ASME Section III Part NB, that deals with construction of Class 1 nuclear equipment, requires that all welding materials that is used in the construction of Class 1 equipment conform to the requirements of NB 2400.
The Certificate Holder must ensure several aspects in procurement of these materials to ensure conformance to NB 2400. One of these aspects is lot classification requirements specified in NB 2420. Another very important aspect is the qualification of every lot, in accordance with NB 2430.
In this article I will discuss about the lot qualification of the welding material in conformance to NB 2400. NB 2400 requires that every lot of the welding material must be subjected to the required tests of NB 2400.
How must this lot qualification be done, what are the tests required (with a special emphasis on mechanical testing portion), what is the acceptance criteria, and a few other insights that I gained in the course of my own official work – forms the content of this article.
In this article we shall lay special emphasis on NB 2431 – which covers the mechanical testing portion of the qualification.
The clauses NB 2431.1 and NB 2431.2 describe in detail the process of preparing a test coupon, required tests, method of removing samples for testing, and the acceptance criteria for each test. Mainly, only tensile, impact, and drop weight test have been specified.
NB 2431.1 and 2431.2 are given as optional to each other; qualification through either one of them is enough; it is not required to do both of them. Let us see in detail all about NB 2431.1 first.
Mechanical Testing As Per NB 2431.1: General Test Requirements
NB 2431.1: The NB 2431.1 establishes a method of lot qualification that is applicable to all kinds of welding materials (as opposed to NB 2431.2 which is applicable only to welding materials classified under SFA 5.1 and SFA 5.5 of ASME Section II, Part C).
Following are the noteworthy points of the mechanical testing specified under NB 2431.1:
Size Of Test Coupon
NB 2431.1 (a) – The thickness and length of test coupons shall be as necessary to cover the tests required by NB 2431.1. No specific size (neither length, nor thickness) is mandated for the test coupon.
Base Material for Preparing The Lot Qualification Test Coupon
NB 2431.1 (b) – The test coupon must be prepared so as to substantially eliminate the effect of base material. The lot qualification aims to test the properties of weld metal alone.
What base metal must be used for lot qualification then? We will see the upcoming paragraphs.
However, the test coupon welding of material to be used for electro-slag process must be done in accordance with a qualified WPS that would be used for production welding. This would of course restrict the choice of base material that can be used for preparing the test coupon.
This raises a question in mind: Is a qualified WPS not necessary for preparing the lot qualification coupons for welding materials intended to be used with processes other than electro-slag process?
The answer is no. The Material Manufacturer or Material Supplier, who generally does the lot qualification testing in the process of certifying a material, does not have to have a qualified WPS that would be used for production welding at the Certificate Holder’s works.
Preparation of the coupon for qualification of all other welding materials (except those for electro-slag) can be done by following the rules given in Section II Part C. However, for electro-slag welding process, a qualified WPS (which would be used for eventual production welding) must be provided to the organization that is performing the lot qualification exercise.
The last line of NB 2431.1 (b) reads that “the base material shall conform to the requirements of Section IX, QW 403.1 or QW 403.4, as applicable.” This line raises a slight confusion as to whether it is applicable to all processes or only the electro-slag process?
This has been clarified by ASME in two Interpretations, III-1-13-28 and III-2-13-04. Though, if one is familiar enough with Section IX, it can be noted that QW 403.1 and 403.4 appear under QW 250 only in the table for electro-slag welding process, so this doubt should not arise.
What about the choice of base material for other processes?
Base Material Choice For Lot Qualification Per NB 2431.1 For Processes Other Than Electro-slag Process
The Lot Qualification test assembly is required to be prepared in accordance with the application SFA specification, as becomes clear in NB 2431.1(d). This means that the base material specified in the applicable SFA specification can be used for preparing the test coupon for Lot Qualification.
It is not necessary to use the same base material as the production. Neither is it necessary to use a base metal of same P –No. as the one in production welding. It is enough to use the base metal specified in Section II C for preparing the test coupon.
Preheat And Inter-pass Temperatures For Lot Qualification Coupon Per NB 2431.1
NB 2431.1 (c) – This paragraph specifies that “welding of the test coupon shall be performed within the range of preheat and interpass temperatures that will be used in production welding.”
This makes an important question arise in mind: What if the electrode is used in several production welds which have different preheat and interpass temperatures? See the following paragraph for an example. What preheat and interpass temperatures should be used for preparing the lot qualification test coupon then?
Supposing that an electrode, say E7018-1 electrode, is used for production welding in the following four conditions: i) without preheat, without PWHT; ii) without preheat, with PWHT; iii) with preheat, without PWHT; and iv) with preheat, with PWHT, in these four different kinds of welds.
In order to comply with NB 2431.1(c), is it required to prepare four separate test coupons with preheat and PWHT combinations as described above?
Answer: No. Only one coupon can be prepared, choosing the highest preheat, inter-pass, and PWHT temperatures that would be encountered in production welding. This has been clarified by ASME in III-1-77-64. The Interpretation III-81-02 further buttresses this point. This Interpretation indicates that the welding of the weld test coupon must not be performed at a lower preheat than the range of preheat specified in production WPS.
Notwithstanding the above, we can completely bypass NB 2431.1 and do lot qualification as per Standard test option provided in NB 2431.2, if the electrode is one classified under SFA 5.1 or SFA 5.5. This would circumvent the question of preheat and interpass temperature. The test coupon only need to comply with SFA 5.1 or SFA 5.5 (as applicable).
PWHT For Lot Qualification Coupon Per NB 2431.1
NB 2431.1 (c) also specifies that the coupon must be tested in the as-welded condition or in the applicable post weld heat treated condition. This means that if the production weld is envisaged to remain as-welded, the lot qualification coupon must also be tested in the as-welded condition. If the production weld is envisaged to be subjected to a PWHT, the lot qualification coupon must also be subject to PWHT before removing samples for mechanical testing.
The term ‘applicable’ here indicates that the PWHT given to coupon must be of similar nature (that is same time and temperatures). Specifically, the holding time for PWHT of coupon must be at least 80% of the cumulative holding time to be applied to the weld metal in production application.
If the production joint undergoes multiple heat treatments in the fabrication, the total time of all heat treatments must be reckoned to calculate the holding time for PWHT of the coupon. This holding time can be applied to the coupon in one cycle.
The cooling rate is an important thing in the PWHT. This is why; the NB 2431.1 (c) specifies that “cooling rate from the post weld heat treatment temperature shall be of the same order as that applicable to the weld metal in the component.”
In my work place, many SMAW electrodes are used in both as-welded joints, as well as joints with PWHT. Such electrodes are qualified through two coupons: first, in which samples for mechanical testing are to be removed in as-welded condition; second – in which samples for mechanical testing are to be removed after a relevant PWHT.
How To Prepare Tensile And Impact Specimens For Lot Qualification
NB 2431.1 (d) – specifies that tensile and impact specimens shall be located, oriented and prepared in accordance with the requirements of applicable SFA specification. Also, requirements of NB 2321.1 and 2321.2 are applicable – these paragraphs define the size of specimens and number of samples and testing standard to be followed.
This raises a question in mind. The SFA specifications generally require testing of sets of five impact specimens, among which the highest and lowest values of energy must be discarded, and the remaining three must be averaged for determining acceptance. However, NB 2331/2332 specify only three specimens.
For lot qualification testing of NB 2431, is it required to do five impact tests to meet the requirement of NB 2331/2332, or is three enough?
The Interpretations III-1-79-29, III-82-60 point us in the direction that the practice of taking five specimens, and discarding the highest and lowest values is the right practice. However, the Interpretation III-1-04-40 again changes the direction, and indicates that it is enough to take three specimens.
So, all in all – three specimens should be enough.
NB 2431.1 (d) also specifies that the longitudinal axis of the impact and drop weight test specimens shall be located at least ¼ t from the surface, where ‘t’ is the thickness of test coupon.
Acceptance Criteria For Tensile Test
NB 2431.1 (e) – specifies that “One all weld metal tensile specimen shall be tested and shall meet the specified minimum tensile strength requirements of the base material specification.”
These are a couple of interesting things to note here.
Firstly, this line only asks ‘minimum tensile strength requirements’, and does not ask yield strength, and % elongation. This firmly establishes that the lot qualification asked by NB 2400 does not require YS and % elongation to be reported (even though the relevant SFA specification might require it). This also finds confirmation in Interpretation III-1-04-35. The Interpretation III-1-78-151 further hammers home this point with absolute clarity.
Secondly, the acceptance criterion for tensile strength obtained in this lot qualification is the strength of the base material to be used in the production joint, and not the strength specified in the relevant SFA specification. This has been made repeatedly clear by ASME through multiple Interpretations. One such Interpretation is III-1-78-48. Another example is III-81-103.
Alright. What if the production weld is between two dissimilar base metals that have different tensile strengths? What then would be the acceptance criteria for the tensile strength during the testing of lot qualification per NB 2431.1?
This question has been addressed in the last line of NB 2431.1 (e), which states that “when base materials of different specifications are to be welded, the tensile strength requirements shall conform to the specified minimum tensile strength requirements of either of the base material specifications.”
Acceptance Criteria For Impact Test
NB 2431.1 (f) – specifies when impact testing is required, and the acceptance criteria for impact tests. Whenever the base material requires impact test per NB 2331 and NB 2332, the welding material needs to undergo impact testing too. The acceptance criteria for the weld would be same as that for the base material. If two different base metals are used, “the weld metal may conform to either of the two requirements.”
Mechanical Testing As Per NB 2431.2: Standard Test Requirements
As an alternative to NB 2431.1, the NB 2431.2 has been provided as the procedure (required tests, method of preparing the coupon, and acceptance criteria) for preparing the test coupon for lot qualification. This alternative is only applicable to carbon steel and low alloy steel electrodes classified under SFA 5.1 or SFA 5.5 respectively.
Most requirements specified in NB 2431.1 remain applicable to NB 2431.2 too. Fundamentally, the focus is directed at the test assembly specified in SFA 5.1/SFA 5.5, with the size suitably increased to accommodate impact and drop weight samples. The welding of the test coupon follows all methodology outlined in the relevant SFA specification.
The main point of difference lies in NB 2431.2 (e). As per this paragraph, one all weld tensile is to be tested, and it must meet the specified minimum tensile strength requirement of the SFA specification for the applicable electrode classification. Note that NB 2431.2(e) says “requirement of the SFA specification”, while NB 2431.1(e) had specified “requirements of the base material specification”.
In essence, the standard test requirements of NB 2431.2 require compliance to SFA specification, while the general test requirements of NB 2431.1 require compliance to base material specification, which is generally found in the relevant SA specification of Section II Part A. This understanding has been further solidified by ASME through an Interpretation, III-1-78-48.
Exemptions To Mechanical Testing During Lot Qualification
The mechanical testing in the lot qualification is basically only tensile and impact tests (and where applicable – drop weight tests) for welding materials that are used to joint P-Nos. 1, 3, 4, 5, 6, 7, and 9. Not all welding materials need to be subjected to these tests, the exemptions are as below (as given under NB 2431):
- Austenitic SS and non ferrous welding material used to join the listed P Numbers,
- Consumable Inserts,
- Welding material used with GTAW for root welding, up to a maximum of two layers, and
- Welding material to be used for joining base metals exempted from impact testing by NB 2311.
With the above context, consider the following question asked in Interpretation III-82-23:
“Question: When using filler metal with the GTAW process for the root bead in open butt welding, is the filler metal subject to the testing requirements of NX-2430?
Reply: Yes. The filler metal is subject to the testing requirement of NX-2430.”
In the context of above question, it must be borne in mind that the exemption granted in NB 2431 for is only for tensile and impact tests, not for chemical analysis and delta ferrite examination (which are covered by NB 2432 and NB 2433, and which are thus part of NB 2430) which are also part of the lot qualification. The reply given to the question in above Interpretation implies this.
Miscellaneous Observations On NB 2431
- NB 2431.1 (d), (e) and NB 2431.2 (e) require an all weld tensile test. Would it be alright if we use the results of a tensile test of a procedure qualification? Remember, that the tensile test of a procedure qualification is generally taken in the transverse direction of the weld. Would the results of a transverse tensile, taken from a procedure qualification coupon, satisfy the requirement of NB 2431?
Yes, it would be enough. However, the tensile strength should meet the requirement of NB 2431.1 (e) or NB 2431.2 (e). This also has the sanction of an Interpretation from ASME. The Interpretation number is III-1-78-53 (qs 2).
- Is it required to do radiographic examination of the test coupon prepared for doing the lot qualification of NB 2430? Does Section III NX require any kind of non destructive examination for lot qualification of welding materials in accordance with NB 2400?
The inspector in my work place insists that the lot qualification must include the radiographic examination of the test coupon, which must meet the acceptance criteria defined in the relevant SFA specification.
I have tried reasoning that NB 2400 does not include any mention of any NDE in its body, and the only tests that need to be included are those that are explicitly specified under NB 2400.
Fortunately, I have found an Interpretation that addresses exactly this matter and rules in my favor. The Interpretation III-1-79-59 is categorical in stating that “there are no Section III non-destructive examination requirements specified for qualifying welding materials for Section III, Division 1 or 2 construction.”
- Should all tests specified in Section II C form part of the lot qualification testing per NB 2400?
No. Only the tests specified in NB 2400 – viz. tensile, impact and chemistry (and delta ferrite, if applicable) are required. The lot qualification testing of NB 2400 requires covering only the tests specified in NB 2430, and not all tests specified in Section II Part C. This also finds firm confirmation in Interpretations III-1-04-25 and III-80-37(qs 2).
So this was all about the mechanical testing of the test coupon, towards meeting the lot qualification requirements of clause NB 2400 of ASME Section III NB for welding materials. Please do leave your thoughts in the comments section below.
Food for thought: How do ASME BPVC welding requirements compare against the RCC (the French Code) requirements? Read this article to find out.