ASME BPVC is a magisterial body of work. Remarkably consistent over several decades, it commands respect across the world for being concise, reliable, authentic, precise, etc. Many adjectives may be deployed to describe it. It is not however perfect. It has constantly evolved over the years, a new edition of all sections comes out every two years now. Many intelligent engineers have contributed to its growth and refinement.
In this article, I have listed a few suggestions for changes to NB-2400 of Section III NB. There is another article on this website titled ‘Suggestion for change under NB-2420’ which talks on a particular subject. That subject is too big to be included in this article; hence it has been made into a different article altogether.
The three proposals in this below article are a result of my observations in my own office work. There may of course be disagreements, rebuttals, etc. by knowledgeable people, but that is welcome. Strenuous discussion only makes the code evolve.
1. Replace ‘Heat’ With ‘Lot’ In Para 1 of NB-2420
The first para under NB-2420 mentions the word ‘heat’ several times. This word should be replaced by the word ‘lot’. The reason is described in the below paragraphs.
Background
The first paragraph of NB-2420 states:
‘The required tests shall be conducted for each lot of covered, flux-cored, or fabricated electrodes; for each heat of bare electrodes, rod, or wire for use with the OFW, GMAW, GTAW, PAW, and EGW (electrogas welding) processes (Section IX, QG-109); for each heat of consumable inserts; for each combination of heat of bare electrodes and lot of submerged arc flux; for each combination of lot of fabricated electrodes and lot of submerged arc flux; for each combination of heat of bare electrodes or lot of fabricated electrodes, and dry blend of supplementary powdered filler metal, and lot of submerged arc flux; or for each combination of heat of bare electrodes and lot of electroslag flux. The definitions in SFA-5.01 and the Lot Classes specified in (a) through (e) below shall apply.’
NB-2420(c), (d), (e) require bare electrodes/rods, consumable inserts, SAW wires, to be of lot class S2. The definition of lot class S2, given at SFA-5.01 of Section II Part C, permits use of controlled chemical composition as an alternative to one heat, in manufacture of lot class S2 consumables. This means that multiple heats, provided their chemical composition is controlled within a range, can be used in the manufacture of a lot, and the resultant quantity would still get counted as a single lot.
While the code intends that each lot of bare electrodes/rods/consumable inserts/SAW wires etc. be subjected to the requisite testing, the use of the word ‘heat’ at several places in the opening sentence of NB-2420 conveys the meaning that each heat used in the making of a lot is required to be tested. This is clearly not the intent of code.
The use of the word ‘heat’ here, in all editions from 2017 onwards, seems to be a continuation of use of the word from the 2015 edition – when all definitions were contained at NB-2420 itself, rather than references to lot classes of SFA-5.01. This use of the word ‘heat’ here, while appropriate in the 2015 and earlier editions, is in error, in all the later editions.
Proposal for change
The first paragraph of NB-2420 is reproduced below. The proposed changes are in red, and deletions are indicated as strike-outs.
‘The required tests shall be conducted for each lot of covered, flux-cored, or fabricated electrodes; for each heat lot of bare electrodes, rod, or wire for use with the OFW, GMAW, GTAW, PAW, and EGW (electrogas welding) processes (Section IX, QG-109); for each heat lot of consumable inserts; for each combination of heat lot of bare electrodes and lot of submerged arc flux; for each combination of lot of fabricated electrodes and lot of submerged arc flux; for each combination of heat lot of bare electrodes or lot of fabricated electrodes, and dry blend of supplementary powdered filler metal, and lot of submerged arc flux; or for each combination of heat lot of bare electrodes and lot of electroslag flux. The definitions in SFA-5.01 and the Lot Classes specified in (a) through (e) below shall apply.’
2. Remove ‘Listed P-Nos.’ From NB-2431(a)
The first para under NB-2431 provides exemptions to mechanical testing for batch qualification of welding material. The phrase ‘listed P-Nos.’ in NB-2431(a) causes some ambiguity. I explain below.
Background
NB-2431 states:
‘Tensile and impact tests shall be made, in accordance with this paragraph, of welding materials that are used to join P-Nos. 1, 3, 4, 5, 6, 7, 9, and 11 base materials in any combination, with the exceptions listed in (a) through (d) below:
(a) austenitic stainless steel and nonferrous welding material used to join the listed P-Numbers;’
The phrase ‘used to join the listed P-numbers’ at the end of above text conveys that the exemption from tensile and impact testing is applicable only for those austenitic SS & non-ferrous welding materials that are used to join P-nos. 1, 3, 4, 5, 7, 7, 9 & 11 base metals, and not for those that are used to join other base metals such as say, P-No. 8, etc.
However, interpretations III-1-77-249 and III-1-79-102 inform that the exemption for these welding materials remains applicable when they are used to join P-No. 8 too. In fact, III-1-77-249 indicates that the exemption remains good regardless of the base metals that are being welded.
In light of the above, one of the following two proposals may be adopted for revising the above clause. The proposed change is indicated in red.
Proposal 1
‘Tensile and impact tests shall be made, in accordance with this paragraph, of welding materials that are used to join P-Nos. 1, 3, 4, 5, 6, 7, 9, and 11 base materials in any combination, with the exceptions listed in (a) through (d) below:
(a) austenitic stainless steel and nonferrous welding material used to join the listed P-Numbers;’
Proposal 2
‘Tensile and impact tests shall be made, in accordance with this paragraph, of welding materials that are used to join P-Nos. 1, 3, 4, 5, 6, 7, 9, and 11 base materials in any combination, with the exceptions listed in (a) through (d) below:
(a) austenitic stainless steel and nonferrous welding material used to join the listed P-Numbers listed under QW-422 of Section IX;’
3. Batch Qualification Coupon Should Be Made with Highest Preheat
NB-2430 requires that the test coupon for batch qualification be made with the same preheat, interpass and PWHT that the production weld is going to see. This however causes some confusion as explained below.
Background:
NB-2431.1(c) states:
‘The welding of the test coupon shall be performed within the range of preheat and interpass temperatures that will be used in production welding. Coupons shall be tested in the as-welded condition, or they shall be tested in the applicable postweld heat-treated condition when the production welds are to be postweld heat treated’
The above paragraph desires that the test coupon prepared for batch qualification of welding material shall be prepared within the range of preheat & interpass temperatures that will be used in production welding.
An electrode (say E7018-1 SMAW electrode) is used in the following five kind of welds in production welding:
- Weld 1: without preheat, without PWHT.
- Weld 2: without preheat, with PWHT of 600°C.
- Weld 3: with preheat of 150°C, without PWHT.
- Weld 4: with preheat of 150°C, with PWHT of 600°C.
- Weld 5: with preheat of 200°C, with PWHT of 600°C.
Does the code require five different test coupons for qualification of this electrode as per NB-2400? While NB-2400 provides no explicit answer to this question, two interpretations III-81-02 and III-1-77-64 provide a guidance that the test coupon must not be prepared with a preheat temperature lower than the preheat of production weld. In other words, if, for example, an electrode lot is going to be used in production welds – i) without preheat, ii) with preheat of 150°C, and, iii) with preheat of 220°C, then the code’s expectation is met if the test coupon is made with a preheat of 220°C.
In other words, in such cases when the electrode is going to be used with multiple preheat temperatures in production weld, it is enough to make just one coupon with the highest preheat among all of them.
This, however, runs counter to the metallurgical considerations and also the understanding established by QW-406.1 of Section IX.
Decrease in Preheat is Detrimental to Mechanical Properties
QW-406.1 stipulates that a decrease in preheat temperature requires requalification of welding procedure. In other words, if a procedure has been qualified with preheat – the production weld made with the help of this procedure must also be made with preheat only. If, however, a procedure has been qualified without preheat – the production weld may be made with preheat too, as doing so would constitute an increase in preheat – which is not prohibited by QW-406.1.
This indicates that decrease in preheat is detrimental to the mechanical properties of a weld. This thinking also conforms to the commonly understood behaviour of carbon steel and low alloy steels. An increase (within a reasonable range) in preheat, on the other hand, is considered beneficial.
Worst Condition Is Not Simulated
Welding qualifications (including batch qualifications of NB-2431) are generally sought to be done by simulating the worst conditions that the production job could potentially witness. [Toughness tests at sub-zero temperatures and long soaking hours in PWHT are examples of this]. If the qualification shows acceptable test results under these worst conditions, it can be inferred that the production welds made under easier conditions will surely possess acceptable mechanical properties.
Pursuant to this thinking, if a batch qualification is done without preheat – and shows acceptable results, it is akin to simulating the worst condition and still passing the test. Among the 5 production welds described on page 1, this qualification simulates weld 1, which is the worst condition among the five. Such a batch, when used in all five kinds of production welds described on page 1, can be expected to have acceptable mechanical properties.
Section III however, under such conditions as described above, stipulates that the batch qualification shall be done with the highest preheat temperature that the production weld might see. By doing this, the code ends up not simulating the worst condition that the job might see, but in fact ends up doing the opposite. Furthermore, there does not appear to be any basis for doing the batch qualification with the highest preheat temperature.
It is therefore proposed that NB-2431.1(c), first two sentences, be revised.
Proposal for change
The proposed revision is as follows (changes are shown in red):
‘The welding of the test coupon shall be performed within the range of preheat and interpass temperatures that will be used in production welding. When the welding material is going to be used with multiple preheat temperatures in production welds, the test coupon shall be prepared with the lowest preheat that will be applied to welding material when it is used in production. Coupons shall be tested in the as-welded condition, or they shall be tested in the applicable postweld heat-treated condition when the production welds are to be postweld heat treated.’
This is all I have to offer, currently. Please do share your thoughts in the comments section below.