ML24213A064
| ML24213A064 | |
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| Issue date: | 07/31/2024 |
| From: | Office of Nuclear Regulatory Research |
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| Download: ML24213A064 (1) | |
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1 NRC Work on Thermal Aging Embrittlement of Austenitic Stainless Steel Weld Metal Implications for Section XI Flaw Evaluation Procedures Working Group on Pipe Flaw Evaluation August 13, 2024
2 Overview
- NRC is investigating thermal aging embrittlement (TE) of austenitic stainless steel weld (ASSW) metal, and recently issued a technical letter report on this topic:
- TLR-RES/DE-2023-14-Rev. 0, Assessment of Thermal Aging Embrittlement of Austenitic Stainless Steel Weld Metals (ML24180A123)
- The NRC work focused on TE effects on fracture toughness, but also looked at TE effects on SCC crack growth rates.
- The report includes an assessment of the Section XI, Appendix C flaw evaluation procedures with respect to TE of ASSW.
3 Fracture Toughness Data for ASSW
- NUREG/CR-6428, Rev. 1 (2018) (NUREG) contains the most comprehensive compilation of thermally aged ASSW FT data.
- Data from PIFRAC program used for unaged lower curves, in NUREG/CR-6428 Rev. 0 (1995). This was mostly SMAW and SAW data. These curves were updated with additional data in NUREG, Rev. 1.
- Lower bound J-R curves for aged material were based on a lot of data from MHI (Hojo, 2014) and a few other studies.
- The J-R curves are mainly from testing in air.
- Some more recent data was not considered in developing the LB curves.
4 Effect of Aging Time and Temperature
- Aging parameter P establishes equivalency of different aging times/temperatures:
= log 1000 19.143 1
+273 1
673
=
=
= °
- Most accelerated laboratory aging is at 400 °C for 10,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />.
- This is equivalent to 54 EFPY at 290 °C (assuming Q of 113 kJ/mol)
- Aging representative of 54 or 72 EFPY at PWR hot leg or pressurizer temperatures requires higher P-numbers.
5 Service Temp. Piping Type EFPY Service Time (hr)
P Accelerated Aging Time (hr) 290 BWR, PWR CL 54 473040 3.96 9145 320 PWR HL 54 473040 4.49 31018 343 PWR PZR 54 473040 4.86 72994 290 BWR, PWR CL 72 630720 4.09 12193 320 PWR HL 72 630720 4.62 41357 343 PWR PZR 72 630720 4.99 97325 Equivalent Accelerated Aging Times at 400 °C for Various Service Temperatures and EFPY Activation Energy = 113 kJ/mol
As aging time increases, the amount of decrease in the J-R curve tends to decrease, such that additional aging time causes little change.
JIC tends to saturate earlier than J at higher crack extensions.
For TE screening, J2.5 has been historically used, e.g. in CASS AMP, but J-value at J2.5 may not be fully saturated at P=4.
Fracture toughness of Type 316L GTAW Weld Material with 8%
ferrite as a function of aging time, from Hojo, 2014. Upper. JIC ;
Lower. J6mm
7 J-R Curves for 316L SMAW welds aged at 400 °C for various times. J-R curves are not saturated at 10,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />.
J-R Curves for 308L SMAW welds aged at 400 ° C for various times. J-R curve are essentially saturated after 10,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />.
Fracture Toughness of Aged SMAW Fracture toughness J-R curves for SMAW weld metal evaluated in NUREG/CR-6428, Rev. 1
9 Comparison of J-R curves for archival Type 308L SMAW weld metal to NUREG/CR-6428, Rev. 1 lower bound curve for SAW and SMAW weld metal
10 Summary -SMAW/SAW Fracture Toughness
- Lower bound FT curve provided in NUREG/CR-6428,Rev. 1.
- An unaged SAW curve was included. Applicability of SAW to the aged curve was established by a statistical study.
- Data not included in NUREG/CR-6428, Rev. 1 is bounded by the proposed lower bound curve, including archival BWR weld material.
NUREG/CR-6428, Rev. 1 Lower Bound GTAW Curve
12 Other Fracture Toughness Data for GTAW
- A few recent studies generated J-R curves not bounded by the NUREG/CR-6428, Rev. 1 lower bound GTAW curve.
- These include Lucas (2011, 2016), I-NERI, 2017, and Hong et. al., 2018
- The materials in these studies also had low unaged fracture toughness
- However, other GTAW J-R curves from Koyama and Hojo, 2014 were either bounded by, or close to the NUREG LB curve.
Comparison of Other GTAW J-R Curves to NUREG/CR-6428 Lower Bound 13
14 Summary - GTAW Fracture Toughness
- A lower bound fracture toughness curve for GTAW is proposed in NUREG/CR-6428, Rev. 1.
- This curve is based on a relatively small amount of data.
15 Effect of the Environment on ASSW Fracture Toughness
- Lower bound curves in NUREG/CR-6428 based on air testing (no environmental effects)
- A few studies have looked at effects of the environment on fracture toughness of ASSW.
- Lucas, 2011 - Tested aged 316L in BWR high O2 environment. Found J-R curve ~ 40%
lower than in air.
- Chen, 2019b Tested aged Type 308L in high purity water (<10 ppb O2) - J-R curve bounded by NUREG SAW/SMAW LB (Slide 5)
- Summary - There is not enough testing on ASSW FT in coolant environments to determine if there is an environmental effect. More testing in PWR or BWR HWC environments would be helpful.
16 ASME Flaw Evaluation for ASSW
- Currently, GTAW welds are treated as wrought material, therefore limit load analysis (C-5000) is used
- Z-factors are provided in C-6330 applicable to flux welds and CASS. The same Z-factor is used for SAW/SMAW welds and CASS (Type CF3, CF8, or CF8M) with ferrite content > 14%,
except for CF8M with ferrite > 25%.
17 This figure shows how the assumed toughness basis for CASS was determined under PVP2017-66100 The current SMAW/SAW line was determined to be bounding for CF-3 and CF-8 CASS with ferrite > 14% and CF-8M with ferrite between 14 and 25%
According to PVP2017-66100, the toughness level represented by this line was based on work in 1986 by Landes and McCabe (EPRI NP-4768), on unaged ASSW.
The SAW/SMAW line has a value of ~
350 kJ/M2.
NUREG LB J2.5 for SAW/SMAW = 177 kJ/m2 NUREG LB J2.5 for aged GTAW = 408 kJ/m2
18 Comparison of PVP2017-66100 Toughness Basis to NUREG LB Aged Values For aged SAW/SMAW, the J2.5 from the NUREG/CR-6428, Rev. 1 LB curve is 177 kJ/m2
, which is close to the line for Ferritic Category 2 steel on the figure.
For aged GTAW, the J2.5 from the NUREG/CR-6428, Rev. 1 LB curve is 408 kJ/m2, which is slightly above the line for SAW/SMAW on the figure.
It therefore appears that the toughness of thermally aged SAW/SMAW welds is not bounded by the current Z-factor.
Based on comparisons with the lower bound toughness curves in NUREG/CR-6428, Rev. 1, o the Z-factors in Section XI, Appendix C, C-6330(a), would be appropriate for flaw evaluation of ASSW welds made using the GTAW process.
o the Z-factors in Section XI, Appendix C for Category 2 ferritic material would be appropriate for flaw evaluation of ASSW welds made using the SAW or SMAW process (flux welds).
19 This figure shows the J-R curve for Type 304 Base Metal SAW (Type 308 weld metal) from EPRI NP-4768 The 550 °F curve has a J2.5 of ~ 250 kJ/m2 This is the minimum J2.5 value from EPRI NP-4768 The SAW/SMAW line in PVP2017-66100 does not match this value
20 Code Case N-906 Code Case N-906 provides an alternative method of predicting the failure mode and determining Z-factors to that provided in Section XI, Appendix C for CASS materials.
The technical basis for the code case uses a mathematical model which correlates toughness with chemical composition developed from fracture toughness tests of 19 heats of aged CASS material (ASME, 2019). The corresponding Z-factors are then determined using equations based on dimensionless-plastic-zone-parameter (DPZP) analysis.
DPZP analysis based on tests of part-throughwall circumferentially cracked pipes, so less conservative than Appendix C method which is based on throughwall-cracked pipes.
It is possible that a similar approach could be applied to flaw evaluation of ASSW piping welds, if a correlation applicable to ASSW is developed.
21 Summary - ASME Code Flaw Evaluation Sec. XI Appendix C procedures do not account for TE of ASSW, with respect to the underlying basis for the Z-factors; The loss of fracture toughness due to TE is significant, and should be appropriately accounted for in flaw evaluation procedures; The Z-factors in Section XI, Appendix C, C-6330(a), which are currently specified for SMAW and SAW welds, Type CF3 or CF8 CASS with ferrite greater than 14%, and Type CF8M CASS with ferrite greater than 14%
but less than or equal to 25%, may be appropriate for flaw evaluation of aged ASSW welds made using the GTAW process, pending confirmation of the fracture toughness basis for these Z-factors.
The Z-factors in Section XI, Appendix C for Type 2 ferritic material would be appropriate for flaw evaluation of aged ASSW welds made using the SAW or SMAW process, rather than the Z-factors from C-6330(a).
WGPFE should provide clarification/better documentation of the toughness basis for the Z-factors of unaged SAW/SMAW welds, as shown by the line in Figure 5-1 (Figure 8 from ASME, 2027), if these Z-factors will be retained in Section XI.
A similar method to ASME Code Case N-906 could potentially be developed as an alternative for flaw evaluation of ASSW piping welds.
22 Effect of TE of ASSW on SCC and Fatigue
- Relative few studies - 4 tested in BWR oxidizing environments, 3 tested in PWR or low-potential environments
- BWR oxidizing environments - aged material SCC CGR is 10X higher, on overage, compared to unaged.
- PWR or low-potential environments, no difference between aged and unaged material.
- Fatigue
- Some experiments have been conducted, but all applied aging at temperatures 593 °C, and were focused on high-temperature reactor operating conditions, so not considered applicable to ASSW in LWRs.
23 Conclusions Thermal aging of ASSW can cause significant reduction in the J-R fracture toughness compared to the unaged condition.
NUREG/CR-6428, provides lower bound fracture toughness curves for thermally aged GTAW welds and SAW/SMAW welds.
The Z-factors in the ASME Code,Section XI, Nonmandatory Appendix C, do not account for thermally aged fracture toughness of ASSW.
Based on comparisons with the lower bound fracture toughness curves in NUREG/CR-6428, Rev.
1:
the Z-factors in Section XI, Appendix C, C-6330(a), would be appropriate for flaw evaluation of ASSW welds made using the GTAW process.
the Z-factors in Section XI, Appendix C for Category 2 ferritic material would be appropriate for flaw evaluation of ASSW welds made using the SAW or SMAW process (flux welds)
The NRC staff requests that ASME open a record to re-evaluate the Section XI, Appendix C flaw evaluation procedures for ASSW to ensure that these procedures account for thermal aging embrittlement of the material.
24 Conclusions (cont)
- More test data would be useful to confirm fracture toughness properties of thermally aged ASSW.
- TE Effects on cracking:
- TE of ASSW can cause about a 10X increase SCC CGR in BWR oxidizing environments, but in PWR environments seems to have no effect.
- There is no data relevant to LWRs on effects of TE on fatigue CGR.
25 Acronyms AERM = aging effect requiring management AMP = aging management program ASSW = austenitic stainless steel weld BWR = boiling water reactor CGR = crack growth rate DPZP = dimensionless plastic zone parameter EFPY = effective full power years FT = fracture toughness GTAW = gas tungsten arc weld J-R curve = J-integral resistance curve LB = lower bound LWR = light water reactor OE = operating experience PWR = pressurized water reactor SCC = stress corrosion cracking SMAW = shielded metal arc weld SAW = submerged arc weld SLR = subsequent license renewal TE = thermal aging embrittlement
26 References ASME, 2019 M. Uddin, C. Sallaberry and G. Wilkowski, Flaw Evaluation Procedure for Cast Austenitic Stainless Steel Materials Using a Newly Developed Statistical Thermal Aging Model, PVP2019-93711, in Proceedings of the ASME 2019 Pressure Vessels & Piping Conference PVP2019, July 14-19, 2019, San Antonio, Texas, USA NUREG/CR-6428, Rev. 1 O. Chopra, Effects of Thermal Aging on Fracture Toughness and Charpy-Impact Strength of Stainless Steel Pipe Welds. NUREG/CR-6428, Revision 1 (AN/EVS-17/3), August 2018 Chen et. Al, 2019a Y. Chen, C. Xu, Y. Yang, W.Y. Chen, B. Alexandreanu, K. Natesan, and A.S. Rao, Cracking Behavior of Thermally Aged Austenitic Stainless Steel Weld, Transaction s, SMiRT-25, Charlotte, NC, USA, August 4-9, 2019, Division I Chen et. al., 2019b Y. Chen, B. Alexandreanu, C. Xu, Y. Yang, K. Natesan, and A. S. Rao, Environmentally Assisted Cracking and Fracture Toughness of an Irradiated Stainless Steel Weld, Proceddings, 19th International Conference on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors l Boston, MA, August 18-22, 2019 l Pages 303-310 EPRI, 1986b Landes, J. D. and McCabe, D. Toughness of Austenitic Stainless Steel Pipe Welds, EPRI NP-4768, Electric Power Research Institute, Palo Alto CA, October 1986 I-NERI, 2017 Thak Sang Byun, Timothy G. Lach, David A. Collins, Changheui Jang, Effect of Thermal Aging in Stainless Steel Welds 2nd Year Progress Report of I-NERI Collaboration M3LW-17OR0402153 PNNL-27013, October, 2017
27 References Hong et. al., 2018 Sunghoon Hong, Hyunmyung Kim, Byeong Seo Kong, Changheui Jang, In Hwan Shin, Jun-Seog Yang, Kyoung-Soo Lee, Evaluation of the thermal ageing of austenitic stainless steel welds with 10 % of -ferrites, International Journal of Pressure Vessels and Piping, Volume 167, 2018, Pages 32-42, ISSN 0308-0161, https://doi.org/10.1016/j.ijpvp.2018.10.006.
Lucas, 2011 Lucas, Timothy R., "The Effect of Thermal Aging and Boiling Water reactor Environment on Type 316L Stainless Steel Welds," Doctoral Thesis, Massachusetts Institute ofTechnology, Cambridge, MA, May 2011 Lucas et. Al., 2011 T. Lucas, R.G. Ballinger, H. Hanninen and T. Saukkonen, Effect of Thermal Aging on SCC, Material Properties and Fracture Toughness of Stainless Steel Weld Metals, Proc. 15th Int. Conf. on Environmental Degradation of Materials in Nuclear Power Systems, TMS, 2011.
Lucas et. al., 2016 Lucas, Timothy, Antti Forsstrm, Tapio Saukkonen, Ronald Ballinger, and Hannu Hnninen. Effects of Thermal Aging on Material Properties, Stress Corrosion Cracking, and Fracture Toughness of AISI 316L Weld Metal.
Metallurgical and Materials Transactions A 47, no. 8 (June 14, 2016): 3956-3970.
28 Questions?