I have been procuring welding consumables for Section III equipment for more than a decade. Section III pertains to nuclear facility components. What should one keep in mind when procuring welding consumables for Section III equipment? How are the requirements any different from welding consumables for fossil boilers or any other pressure equipment?
A raw perception says that the ‘nuclear’ aspect must mean that the requirements must be more stringent.
Stringent? In what way? Higher values of UTS? lower impact temperatures? We will see later in this article.
Many people think that the only thing that differentiates nuclear consumables from the ordinary variety is that – every heat/ batch/ lot of consumables must be subjected to the requisite tests; whereas, for ordinary consumables – this requirement does not apply. However, this perception is not quite accurate. There are several other differences.
This article explores various aspects that the welding engineer must keep in mind when drawing up the specification for welding material for Section III equipment. Although requirements for welding material are similar across various subsections of Section III, this article keeps a special focus on subsection NB, since that is the one I have most exposure to.
I have referred to the purchasing organization as the contractor, in this article. Generally, the contractor is a ASME Certificate Holder. However, this is not mandatory; any company not having a ASME certificate can also fabricate component to Section III provided:
i) Customer or national laws does not require the contractor to be an ASME Certificate
Holder.
ii) Contractor is not using the ASME certification mark.
Which Clause Addresses Welding Materials in Section III?
In Section III, subsection NB – clause NB-2400 addresses requirements for welding material. In other Sections too, the requirements are specified at the same location; that is – NC-2400, ND-2400, NE- 2400, and so on. The requirements for welding consumables are mostly same in all subsections of Section III.
What Welding Materials Are Permitted for Section III Construction?
NX-2400 of Section III states that the all welding material shall conform to requirements of welding material specification of Section IX. The Section IX by itself does not specify any requirements for welding material, it only points towards relevant SFA specifications of Section II Part C. In other words, welding material used for Section III construction shall conform to the relevant SFA specification.
In addition, any other filler metal as specified in Section IX is also permitted. This phrase points towards all those fillers that are not classified under Section II C. For such filler metals, Section IX requires that the WPS must specify the trade name of the filler (QW-404.33), and procedure and performance qualifications be done with the exact same grade (not the same trade name) of filler.
When impact toughness is a requirement (which it generally is, most of the time for Section III construction), such filler metals (i.e. non-ASME grade fillers) and also the fillers having a G-suffix are required to be of not just the same grade, but also the same trade name. [QW-404.12].
In short, NX-2400 says that all requirements of Section IX (of which, there aren’t too many) must be respected, in the matter of filler metals.
Are All Section II C Tests Required to be Done?
This is the question that generates a lot of argument. NX-2420 says that the required tests should be done for each lot/ heat of the consumable. Later, NB-2430 defines the required tests in the form of only three tests: tensile, impact and chemical composition tests. However, some code users misconstrue the term required tests to mean that ALL tests specified by the relevant SFA
specification are required to be done, in addition to the tests specified under NB-2430.
While the conservative minded inspector/ customer would like to do as many number of tests as possible, the contractor would like to keep the tests to a minimum, to save on testing costs as well as to keep things simple. What does the Code intend, however?
Well, the Code DOES NOT require all tests of Section II C to be repeated as part of the lot qualification of NB-2420. This is confirmed by a number of interpretations such as III-1-04-25, III-1-83-245, III-1-83-53, III-80-37 (qs 2), etc.
However, Contractor May Choose to Do Section II C Tests Too
The interpretations mentioned above leave no room for doubt. It is clear that Section III does not require all Section II C tests to be done in order to satisfy NB-2420.
However, as with any code requirement, Code specifies only the minimum requirements. The code user is free to adopt any requirements over and above those given in code, as long as the minimum requirements of code are met. Therefore, if a contractor CHOOSES to specify to the welding material supplier that all SFA tests be done too (in addition to those in NB-2431), it would neither foul the code in any way, nor would it be in any way an uncommon thing.
In my opinion, it is better to do all tests specified in SFA as well. This is because the testing defined under NB-2430 specifies only minimal testing, and in the matters of nuclear – it is better to spend some extra money, do some extra tests, and be safe, rather than save a few lousy bucks, have a doubt fester in your mind, and be sorry later.
In fact, the interpretation III-82-10 provides a similar guidance. It indicates that the contractor should have a copy of the test data generated during classification. In other words, the contractor should have a Certificate of Conformance on the file.
Note (1): The certificate of conformance is a typical test report, not necessarily containing the results from the same heat that is being supplied. The results contained on this report are typical results that can be expected from that trade name, and which the supplier established during classification tests earlier.
Note (2): For fillers that are not classified under Section II C, a certificate of conformance is not applicable. Such fillers should be bought with the full range of tests done, as defined in material specification, for each lot.
In conclusion, although interpretation III-82-10 advises to keep on file a certificate of conformance, it is a good idea to have supplier perform all tests of relevant SFA specification, in addition to tests specified under NB-2430, and have him state these results on the CMTR. Alternatively, contractor can choose do these tests himself, at his works.
Common Pitfalls
We saw above that some contractors ask the supplier to do all tests of SFA specification too, in addition to the testing required by NB-2430. Although, this is quite fine – this practice has a pitfall.
The NB-2430 requires that the qualification coupon should be made in the same heat treatment condition as the job. If the job is going to remain as-welded, the coupon should remain as-welded too. If the job is going to see some PWHT, the coupon should be subjected to PWHT too. The SFA specifications, however, commonly specify testing in as-welded condition only.
The mistake that many procurement engineers tend to make is that they specify the coupon to be made with PWHT (required by NB-2430), and that the results should meet the requirements of both NB-2430 as well as SFA specification. While the coupon must meet NB-2430 requirement in PWHT condition, it is a mistake to think that it must meet SFA values too.
For example, SFA-5.18 specifies that ER70S-6 must meet impact energy of 27 Joules at -30°C. There is no PWHT specified in SFA 5.18. This rod is going to be used for a Section III application which requires 68 Joules at +18°C, after a PWHT at 600°C.
Now, if the engineer specifies that the weld metal must meet 68 Joules at +18°C, and also 27 Joules at -30°C, after a PWHT of 10 hours at 600°C, it is not quite right. Although, the weld metal might sometimes meet both requirements, but it is not right to specify in this manner.
The right way to specify would be to ask for 27 Joules at -30°C in as-welded condition, and 68 Joules at +18°C in PWHT condition.
I have written a whole different article on how to qualify welding material for Section III NB requirements. It can be found here.
Does The Electrode Supplier Need To Have Any Specific Credentials?
Section III, at subsection NCA-3800, has specified system requirements that a Material Organization(MO) is expected to meet. There are two kinds of MOs: Certified MO and Qualified MO.
The certified MO is evaluated by ASME, and a Quality System Certificate (QSC) is issued to them if they meet the requirements of Section III. A certified MO is a QSC holder, and such companies have some reputation associated with them. A buyer can rely on the supplier to understand all requirements of Section III. If you buy from such supplier, you do not have to undertake the struggle to explain each requirement in detail to the supplier. However, certified MOs are somewhat expensive – owing to the expenditure they incur on acquiring the QSC.
Another route is Qualified MO. The Certificate Holder (that is the component fabricator or manufacturer) can evaluate a supplier for a limited scope quality program. This evaluation is to verify the supplier’s capability to supply material meeting the requirements of NB-2400.
Now, how does one go about this evaluation – is a weighty question.
Well, the idea is to verify the supplier’s capability to meet NB-2400 requirements. One of the main requirements of NB-2400 is the lot class. Per NB-2400, the covered electrodes are required to be of lot class C3; the bare solid rods are required to be of lot class S2; fluxes are required to be of lot class F2; and so on. So, a good starting point in the supplier’s evaluation would be to look for such controls in supplier’s Quality Manual and the related subsidiary documents.
For example, the lot class C3 means that the electrode must have been produced in a single uninterrupted production schedule, in a quantity not exceeding 45 tons, the core wire must be of single heat or of controlled chemical composition, and the covering flux must be of a single wet mix or of controlled chemical composition.
Does the supplier’s QA program have requisite controls to assure these requirements? Does the supplier understand these things properly?
Then, the evaluation of the quality program of supplier is not to be limited to verify compliance to NB-2400 alone. There are a number of other things as well. NCA-3800 lists down in some detail how this evaluation can be done, responsibilities of the Material Organization.
A very handy way to evaluate a supplier’s quality program would be to take a printout of NCA-4200; and verify one by one whether the supplier’s QMS (Quality Management System) has adequate controls to satisfy each of the points under NCA-4200. Besides a review of Quality Manual, an audit can be done to establish whether the supplier actually implements what has been specified in Quality Manual.
Who Should Do The Batch Testing?
When procuring welding consumables for Section III requirements, it is required to do batch testing satisfying the requirements of NB-2400. The information required to satisfy NB-2400 is not standard information, and known only to the manufacturer of the component. For example, if a supplier produces E7018-1 electrode in full compliance of SFA 5.1 of Section II part C, it would not be enough for him to satisfy NB-2400.
He would not know the drop weight test temperature, preheat/ inter-pass temperature requirements, PWHT parameters, impact test temperature and acceptance criteria, etc. This is why, NB-2410 helpfully provides a list of information that the Certificate Holder must provide to the supplier.
Now, is it necessary that only the supplier should do all testing?
No. Although this is generally the case, the Code does not necessarily require this. The Code permits that the requisite testing (to satisfy NB-2400) can be done by the Certificate Holder too. This is provided under NCA-3860.
Interesting Observations Regarding Batch Testing
Here are a few interesting observations regarding lot qualification to NB-2400, which appear only at a close look at the code.
Needs to Meet Requirement of Base Metals to be Welded
An interesting thing that comes out from the paragraphs contained under NB-2400 is that the tensile and impact tests need to meet the requirements of the base metal that is going to be used in the production welding, not those specified in Section II C. This is interesting. This expectation gets reaffirmed through interpretations III-1-78-48, III-81-103.
[Note: If the production weld comprises of dissimilar base metals, then the welding consumables of NB-2400 may conform to the weaker of the two. This is similar to the guidance given by Section IX for dissimilar metals.]
NB-2431.2, which provides an alternate to this requirement, says that the coupon can be prepared in accordance with SFA requirements alone (i.e. need not satisfy preheat, inter-pass, & PWHT requirements of NB-2431.1); and it is enough if SFA requirements are satisfied (i.e. UTS & impact need not be equal to those of production base metal). This paragraph, NB-2431.2, however – is applicable only for carbon steel and low alloy steel SMAW electrodes of SFA 5.1 and SFA 5.5.
Is It Required To Use The Same Base Metal As In Production Weld?
We saw above that the UTS & impact is required to meet the requirements of base metal that is going to be used in production weld, not those of SFA specification. Does this mean that the base metal taken for lot qualification under NB-2400 must be of same grade as the production weld? Or at least of same P-number?
No. NB-2431.1(b) begins by saying that the base metal chosen must be such that the effect of base metal on test results is substantially eliminated. This in effect means that one is free to choose a base metal conforming to SFA requirements.
Though, it must be kept in mind that if electro-slag process is used, the same paragraph specifies that the coupon must be used using a qualified WPS. This means that the choice of base metal gets dictated by the WPS.
How Is The Coupon Of NB-2430 Different From The Coupon of SFA Specification?
Well, the coupon made for NB-2430 can be same as the groove weld coupon defined under SFA specifications, except for the following two differences:
- Preheat and inter-pass temperatures must be same as those of production welding. If production welding has several different preheat and inter-pass temperatures, then it is enough to make the coupon with the highest preheat & inter-pass temperature that will be observed in production welding (III-1-77-64, III-81-02, III-1-83-202(qs 3)).
- PWHT of the coupon must be essentially same as that in production welding, and the soaking time shall not be less than 80% of the total soaking time that the production weld will ever see in its’ lifetime (this should include any PWHT that the job might see in repairs in its’ service too). This requirement is same as the QW-407.2 – which is a supplementary essential variable in Section IX.
Another Interesting Requirement
Another interesting thing specified in the paragraphs under NB-2400 is that the only UTS is required to be checked during the batch qualification. That is, yield strength and percentage elongation are not required to be checked.
This gets further reaffirmed through interpretations III-1-04-35 and III-1-78-151.
The UTS thus found must equal or exceed that of the base metal to be used in production.
Batch Qualification With Each Process
NB-2400 requires that the test coupon for batch testing be prepared with each processes with which the consumable is going to be used in the production welding. For example, if a heat of ER70S-6 is going to be used with manual-GTAW and GMAW process, separate test coupons must be made – one with manual GTAW, another with GMAW.
Impact Testing – Lowest Service Temperature
The test temperature, energy absorbed and lateral expansion for impact tests have to be determined carefully.
In a vessel, when a pressure retaining material exceeds 5/8 inch (16 mm) in thickness, NB-2331 requires establishing a T NDT by doing drop weight tests (2 specimens shall show no-break), followed by impact tests at a temperature no higher than T NDT + 60°F (T NDT + 33°C) (all 3 specimens should show 50 ft-lb (68 Joules) & 35 mils (0.89 mm) lateral expansion).
When a welding material is to qualified to join such material, this testing should be done for welding material too. In the course of batch testing under NB-2400, the drop weight test and impact tests as described above should be done. The temperature and acceptance criteria should be furnished by the manufacturer to the supplier who does the testing.
The temperature at which the impact testing is done is taken as Lowest Service Temperature (LST), and is a very important parameter for the welding engineer to hold on to.
Lowest Service Temperature – Who Specifies It
LST is determined at the design stage. Commonly, it is furnished by the owner (the agency that operates the power plant) of the equipment to the certificate holder who fabricates the component.
All three of the following must be subjected to DWT at T NDT , followed by impact test at T NDT + 60°F (T NDT + 33°C) [also called Lowest Service Temperature]:
- Base metal to be used for fabrication
- Welding material to be used for joining such impact-requiring materials.
- WPS qualification in accordance with NB-4335.
Retesting is permitted in all three qualifications. If the acceptance criteria of 50 ft-lb (68 Joules) & 35 mils (0.89 mm) lateral expansion is not met, Section III NB also provides further provisions to do further testing to determine a new LST than the initially intended temperature. The welding engineer who procures welding material must be mindful of not just the design specification, but also the WPS with which he intends to use that welding material.
Three Specimens – Not five
SFA specifications generally specify five impact specimens, of which the lowest and highest values must be discarded. The average energy value of the remaining three, and the minimum among the three must not be lower than the corresponding values specified in SFA specification.
Section III at NB-2320, however, specifies that only three specimens be taken.
So, when a welding material is being batch tested in accordance with NB-2400, how many specimens should one take? Three? Or five?
Section III NB does not address this directly. So, we try to extract an answer from interpretations.
There are two relevant interpretations which point to two different lines of thinking.
ASME Interpretation III-1-04-40 indicates that it is okay to take three specimens and compare the results to the acceptance criteria of NB-2431. Whereas, interpretation III-82-60 indicates that it is okay to take five specimens, discard the highest and lowest values, and compare the results to the acceptance criteria of NB-2431.
It appears that there isn’t a consensus in Section III. So, as a code user – I think it would be alright to use either among the two practices.
Impact Testing At Lower Temperature Is Alright
Commonly, the lowest service temperature is specified in the region of 10°C to 30°C.
Suppose that it is +20°C. This means that the welding engineer who procures welding material must specify impact test at +20°C, with an acceptance criteria of 50 ft-lb (68 Joules) & 35 mils (0.89 mm) lateral expansion.
But the SFA specifications generally specify impact requirements at far lower temperatures. For example, the impact test requirement for E7018-1 electrode is 20 ft-lb (27 Joules) at -45°C.
If a batch exhibits 50 ft-lb (68 Joules) & 35 mils (0.89 mm) lateral expansion at -45°C, then impact tests at +20°C are not required. The material meets the Section III requirements if the impact values are found to be meeting at a temperature lower than the specified LST.
So, this is all i have to offer on this subject at the moment. Please feel free to ask questions, offer suggestions or criticism. You can share your thoughts in the comments section below.