Essential Variables for WPS Qualification as per ASME Section IX

Article II of ASME Section IX deals with welding procedure qualifications. Under QW-250 of Article II are contained a number of tables, titled QW-252 to QW-267. Each table is dedicated to a process such as SMAW, SAW, GTAW, and so on. These tables contain a listing of essential variables, supplementary essential variables and non-essential variables for that process.

This article discusses essential variables for SMAW process. We shall see, in general, how each variable is applied; and some interesting insights pertaining to these variables.

Base Metal Thickness (T)

Addressed at QW-403.8, which further refers to QW-451, this is one of the most important essential variables, and is applicable for most of the welding processes. The QW-451 is basically a table, which establishes the qualified base metal thicknesses for various test coupon thicknesses. This table looks something like this.

Thickness of the test coupon (T) Inch (mm)Range Qualified, of Base Metal Thickness
MinimumMaximum
Less than 1/16 (1.5)T2T
1/16 to 3/8 (1.5 to 10)1/16 (1.5)2T
Over 3/8 (10), but less than ¾ (19)3/16 (5)2T
¾ (19) to less than 1½ (38)3/16 (5)2T
1½ (38) to 6 (150)3/16 (5)8 (200) [see note 1 below]
Over 6 (150)3/16 (5)1.33T

Note (1): This maximum qualified BM thickness up to 8 inch (200 mm) is applicable only for GTAW, GMAW, SMAW, SAW, and PAW processes. For all other processes, maximum qualified BM thickness is 2T only.

Exemptions

Following exemptions to the above table are permitted by Section IX:

  1. For partial penetration groove welds: there is no upper limit on the qualified base metal thickness if the procedure qualification was done on a test coupon of thickness 1½ inch (38 mm) or more.
  2. For repairs and build ups: there is no upper limit on the qualified base metal thickness if the procedure qualification was done on a test coupon of thickness 1½ inch (38 mm) or more.
  3. For production welds with dissimilar base metal thickness: thinner member shall meet QW-451. However, there is no upper limit on the qualified base metal thickness if the procedure qualification was done on a test coupon of thickness:
  • ¼ inch (6 mm) or more for P-No. 8, 41-46, 49, 51-53, 61-62.
  • 1½ inch (38 mm) or more for all other P-Nos.

4. For fillet welds: a groove weld qualification qualifies ALL base metal thicknesses. All the a above exemptions are provided under QW-202.

Further Restrictions

The limits of qualified base metal thickness established by the table QW-451.1 are further restricted under following circumstances:

  1. Supplementary essential variables (SEVs): When a SEV specifies a qualified BM thickness that is more restrictive than QW-451, the limit established by SEV takes precedence over the limit permitted by QW-451. For example, when impact toughness is specified, the qualified minimum BM thickness is 5/8 inch (16 mm) or the actual test coupon thickness, whichever is less. When test coupon thickness is ¼ inch (6 mm) or less, the qualified minimum BM thickness is ½ T. This is a SEV, specified at QW-403.6.
  2. When a pass is greater than ½ inch (13 mm): When any pass in a test coupon is more than ½ inch (13 mm) thick, the maximum qualified BM thickness is 1.1 times the test coupon thickness. This is an essential variable, specified at QW-403.9.

Under both the circumstances described above, the qualified base metal thickness established by QW-451 gets further restricted due to the above stated variables.

Does Not Pertain to Weld Metal Thickness

This variable is applicable only to base metal thickness, and has nothing to do with weld metal thickness (t), which is covered elsewhere at QW-404.30, QW-404.32, etc. This matter is the subject Interpretation IX-83-127 (qs 1) too.

When A Pass Is More Than ½ inch (13 mm) Thick

Though already mentioned a few paragraphs above, this variable is again stated here as it is an essential variable.

Stated at QW-403.9, this variables states that – for single or multi-pass welding, when the observed thickness of any pass in the preparation of qualification test coupon is greater than ½ inch (13 mm) thick, the maximum qualified thickness is 1.1 times the coupon thickness.

Interpretation IX-04-09 & IX-83-152 present examples making use of this variable.

It should be noted that the thickness indicated here refers to thickness/ depth of the bead in a groove weld coupon, and has nothing to do with width of the bead. For a single pass fillet weld coupon, this thickness refers to the throat of the weld. [IX-78-48, IX-78-35 (qs 2), IX-82-17].

P-Number of The Base Metal

This is another important essential variable, stated at QW-403.11, which refers us to QW-424. The QW-424 shows a table which looks like below. A test coupon qualified with a BM shown on left hand side of a row qualifies for all base metals shown on the right hand side of that row.

Base Metal Used in Test CouponQualified Base Metals
One metal from a P‐Number to any metal from the same P‐NumberAny metals assigned that P‐Number
One metal from a P‐Number to any metal from any other P‐NumberAny metal assigned the first P‐Number to any metal assigned the second P‐Number
One metal from P‐No. 15E to any metal from P‐No. 15EAny P‐No. 15E or 5B metal to any metal assigned P‐No. 15E or 5B
One metal from P‐No. 15E to any metal from any other P‐NumberAny P‐No. 15E or 5B metal to any metal assigned the second P‐Number
One metal from P‐No. 3 to any metal from P‐No. 3Any P‐No. 3 metal to any metal assigned P‐No. 3 or 1
One metal from P‐No. 4 to any metal from P‐No. 4Any P‐No. 4 metal to any metal assigned P‐No. 4, 3, or 1
One metal from P‐No. 5A to any metal from P‐No. 5AAny P‐No. 5A metal to any metal assigned P‐No. 5A, 4, 3, or 1
One metal from P‐No. 5A to a metal from P‐No. 4, or P‐No. 3, or P‐No. 1Any P‐No. 5A metal to any metal assigned to P‐No. 4, 3, or 1
One metal from P‐No. 4 to a metal from P‐No. 3 or P‐No. 1Any P‐No. 4 metal to any metal assigned to P‐No. 3 or 1
Any unassigned metal to the same unassigned metalThe unassigned metal to itself
Any unassigned metal to any P‐Number metalThe unassigned metal to any metal assigned to the same P‐Number as the qualified metal

It should be noted that the choice of base metal in the test coupon as established by the above table gets further restricted by QW-403.5 – which pertains to Group Number, when impact toughness is a consideration. In other words, when toughness is a consideration, QW-403.5 applies in addition to QW-403.11, not instead of QW-403.11. [IX-79-92 (qs 1)]

I have dealt with the subject of P-numbers in a separate article, where I have discussed a number of interesting observations pertaining to this matter. The article may be found here.

Other essential Variables Need to be Satisfied Too

It should be kept in mind that while the above table allows a few liberties, these liberties are only applicable when all other essential variables are satisfied too.

For example, consider a procedure qualification test coupon qualified with P-No. 5A metal welded to P-No. 5A metal, with a PWHT of 1400°F. Now, as per above table, this PQR qualifies welding of P-No. 5A to P-No. 1 too. However, the temperature 1400°F lies between lower transformation temperature and upper transformation temperature for P-No. 1 metals. This, as per QW-407.1, requires a separate qualification.

Hence, the aforementioned PQR cannot be used to support P-No. 5A + P-No. 1 base metal combination.

Moral of the lesson is – when evaluating applicability of a PQ for a production weld, all essential variables should be satisfied. Judgment should not be based on a single essential variable alone.

This interesting example formed the subject of interpretation IX-83-11.

F-Number

F-number is assigned to filler metals. The F-numbers of various filler metals are tabulated under QW-432. It should be noted that only the filler metals that are classified in ASME Section II Part C get assigned a F-number.

Defined at QW-404.4, this variable says that a change from a F-number to any another F-number or any other filler metal not listed under QW-432 requires requalification of procedure.

The first part of the variable is simple enough, and is easy to understand. Simply speaking, one must use a filler of same F-number as was used in the procedure qualification test coupon. Speaking another way, any filler can be used in production (even of a different classification) as long as it has the same F-number as was used in test coupon welding.

However, what happens when the filler used in test coupon does not have a F-number? What does the phrase ‘any other filler metal’ in QW-404.4 mean?

The term ‘any other filler metal’ refers to filler that does not have a F-number. When such a filler is used in test coupon welding, it is necessary to use filler of the same grade in the production welds (although, filler of same trade name is not necessary).

Another thing worth noting here is that although it is permitted to use a filler of different classification (as long as F-number is same), it does not mean that one indiscriminately substitute filler metals without regard to other aspects of welding.

A-Number

A-number is the assigned to weld metal composition. There are several ways to determine the A-number, defined at QW-404.5. The most common method is to check the chemical composition of the weld deposit in the test coupon, and compare it against the table under QW-442.

This variable is defined at QW-404.5, and says that a change of one A-number to another requires requalification. But, A-No. 1 qualifies A-No. 2 and vice-versa.

Another thing worth noting is that A-number is assigned only to ferrous base metals. So, when the PQR has been done with a non-ferrous metal, this variable is to be disregarded.

In essence, A-number is Section IX’s way of preventing indiscriminate substitution of filler metals on the WPS. By limiting the choice of filler to only those that can produce the same A-No. narrows down the choice of fillers for the welding engineer/fabricator. In a way, this also makes the process of choice of filler easier.

I have written a whole article on this website, detailing the finer nuances of A-number in a separate article. The article can be found here.

Deposited Weld Metal Thickness

This is another all-important essential variable. Defined at QW-404.30, which further points us to QW-451, this variable says that a change in deposited weld metal thickness beyond that qualified in accordance with QW-451 will attract requalification.

Under QW-451, qualified range for deposited weld metal thickness has been given for various test coupon thicknesses. The table is reproduced below (only with respect to weld metal thickness).

Thickness of the test coupon (T)
Inch (mm)
Maximum qualified thickness (t) of deposited weld metal
Less than 1/16 (1.5)2t
1/16 to 3/8 (1.5 to 10)2t
Over 3/8 (10), but less than ¾ (19)2t
¾ (19) to less than 1½ (38)2t if t < ¾ (19)
¾ (19) to less than 1½ (38)2T if t > ¾ (19)
1½ (38) to 6 (150)2t if t < ¾ (19)
1½ (38) to 6 (150)8 (200) if t > ¾ (19) [see Note (1) below]
Over 6 (150)2t if t < ¾ (19)
Over 6 (150)1.66T if t > ¾ (19)

Note (1): This maximum qualified weld metal thickness up to 8 inch (200 mm) is applicable only for GTAW, GMAW, SMAW, SAW, and PAW processes. For all other processes, maximum qualified weld metal thickness is 2t only.

A notable thing here is that there is no limit for deposited thickness on the lower side. That is, there is no minimum qualified weld metal thickness, only a maximum.

Preheat Temperature

This is an interesting variable that is commonly misunderstood by code users the world over. I have written a separate article detailing the intricacies of this variable. The article may be accessed here.

Post Weld Heat Treatment

This is another important essential variable, that is found at QW-407.1. I have written a separate article detailing interesting aspects of this variable. The article may be accessed here.

Use of Thermal Process for P-No. 11A & 11B

Addressed at QW-410.64, this variable is applicable only when P-No. 11A or P-No. 11B base metals are used, and the base metal thickness is less than 5/8 inch (16 mm), and the production joint is part of a vessel (i.e. not pumps/valves/fittings).

This variable says that when the production weld is going to be prepared by a thermal process, the weld groove in the test coupon shall also be prepared by the same thermal process(es). This also includes back gouging when it is employed during production.

This was all. Thanks for reading. Please share your thoughts in the comments section below.

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