ML101880091
| ML101880091 | |
| Person / Time | |
|---|---|
| Issue date: | 03/31/2011 |
| From: | Stevens G NRC/RES/DE/CIB |
| To: | |
| Bayssie Mekonen/RES 251-7489 | |
| Shared Package | |
| ML101870548 | List: |
| References | |
| DG-1222 RG 1.50, Rev 1 | |
| Download: ML101880091 (17) | |
Text
Staff Responses to Public Comments on Draft Regulatory Guide DG-1222, Control of Preheat Temperature for Welding of Low-Alloy Steel dated June 2009 (Proposed Revision 1 of Regulatory Guide 1.50 dated May 1973)
- 1. James H. Riley, NEI (NEI) 2. David Egonis, NextEra Energy (NEE) 3. J. A. Gresham, Westinghouse (W)
(ML092220038) (ML091900368) (ML092370502) 4a. Scott Presler, FPL Duane Arnold (FPL) 4b. Ron Clow, XE Nuclear (XEN) 4c. Nick Mohr, Duke Energy (via S. Findlan, EPRI) (via S. Findlan, EPRI) (via S. Findlan, EPRI)
(ML092860604) (ML092860604) (ML092860604) 4d. Alex Gutierrez, PG&E (PGE) 4e. Neal Chapman, Entergy (ENT) 4f. Phil Flenner, EPRI (EPRI)
(via S. Findlan, EPRI) (via S. Findlan, EPRI) (via S. Findlan, EPRI)
(ML092860604) (ML092860604) (ML092860604)
- 5. David P. Helker, Exelon (EXLN)
Public Comments NRC Response The notices requested comments on all of these draft regulatory Extension to October 1, 2009 granted per NRC 7590-guides by August 31, 2009. NEI and EPRI are collecting and 01-P dated August 11, 2009 (ML092230530).
consolidating industry comments on these draft guides, but it has become apparent that it will not be possible to complete a comprehensive review of all of these documents in the time NEI-1 available, The information contained in these draft guides is important to the industry's work on primary system materials and it is important to carefully evaluate the changes proposed. NEI is therefore requesting a 30-day extension of the public comment period on these draft guides until October 1, 2009, to allow adequate time to complete and document our review.
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Public Comments NRC Response Has there been any industry experience of failed welds that would Delayed hydrogen-induced cracking in welds is a very lend to this decision by the NRC to impose a post pre-heat well documented phenomenon and is documented in maintenance for a minimum of four hours. available literature. RG 1.50 recommends that the preheat temperature be maintained until a PWHT has been performed. Over the years, the NRC has received and accepted alternatives, to the above recommendation, to use post weld baking as a means to ensure that cracking does not occur between the completion of welding and PWHT. The staffs intention is to include an alternative to the current recommendation that welds be maintained at the NEE-1 preheat temperature until PWHT is performed. The post weld backing alternative described in DG-1222 is consistent with alternatives approved for various reactor designs reviewed by the NRC.
There are components may require a 200° preheat but do not No. RG 1.50 does not apply to those low alloy steel require PWHT. Does this rule still apply? components that are exempt from PWHT by ASME Code.
The term low-alloy can cover a number of materials. Does the Yes. Please refer to the NRC Response to Comment NRC have any specific materials of concern? EPRI-1 below.
The second sentence of the third paragraph under Procedure The second sentence in the third paragraph under Qualification states: "The level of hydrogen in weld filler metal is Procedure Qualification of Section B, Discussion, of low enough to preclude adverse effects in the weld, ..." This is not the RG is changed as follows:
W-1 true for all weld filler metals, and further depends on the type of steel being welded. It is suggested this sentence be changed to: The level of hydrogen in weld filler metal may be low "The level of hydrogen in weld filler metal can be low enough to enough to preclude adverse effects in the welds preclude ..... "
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Public Comments NRC Response Regulatory Position 2 specifies maintaining preheat until PWHT or The last sentence in the third paragraph under a hydrogen bakeout. It is suggested that the remainder of the Procedure Qualification of Section B, Discussion, of document be made consistent with this. It is suggested that the RG is changed as follows:
paragraph 3 of the last sentence under Procedure Qualification be changed to: "Therefore, the minimum preheat temperature should Therefore, the minimum preheat temperature should be established to ensure a desirable cooling rate for the weld, and be established to ensure a desirable cooling rate for the this temperature should be maintained until a post weld heat weld, and this temperature should be maintained until a treatment, or a hydrogen bakeout has been achieved." Also, it is postweld heat treatment or a post weld hydrogen W-2 suggested that the last sentence under Production Welds be bakeout has been achieved.
changed to read: "To ensure that the welds will be acceptable, the metal temperature should be monitored during the welding process The last sentence under Production Welds of Section and through post weld heat treatment or hydrogen bakeout." B, Discussion, of the RG is changed as follows:
To ensure that the welds will be acceptable, the metal temperature should be monitored during the welding process and through post weld heat treatment or post weld hydrogen bakeout.
The concern is with the post-weld preheat maintenance The comment has been addressed by a change to requirement. This is something new and would require a weld Regulatory Position 2 of the RG. Revision 0 of RG 1.50 program revision at DAEC. Currently this is only applicable to states the following in Section C, Regulatory Position:
some P#s in B31.1.
- 2. For production welds, the preheat temperature should be maintained until a post-weld heat treatment has been performed.
FPL-1 Revision 1 of RG 1.50 allows for a postweld hydrogen bakeout in lieu of maintaining the preheat temperature until final postweld heat treatment.
Also, please see the NRC Responses to the DUKE and EPRI comments below.
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Public Comments NRC Response RG-1.50 should be revised in accordance with the EPRI WRTC Please see the NRC Responses to the EPRI comments XEN-1 (RRAC) efforts and findings in the way of PWHT and pre heat below.
requirements.
The main change is the inclusion of a post weld hydrogen bakeout Any welding process involving a welding flux has the and an associated soak time if preheat maintenance is not done. potential to contain moisture in the welding flux.
Moisture in the welding flux increases the susceptibility (Assuming a WPS is qualified in accordance with Section IX and of the weld to hydrogen induced cracking. When DUKE-1 Section III as specified by the Reg. Guide) Comments are as discussing welding fluxes, Section B of RG 1.50 applies follows: to any and all welding processes utilizing a welding flux.
Part B, 3rd paragraph) When discussing welding fluxes what welding processes are being discussed?
Part B, 3rd paragraph) Are Low hydrogen SMAW electrodes which Yes.
have been tested to have low levels (H4) of hydrogen and properly DUKE-2 controlled before welding included in the description "welding fluxes"?
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Public Comments NRC Response Part C, item 2.) With proper use of low hydrogen processes and As discussed in Section B, Discussion, of the RG, the welding filler material, if employed, should negate the need for level of hydrogen in weld filler metal is low enough to hydrogen bakeout and soak as the predominant source for preclude adverse effects in the welds, but greater hydrogen is controlled to a low level. Please explain why the use quantities of hydrogen can be present in the weld of low hydrogen processes and filler materials as one of the main region from the dissociation of moisture in hygroscopic ways to control hydrogen are not discussed as a mitigation welding fluxes or from adsorption on metal surfaces if technique. the welding fluxes and surfaces have not been properly dried before weld deposition.
Using an H8, or even an H4, electrode with controlled diffusible hydrogen alone provides no guarantee of eliminating problems related to hydrogen during or after welding. In addition to the electrode, several other factors can influence the diffusible hydrogen level and the potential for cracking. These should be considered DUKE-3 as well:
- base metal surface condition (contamination from oils, grease, dirt, moisture, acid, rust and other hydrogen containing materials can increase hydrogen levels);
- welding shielding gas (higher moisture content results in higher hydrogen levels);
- welding procedures (electrical stickout, arc voltage, wire feed speed and other parameters can influence diffusible hydrogen).1 1
Selecting Filler Metals: Low Hydrogen, Key Concepts in Welding Engineering, R. Scott Funderburk, see http://www.lincolnelectric.com/knowledge/articles/content/fillermetals.asp.
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Public Comments NRC Response Part C, item 2.) If PWHT is to be done and low hydrogen The hydrogen will likely diffuse at the post weld heat processes and/or low hydrogen filler materials used, the treatment (PWHT) temperature. However, the DUKE-4 associated soak and preheat maintenance should not be required likelihood of hydrogen induced cracking is increased if as the small amount of hydrogen will diffuse at the PWHT the minimum preheat temperature is not maintained temperature. prior to PWHT.
In Section C2 of the regulatory position, there is only one exception GTAW and GMAW are generally thought of as when the preheat temperature doesn't need to be maintained producing welds with low levels of diffusible hydrogen.
before the final PWHT. That exception is only when a hydrogen However, factors such as moisture in the shielding gas bake out is performed. However, in cases where a low hydrogen and contamination of welding surfaces can lead to welding process is used (i.e., GTAW or GMAW with solid wire), unacceptable levels of hydrogen in the weld.
there shouldn't be any significant amounts of hydrogen in the weld or HAZ. In these cases, it should be allowed to slowly cool the Also, please see the NRC Responses to the EPRI PGE-1 weld to room temperature prior to the final PWHT. Another comments below.
example would be in the case where a sufficient weld deposit has been applied (i.e., 3/8" or 25% of the groove is filled) and the weld is allowed to slowly cool to room temperature. In both of the latter cases, if welding has not been completed (due to end of shift), then the welds can be inspected prior to resuming any welding and the required preheat applied.
In Section C4, it is not clear whether the weld is acceptable if the Position 4 of Section C, Regulatory Position, of the RG soundness is verified by an acceptable examination procedure. is changed as follows:
This sentence can be reworded for better clarification.
- 4. If Regulatory Positions 1, 2 and 3 above are not PGE-2 met, the weld is subject to rejection. However, the soundness of the weld may be demonstrated by an acceptable examination procedure meeting the acceptance criteria specified in ASME Code,Section III.
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Public Comments NRC Response The wording in 2 requires a hydrogen bake out of all CrMo welds Regulatory Position 2 states that the preheat for 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. temperature should be maintained until final PWHT or a ENT-1 post weld hydrogen bake out has been performed. This does not apply to those low alloy steel components that are exempt from PWHT by ASME Code.
The wording in 4 states if we don't do steps 1-3 we need to an Correct.
"acceptable" soundness examination. Soundness usually equals ENT-2 volumetric. Since underbead cracking is what is specifically mentioned surface exams are likely out.
The real concern is the 4 hour4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> post bake out. We have done a lot Please refer to the NRC Response to Comment NEE-1 of work (EPRI, ASME Code, others) to get unneeded PWHT and above.
ENT-3 post-bake out of our Codes and here it is reintroduced at a lower temperature without any cited value.
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Public Comments NRC Response No definition is given for the term "Low-Alloy Steel." The Keys to Currently operating reactors, approved new reactor Metals database defines low alloy steels as follows: designs and new reactor designs currently under review by the NRC use P Nos. 3, 4 and 5A materials in ASME "Low-alloy steels constitute a category of ferrous materials that Code Class 1, 2, and 3 components. P Nos. 5B, 5C exhibit mechanical properties superior to plain carbon steels as the and 15E materials are not used.
result of additions of alloying elements such as nickel, chromium, and molybdenum. Total alloy content can range from 2.07% up to The first sentence of Section C, Regulatory Position, of levels just below that of stainless steels, which contain a minimum the RG is changed as follows:
of 10% Cr."
EPRI-1 Weld fabrication for low-alloy steel (P Nos. 3, 4, and For the purpose of ASME Codes therefore, low alloy steels may 5A) components should comply with the fabrication typically be considered as applying to the materials in P Nos. 3, 4, requirements specified in Section III and Section IX of 5A, 5B, 5C, and 15E. (Note: P No. 15E is a new P No. designation the ASME B&PV Code supplemented by the following:
for Grade 91 materials; formerly categorized as P No. 5B, Group 2 prior to the 2009 Addenda to the ASME Codes.) It is suggested that the Draft Regulatory Guide DG-1222 include a definition of low alloy steels as those included in P Nos. 3, 4, 5A, 5B, 5C, and 15E.
The P No. 15E is suggested since the 2009 Addenda will likely be issued prior to the issue of Revision 1 to RG 1.50.
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Public Comments NRC Response In the 3rd paragraph under Procedure Qualifications under The third paragraph under Procedure Qualification of Section B DISCUSSION, the statement is made that Prolonged Section B, Discussion, of the RG is changed as follows:
time at the preheating temperature can prevent or interrupt local hardening and assist in reducing the adverse effects of a potential Embrittlement of metal in the weld area as the hydrogen gradient. This statement is misleading in that it is result of the presence of hydrogen generally occurs at technically inaccurate to state that Prolonged time at the lower temperatures and may be prevented by preheating temperature can prevent or interrupt local hardening. prolonging the time the weldment is maintained at Research at EPRI has shown that the local hardening on carbon preheating temperature or by performing a postweld and low alloy steels can be reduced with the use of the heat treatment. Preheating at the recommended recommended preheat during welding but some hardening will still temperatures can reduce the local hardening by occur with each welding pass (subsequent welding passes will reducing the cooling rate during welding. Prolonged further reduce the local hardness). The statement however time at the preheating temperature can assist in EPRI-2 included the information that Prolonged time at the preheating reducing the adverse effects of a potential hydrogen temperature ... can assist to reduce the adverse effects of a gradient. This gradient would disappear by means of potential hydrogen gradient which is technically correct. The diffusion of the hydrogen before the weldment is following revision to the draft proposed sentence is suggested: returned to room temperature. Therefore, the minimum preheat temperature should be established to ensure a Section B DISCUSSION, Procedure Qualification, 3rd Paragraph desirable cooling rate for the weld, and this temperature should be maintained until a postweld heat treatment Prolonged time at the preheating temperature can prevent or has been achieved.
interrupt local hardening and assist in reducing the adverse effects of a potential hydrogen gradient. Preheating at the recommended temperatures can also reduce the local hardening by reducing the cooling rate during welding.
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Public Comments NRC Response The last paragraph under Procedure Qualifications under The fourth paragraph under Procedure Qualification of Section B DISCUSSION, concerns the specification of an Section B, Discussion, of the RG is changed as follows:
interpass temperature. While it is true that the interpass temperature does potentially affect impact toughness properties, a In addition to the minimum preheat temperature, a requirement to apply the specific control of the interpass maximum interpass temperature must be specified per temperature when toughness is not required to be controlled is the requirements of ASME Section IX if toughness is a often unnecessary and an excessive requirement that will add requirement of the construction Code or the design. For costs during any welding operation on these materials. The low alloy steel welds not requiring impact testing, a requirements of ASME Section IX, Welding and Brazing maximum interpass temperature should be selected on Qualifications adequately control the interpass temperature by the basis of such influencing factors as the chemical qualification. The rules require the qualified welding procedures composition of the steel.
thus qualified to be followed when impact toughness is a requirement of the construction code. The following revision to the draft proposed paragraph is suggested:
Section B DISCUSSION, Procedure Qualification, Last EPRI-3 Paragraph In addition to the minimum preheat temperature, a maximum interpass temperature must should be specified per the requirements of ASME Section IX if toughness is a requirement of the construction Code or the design. If the weld metal transforms at too high a temperature, the required mechanical properties for the metal may not be met. The maximum interpass temperature varies for different steels, as does the minimum preheat temperature, and For other low alloy steels, a maximum interpass temperature, if required, should be selected on the basis of such influencing factors as the chemical composition of the steel.
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Public Comments NRC Response Under Section C REGULATORY POSITION, the statement is Please refer to the changes to Position 1 in the made in item 1 that The procedure qualification should require the response to this comment below.
following: This is not the purpose of a procedure qualification per the rules of ASME Section IX. The procedure qualification record only provides a documentation of the actual required parameters used during the welding of the qualification coupon and the procedure qualification is an activity rather than a statement of the requirements. To be correct, it is the welding procedure specification (WPS) where the requirements for welding are specified.
Under Section C REGULATORY POSITION, the statement is Please refer to the NRC Response to Comment EPRI-3 made in 1a that A minimum preheat and a maximum interpass above.
temperature should be specified. As stated above, a maximum interpass temperature is not always necessary for some low alloy steels.
EPRI-4 Also, the statement is made in 1b that The welding procedure Position 1, Section C, Regulatory Position, of the RG is should be qualified at the minimum preheat temperature. ASME changed as follows:
Section IX, essential variable QW406.1, specifies that the minimum preheat temperature shall be specified in the WPS and 1. The Welding Procedure Specification (WPS) should that it cannot be decreased more than 55ºC (100ºF) from the specify a maximum interpass temperature and a preheat temperature qualified. The proposed draft statement could minimum preheat temperature that is equal to the be interpreted as requiring the preheat temperature qualified as the minimum preheat temperature and maximum interpass minimum temperature that could be used or specified within the temperature used during procedure qualification WPS. This could cause increased costs associated with controlling the minor deviations from the target preheat temperature with no appreciable effect on the weld quality. Since this is already covered in ASME Section IX, this statement is not needed.
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Public Comments NRC Response Please refer to the changes to Position 1 in the The following revisions to the draft proposed statements are response to this comment above.
suggested:
Section C REGULATORY POSITION, Item 1
- 1. The welding procedure specification (WPS) qualification should specify require the following:
- a. A minimum preheat and a maximum interpass temperature, if required should be specified.
- b. The welding procedure should be qualified at the minimum preheat temperature.
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Public Comments NRC Response Under Section C REGULATORY POSITION, item 2 states: Please see the NRC Response to Comment NEE-1 above.
For production welds, the preheat temperature should be maintained until final postweld heat treatment or a hydrogen In addition, Position 1, Section C, Regulatory Position, bakeout is performed between 200 and 400ºC (400 and 750ºF) for of the RG is changed as follows:
a minimum of four hours after which the component may be slowly cooled to ambient temperature prior to the performance of the final 2. For production welds, the preheat temperature post weld heat treatment. The post weld hydrogen bakeout should be maintained until final postweld heat treatment temperature and soak time should be based on the materials being or a post weld hydrogen bakeout is performed between welded, geometry, and the welding process used. 200 and 400ºC (400 and 750ºF) for a minimum of four hours after which the component may be slowly cooled Similarly to the discussion concerning the need for maximum to ambient temperature prior to the performance of the interpass temperature control, there is little need for hydrogen final post weld heat treatment. The post weld hydrogen bakeout for many low alloy materials. It is therefore essential that bakeout temperature and soak time should be based on any controls proposed that may require hydrogen bakeout be the materials being welded, geometry, and the welding applied only to those materials which require it in order to reduce process used.
EPRI-5 the possibility of hydrogen cracking after welding. Applying this rule to materials and welds which do not need it and which maintaining the preheat until the PWHT is performed would be an extraordinary cost due to time and effort with no appreciable benefit.
There are many factors which influence the need to perform a hydrogen bakeout at the conclusion of a weld if the preheat is not to be maintained. Included is the hardenability of the material, the thickness, the welding process, the welding technique used (heat input and multiple passes) and the moisture content of the flux (if the process involves a flux).
The current Codes provide relatively little restriction based on the imposition of a hydrogen bakeout on low alloy materials. ASME Section III contains nonmandatory preheat rules in Appendix D but there is only a cautionary statement (Para. D-1120) that mentions Page 13 of 17
Public Comments NRC Response the need to consider the effect of reducing the preheat below the stated temperature.
More explicit rules exist in ASME B31.1, Para. 131.6.1 where low alloy materials of P Nos. 3, 4, 5A, and 5B are addressed (there are no P No. 5C materials in B31.1 and P No. 15E (now P No. 5B) will not be addressed until the 2009 addenda). The current Para.
131.6 in B31.1 applies to the interruption of welding (preheat) before the welding is completed but the primary concern is the potential for hydrogen cracking once an adequate weld has been deposited for structural purposes. For P Nos. 3, 4, and 5A, the only requirement is that the weld be slowly cooled to room temperature; no intermediate heat treatment is required. For P No.
5B materials, an intermediate heat treatment is required prior to cooling but is not specified other than being adequate.
The current B31.1, Para. 131.6.1 is shown as follows:
Current B31.1-2007 with 2008 Addenda, Para. 131.6.1:
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Public Comments NRC Response ASME B31.1 has processed a recent change to Para. 131.6.1 that is scheduled for publication in the 2009 addenda to the 2007 edition. The revised Para. 131.6 to be published in the 2009 addenda to ASME B31.1-2007 adds a new Para. 131.6.2 which allows the omission of the intermediate heat treatment for the P No. 5B materials provided the process and electrode moisture content meets certain criteria. The approved change to B31.1, Para. 131.6. is shown as follows:
B31.1 Para. 131.6.2 Revision approved for publication in 2009 Addenda.
This change reflects the ability to control hydrogen cracking through use of processes that will not introduce excessive levels of diffusible hydrogen into the weld.
In Ref. [1], the statement, The use of a 350 Hv hardness criterion implies a significant tolerance to variations in hydrogen content.
Cracking tests show that 350 Hv is a safe level even when using rutile coated electrodes. In Ref. [2], it has been shown that the average maximum hardness in the HAZ of typical multipass welds in P No. 1, 3, 4, and 5A materials is below 350 Hv, even on 1.5 in.
thick materials.
This discussion reflects the ability to control hydrogen cracking using methods other than the methods that are proposed in the draft regulatory guide, Section C, Item 2.
It is therefore suggested that this item be revised as follows:
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Public Comments NRC Response Section C REGULATORY POSITION, Item 2:
For production welds on low alloy materials that are susceptible, the preheat temperature should be maintained until final postweld heat treatment or other process controls should be used to minimize the potential for hydrogen cracking. Controls such as slow cooling, processes with low hydrogen potential, electrodes with low diffusible hydrogen content, welding multiple pass techniques and heat input controls to minimize potential high hardnesses, or a hydrogen bakeout may be used. If needed, the a hydrogen bakeout should be is performed between 200 and 400ºC (400 and 750ºF) for a sufficient time to remove the excess hydrogen, minimum of four hours after which the component may be slowly cooled to ambient temperature prior to the performance of the final post weld heat treatment. The post weld hydrogen bakeout temperature and soak time should be based on the materials being welded, geometry, and the welding process used.
REFERENCES
[1] Graville, Brian A., The Principles of Cold Cracking Control in Welds, p.127, Dominion Bridge Company, Limited, 1975.
[2] McGehee, A., Flenner, P., Investigation of Thickness Limits for Post Weld Heat Treatment (PWHT) Exemption, Interim Report 1011535, Section 3, Electric Power Research Institute, 2005.
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Public Comments NRC Response Section C, "Regulatory Position" Please refer to the NRC Response to Comment EPRI-5 above.
Item 2 states that: "For production welds, the preheat temperature should be maintained until final postweld heat treatment or a hydrogen bakeout is performed between 200 and 400 °C (400 and EXLN-1 750 °F) for a minimum of four hours.... " Exelon considers the minimum "four-hour" period to be excessive for leaner material chemistries, thinner materials, and standard groove weld joint design and high heat input weld processes. Exelon suggests that a graduated scale approach be considered to allow for the variance associated with the factors previously discussed.
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