This article discusses about the subject of A-Number on a WPS. The article begins with an introduction of A-number, then explains how to apply it vis-à-vis the rules of ASME Section IX. It goes on to explain various methods of determining A-number. Towards the end of the article, it explores a little the question of whether it is essential to indicate A-number on the WPS.
What is A-Number?
A-Number is indicative of the nominal chemical composition of the weld metal deposited. QW 442 lists twelve types of nominal compositions, each of which has been assigned an A-Number. Whenever a PQR is made, the chemical composition of the weld metal deposited in the test coupon has to be checked.
This composition has to be compared with the twelve compositions given in QW 442, and the matching composition has to be identified. The A-Number listed in QW 442 for that composition has to be then noted in the PQR.
When this PQR has to be used to support a WPS for a production joint, the filler metal for the production WPS has to be chosen in such a manner that the A-Number of the weld deposited by that filler would be the same as the one recorded in the PQR (except that qualification with A-No. 1 shall qualify for A-No 2 too, and vice versa).
In this manner, Section IX restricts the choice of filler metals that can be used in production with the support of a PQR.
The stipulations regarding A-Numbers have been enshrined in QW 404.5, which appears as an essential variable for most of the processes under QW 250.
How To Apply A-Number In Accordance With Section IX
Consider an example. A PQR has been qualified using E7018 electrode. Can this PQR be used for a production joint using a low alloy steel electrode, say E9018-B9?
E9018-B9 is a LAS electrode classified under SFA 5.5, having F-Number 4. So, QW 404.4 – which governs change of F-Number, is satisfied. However, the A-Number of the weld deposited by E9018-B9 is A-No. 5, while A-Number of weld metal deposited by E7018 is A-No. 1.
Due to this difference in A-Numbers, therefore, E9018-B9 electrode cannot be used in a joint that is supported by the PQR mentioned in the preceding paragraph. The reverse is not allowed too.
Consider another example; a PQR has been qualified with E7018 electrode. Can this PQR be used to write a WPS using E7018-A1 electrode?
The E7018 electrode is a carbon steel electrode classified under SFA 5.1 of ASME Section II Part C. Its’ F-Number as per QW 432 is 4, and the chemical composition of the weld metal deposited by E7018 would classify under A-Number 1.
The E7018-A1 is a carbon-molybdenum steel, classified under SFA 5.5, and its F-Number and A-Number are 4 and 2 respectively.
The QW 404.4 is satisfied, since the F-Numbers of both the electrodes is same. But the A-Numbers are different. However, this case qualifies for the exemption given in Section IX (qualification with A-No. 1 shall qualify for A-No 2 too and vice versa). Hence, QW 404.5 is satisfied too.
Thus, the PQR qualified with E7018 can be used for welding with E7018-A1 electrode as well. However, other considerations such as metallurgical compatibility, PWHT requirements, strength requirements would also have to be kept in mind.
A similar question appeared in Interpretation IX-86-84; the reply given by Committee matches with the above understanding.
So, this (restriction through A-Numbers) is a mechanism by which Section IX restricts indiscriminate substitution of alternate filler metals in WPSs.
A-Numbers Are Only For Ferrous Metals
A notable thing about A-Numbers is that they have been envisaged only for ferrous metals (that is, which have iron in the majority in their composition). Hence, this regulation is only possible for steel and steel based alloys.
For other alloy systems (such as nickel alloys, aluminium alloys, titanium alloys, zirconium alloys etc.), developing A numbers might not have been easy.
Therefore, for such cases, one has to apply other engineering considerations (such as metallurgical compatibility, strength requirements etc.) when considering substitution of the filler used in PQR with some other filler metal.
How To Determine A-Number
QW 404.5 discusses this matter in some detail. For various processes, separate paragraphs have been allotted.
The chemical analysis of the weld deposit taken from the procedure qualification test coupon is a common method, and applicable for all processes. This analysis has to be matched against the twelve compositions given in QW 442, and the matching composition gives us the A-Number.
The chemical analysis of the weld deposit prepared according to the filler metal specification, such as a weld pad, can also be used. In fact, for SMAW process – the composition enshrined in the filler metal specification itself, such as an SFA specification, can be used (pretty easy, right?).
The chemical composition reported in the supplier’s certificate of compliance, such as MTC etc., can also be used to determine A-Number. However, the following needs to be kept in mind:
- For SAW process: The flux used for preparing the weld deposit should be the same as the one used in procedure qualification.
If the composition stated on certificate of compliance given by supplier is used for A-Number determination, the supplier should have used the same flux (as that used in procedure qualification) to arrive at the reported weld metal chemistry. This is generally no problem if both wire and flux have been sourced from the same supplier.
It is to be noted that the term “same flux” signifies same trade name. The Committee made this clear through an Interpretation, IX-20-17. This remains in force even if the wire-flux combination has been classified under an SFA specification.
- For GMAW process: The shielding gas used in preparing the weld deposit should be the same as the one used in procedure qualification.
If the certificate of compliance given by supplier is used for A-Number determination, the supplier should have used the same shielding gas (as that used in procedure qualification) to arrive at the reported weld metal chemistry.
The methods given by QW 404.5 to determine A-Number are liberal enough, and do not require any overt effort. Simply consulting the supplier’s certificate of compliance and matching with QW 442 does the job.
Best Method To Determine A-Number
However, I think that the analysis of the actual weld deposit taken from the procedure qualification test coupon is the most authentic method. Code thinks along these lines too. In replying to a question in an Interpretation (IX-81-44), the Committee made its preference clear.
What If The Chemical Composition Does Not Match With Any Of The A-Numbers in QW 442?
Sometimes the chemical composition of a weld deposit does not conform to any of the listed A-Numbers in QW 442. The last paragraph of QW 404.5 comes in handy for such situations. This paragraph provides that the nominal chemical composition of the weld deposit can also be listed on the WPS/PQR in lieu of A-Number.
Is It Essential To State A-Number On WPS?
Yes. A-Number is an essential variable. In accordance with the rules of Section IX, all essential variables must be addressed on a WPS.
Sometimes an organization may not want to write the chemical composition on the WPS to protect the confidentiality of the composition. Assume that the weld metal chemistry is not matching with any of the A-Numbers either.
Then, in lieu of writing the actual chemical composition, is it alright to indicate the chemistry by referring the trade name or material specification of the filler metal (so that the chemical composition of the weld metal is firmly defined)? Someone asked such a question to ASME in 1981 through an Inquiry.
The Committee responded through the Interpretation IX-ER10-IX-05, replying in the affirmative.
The last paragraph of the QW 404.5 in the present day Edition of Section IX provides that mentioning the AWS classification (except for the “G” suffix classifications), or the manufacturer’s trade name, or any other established procurement document, (so that the chemistry of the weld deposited in procedure qualification can be zeroed down to the maximum extent possible) is also permissible in lieu of A-Number.
Quite possibly, this paragraph got introduced in Section IX in response to the above Interpretation.
Note the words “established procurement documents” in the above paragraph. This means reference to some procurement document, such as the TDC issued with the Purchase Order, can also be made in the WPS, so that the chemical composition of the weld deposited by a filler metal is firmly established. [TDC: Technical Delivery Conditions]
Now, say, a WPS has been qualified by referencing a procurement document in the WPS in lieu of A-Number, to satisfy QW 404.5. Does this WPS need to be re-qualified every time a fresh purchase is made to the referred procurement document?
No, requalification would not be necessary for this. As long as the technical requirements remain same in the procurement document (that is, chemical composition desired from the filler metal is unchanged), requalification of the procedure is not required. Responding to a similar question, the Committee replied likewise, in Interpretation IX-89-34.
It is to be noted that this discussion is applicable for filler metals not covered by any of the SFA specifications as well.
So this was all about A-Number on a WPS. Would you like to add something? Do leave your thoughts in the comments section below.
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