Coupons were taken from the pressure vessel beltline weld of the canceled Greifswald WWER-440/213 nuclear power plant. Each (through the wall thickness) coupon contained the base metal (chromium-molybdenum-vanadium low alloy 15Ch2MFA steel), the multilayer welding seam (10ChMFT steel) and the two-layer cladding (25 chromium/13 nickel non-stabilized austenitic stainless steel Sv 07Ch25N13 and at least 2 passes of 18 chromium/10 nickel niobium stabilized Sv 08Ch18N10G2B austenitic stainless steel). Microstructure in different positions of the pressure vessel wall containing beltline weld is shown in Fig. 1. Series of instrumented indentations across the pressure vessel wall in the base metal, the weld and the cladding were carried out on Inspekt 20kN testing machine at room temperature using 2.5 mm diameter tungsten carbide ball. Ball displacement was measured by high sensitivity LVDT sensor fixed on the side of the contact tip.
Fig. 1. Microstructure in different positions of the pressure vessel wall containing beltline weld.
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Petr Haušild et al. / Procedia Materials Science 12 (2016) 7 – 11
The Automated Ball Indentation (ABI) test is based on multiple instrumented indentation cycles (at the same penetration location) on a polished metallic surface by a spherical indenter. Each cycle consists of indentation, unload and reload sequences and the following set of equations is iteratively solved to determine the flow curve from the ABI data, see Haggag et al. (1990, 1997):
σ repr =
4P δπ d p 2
(1)
d p D
(2)
ε repr = 0.2
(3)
For the strain rate sensitivity calibration, the JRQ steel (with known tensile test) was used. JRQ steel is IAEA reference reactor pressure vessel material of the A533B-1 steel type as it shows relatively large changes in mechanical properties when exposed to neutron irradiation. The calibration was performed directly on the head of broken tensile specimen so that the local stress–strain behavior obtained by ABI could be related to the actual tensile curve. The experimental setup and the calibration procedure are illustrated in Fig. 2.
Fig. 2. ABI experimental setup (left) and the comparison of tensile test with ABI estimated stress-strain relation (right).
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Petr Haušild et al. / Procedia Materials Science 12 (2016) 7 – 11
3. Results The representative true stress and plastic strain curves determined by the ABI test in the different positions through the wall thickness of WWER 440 nuclear reactor pressure vessel are shown in Fig. 3. The elastic-plastic material properties (true stress - plastic strain curves) presented well defined power law hardening in the base metal, weld and cladding, which proves the robustness of the method.
Fig. 3. Positions of the measuring points through the wall thickness of WWER 440 nuclear reactor pressure vessel and the representative stress and plastic strain curves determined by ABI tests in the base metal, the weld and the cladding.
As expected, a multilayer welding seam presented largest scatter, especially in the hardening exponent (slope of true stress – plastic strain curves). Obtained true stress–plastic strain curves in the cladding are in good agreement with the tensile and compression tests carried out on small-size samples taken from the cladding by Materna et al. (2014). ABI estimated yield (0.2 proof) stress (R p0.2) and ultimate tensile strength (Rm) along the wall thickness in the base metals and across the weld seam are compared with the values imposed by the standard (see e.g. Timofeev et al. 2010) in Fig. 4. As it can be seen in Fig. 4, all ABI estimated values meet the requirements (for both minimum yield stress and ultimate tensile strength) imposed by the standard in the base metal as well as in the weld.
Petr Haušild et al. / Procedia Materials Science 12 (2016) 7 – 11
Fig. 4. Yield (0.2 proof) stress and ultimate tensile strength estimated by ABI in base metal (15Ch2MFA) and weld (10ChMFT) compared with the values imposed by standard (lines).
4. Conclusions So-called automated ball indentation technique was successfully implemented and the results obtained by ABI were in a good agreement with results obtained by tensile test on reference material. Mechanical properties in form of the stress-strain behavior were determined by ABI technique in the weld, base metal and cladding of WWER 440 reactor pressure vessel wall. In all measured positions, ABI estimated yield stress and ultimate tensile strength meet the required values imposed by the standard in the base metal as well as in the weld. The main advantage of the ABI technique is that it estimates the properties on a small volume of material (few grains), which is particularly useful in testing e.g. welds and/or irregularly shaped heat affected zones. Especially in our case, a multilayer welding seam presented a large scatter which can hardly be assessed by conventional tensile testing. Acknowledgements This work was carried out with the financial support of Technology Agency of the Czech Republic in the frame of the research project TA03011266. References Haggag, F.M., Nanstad, R.K., Hutton, J.T., Thomas, D.L., Swain, R.L., 1990. Use of automated ball indentation to measure flow properties and estimate fracture toughness in metallic materials, Applications of automation technology to fatigue and fracture testing, ASTM STP 1092, A.A. Braun, N.E. Ashbaugh, and F.M. Smith, Eds., American Society for Testing and Materials, Philadelphia, pp. 188-208. Haggag, F.M., Wang, A., Sokolov, M.A., Murty, K.L., 1997. Nontraditional methods of sensing stress, strain, and damage in materials and structures, ASTM STP 1318, Lucas G, Stubbs D, Eds., American Society for Testing and Materials, Philadelphia, pp. 85-98. Materna, A., Nohava, J., Haušild, P., Oliva, V., 2014. Determination of the elastic-plastic properties of 15Kh2MFA steel with austenitic cladding. Key Engineering Materials 586, 43-46. Timofeev, B., Brumovský, M., Von Estorff, U., 2010. The certification of 15Kh2MFA/15Cr2MoVA steel and its welds for WWER reactor pressure vessels, European Commission, EUR 24581 EN.
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ScienceDirect Procedia Materials Science 12 (2016) 7 – 11
6th New Methods of Damage and Failure Analysis of Structural Parts [MDFA]
Characterization of Reactor Pressure Vessel Steel by ABI Testing Petr Haušilda,*, Jan Siegla, Aleš Maternaa, Miloš Kytkaa,b, Radim KopĜivaa,b a
Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Department of Materials, Trojanova 13, 120 00 Praha 2, Czech Republic b
ÚJV ěež, a. s., Hlavní 130, 250 68 Husinec – ěež, Czech Republic
Abstract Microstructure of the base metal, the multilayer welding seam and the two-layer cladding was characterized through the thickness of the WWER 440 reactor pressure vessel wall. Mechanical properties were determined by performing a series of instrumented indentations across the weld at room temperature. The results were treated by so-called automated ball indentation technique. Mechanical properties obtained by instrumented indentation from the local stress-strain behavior were compared with minimum values required by the standard. © 2016 Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license © 2014The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the VŠB - Technical University of Ostrava, Faculty of Metallurgy and Selection and peer-review under responsibility of the VŠB - Technical University of Ostrava,Faculty of Metallurgy and Materials Engineering. Materials Engineering
Keywords: instrumented indentation, automated ball indentation, reactor pressure vessel steel
1. Introduction The reactor pressure vessel is one of the key safety components in the complex safety assessments of nuclear power plants. The reactor pressure vessel cannot be replaced (from both technical and economical reasons) so it often becomes the component determining the operational safety of the nuclear power plants. WWER 440 nuclear reactor pressure vessel is fabricated by welding of thick walled ring-type components. The pressure vessel wall is composed of the base metal, the multilayer welding seam and the two-layer cladding (Fig. 1).
* Corresponding author. Tel.: +420-224-358-514; fax: +420-224-358-523. E-mail address: [email protected]
2211-8128 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the VŠB - Technical University of Ostrava,Faculty of Metallurgy and Materials Engineering doi:10.1016/j.mspro.2016.03.002
8
Petr Haušild et al. / Procedia Materials Science 12 (2016) 7 – 11
Nomenclature
ırepr İrepr P D dp hp E1, E2
representative stress representative strain load ball diameter indentation (plastic) diameter (taking into account material pile-up) indentation (plastic) depth Young’s modulus of specimen and indenter, respectively
Mechanical properties can present a gradient through the wall thickness, which can hardly be assessed by conventional testing such as standard tensile or Charpy tests. Other techniques using e.g. miniature sample size or indentation are therefore necessary to be employed. In the present paper, the mechanical properties are determined by performing a series of instrumented indentations i.e., so-called automated ball indentation technique. 2. Material and experimental details Coupons were taken from the pressure vessel beltline weld of the canceled Greifswald WWER-440/213 nuclear power plant. Each (through the wall thickness) coupon contained the base metal (chromium-molybdenum-vanadium low alloy 15Ch2MFA steel), the multilayer welding seam (10ChMFT steel) and the two-layer cladding (25 chromium/13 nickel non-stabilized austenitic stainless steel Sv 07Ch25N13 and at least 2 passes of 18 chromium/10 nickel niobium stabilized Sv 08Ch18N10G2B austenitic stainless steel). Microstructure in different positions of the pressure vessel wall containing beltline weld is shown in Fig. 1. Series of instrumented indentations across the pressure vessel wall in the base metal, the weld and the cladding were carried out on Inspekt 20kN testing machine at room temperature using 2.5 mm diameter tungsten carbide ball. Ball displacement was measured by high sensitivity LVDT sensor fixed on the side of the contact tip.
Fig. 1. Microstructure in different positions of the pressure vessel wall containing beltline weld.
9
Petr Haušild et al. / Procedia Materials Science 12 (2016) 7 – 11
The Automated Ball Indentation (ABI) test is based on multiple instrumented indentation cycles (at the same penetration location) on a polished metallic surface by a spherical indenter. Each cycle consists of indentation, unload and reload sequences and the following set of equations is iteratively solved to determine the flow curve from the ABI data, see Haggag et al. (1990, 1997):
σ repr =
4P δπ d p 2
(1)
d p D
(2)
ε repr = 0.2
(3)
For the strain rate sensitivity calibration, the JRQ steel (with known tensile test) was used. JRQ steel is IAEA reference reactor pressure vessel material of the A533B-1 steel type as it shows relatively large changes in mechanical properties when exposed to neutron irradiation. The calibration was performed directly on the head of broken tensile specimen so that the local stress–strain behavior obtained by ABI could be related to the actual tensile curve. The experimental setup and the calibration procedure are illustrated in Fig. 2.
Fig. 2. ABI experimental setup (left) and the comparison of tensile test with ABI estimated stress-strain relation (right).
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Petr Haušild et al. / Procedia Materials Science 12 (2016) 7 – 11
3. Results The representative true stress and plastic strain curves determined by the ABI test in the different positions through the wall thickness of WWER 440 nuclear reactor pressure vessel are shown in Fig. 3. The elastic-plastic material properties (true stress - plastic strain curves) presented well defined power law hardening in the base metal, weld and cladding, which proves the robustness of the method.
Fig. 3. Positions of the measuring points through the wall thickness of WWER 440 nuclear reactor pressure vessel and the representative stress and plastic strain curves determined by ABI tests in the base metal, the weld and the cladding.
As expected, a multilayer welding seam presented largest scatter, especially in the hardening exponent (slope of true stress – plastic strain curves). Obtained true stress–plastic strain curves in the cladding are in good agreement with the tensile and compression tests carried out on small-size samples taken from the cladding by Materna et al. (2014). ABI estimated yield (0.2 proof) stress (R p0.2) and ultimate tensile strength (Rm) along the wall thickness in the base metals and across the weld seam are compared with the values imposed by the standard (see e.g. Timofeev et al. 2010) in Fig. 4. As it can be seen in Fig. 4, all ABI estimated values meet the requirements (for both minimum yield stress and ultimate tensile strength) imposed by the standard in the base metal as well as in the weld.
Petr Haušild et al. / Procedia Materials Science 12 (2016) 7 – 11
Fig. 4. Yield (0.2 proof) stress and ultimate tensile strength estimated by ABI in base metal (15Ch2MFA) and weld (10ChMFT) compared with the values imposed by standard (lines).
4. Conclusions So-called automated ball indentation technique was successfully implemented and the results obtained by ABI were in a good agreement with results obtained by tensile test on reference material. Mechanical properties in form of the stress-strain behavior were determined by ABI technique in the weld, base metal and cladding of WWER 440 reactor pressure vessel wall. In all measured positions, ABI estimated yield stress and ultimate tensile strength meet the required values imposed by the standard in the base metal as well as in the weld. The main advantage of the ABI technique is that it estimates the properties on a small volume of material (few grains), which is particularly useful in testing e.g. welds and/or irregularly shaped heat affected zones. Especially in our case, a multilayer welding seam presented a large scatter which can hardly be assessed by conventional tensile testing. Acknowledgements This work was carried out with the financial support of Technology Agency of the Czech Republic in the frame of the research project TA03011266. References Haggag, F.M., Nanstad, R.K., Hutton, J.T., Thomas, D.L., Swain, R.L., 1990. Use of automated ball indentation to measure flow properties and estimate fracture toughness in metallic materials, Applications of automation technology to fatigue and fracture testing, ASTM STP 1092, A.A. Braun, N.E. Ashbaugh, and F.M. Smith, Eds., American Society for Testing and Materials, Philadelphia, pp. 188-208. Haggag, F.M., Wang, A., Sokolov, M.A., Murty, K.L., 1997. Nontraditional methods of sensing stress, strain, and damage in materials and structures, ASTM STP 1318, Lucas G, Stubbs D, Eds., American Society for Testing and Materials, Philadelphia, pp. 85-98. Materna, A., Nohava, J., Haušild, P., Oliva, V., 2014. Determination of the elastic-plastic properties of 15Kh2MFA steel with austenitic cladding. Key Engineering Materials 586, 43-46. Timofeev, B., Brumovský, M., Von Estorff, U., 2010. The certification of 15Kh2MFA/15Cr2MoVA steel and its welds for WWER reactor pressure vessels, European Commission, EUR 24581 EN.
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