Substitution of steels with same P-number in welding 3

[RNDguy]

I need to know of resources giving tips and sharing experience on the question of when is it safe to substitute a parent metal with same P-number but a different group number without bringing into question the validity of PQR & WPS. For instance, SA-414 grade C is qualified to be welded to SA 240 type 316L under a certain PQR with GTAW. When can one substitute SA-414 grade G instead of grade C in the same circumstances (grade G is group 2, grade C is group 1, both under P-number 1)? What may be the drawbacks of doing this if any? Thanks.

 

[metengr]

Here is some Background;
The qualification of a welding procedure specification (WPS) under the ASME Boiler and Pressure Vessel Code is based on the requirements in QW-256 of Section IX of the Code. To summarize, QW-256 is a table that lists welding variables for the GTAW process. The table contains three groups of welding variables that can be used to qualify a WPS with a weld coupon. The welding variables are as follows; Essential, Supplementary Essential and Nonessential. As you might guess, the Essential list of welding variables has a direct impact on qualification of the WPS, and are commonly used in WPS qualification where impact testing or notch toughness fracture behavior of base materials is not a concern after welding. Any change in the list of Essential welding variables would require re-qualification of a WPS. The Procedure Qualification Record (PQR) is a record of the welding variables (Essential or Supplementary Essential or Nonessential) from QW-256 to weld a coupon.

In paragraph QW-403 of QW-256, which is applicable to base materials, you have several welding variables that refer you to changes that would require re-qualification of an existing WPS. Specifically, QW-403.5 refers to a change in group number for base material with the same P-No. For impact testing of base material, the Supplementary Essential welding variables in QW-256 must also be included and treated as Essential Welding variables for qualifying a WPS. Thus, an existing WPS that was not qualified for impact testing would need to be re-qualified to allow for a change in base materials with a different Group No. For applications where impact testing is not performed or required, a change in base metal Group No is not considered an essential welding variable. This means that a WPS qualified to only the Essential welding variables listed in QW-256 will permit substitution of different Group No base metals under the same P-No.

The reason for having the Supplementary Essential welding variables applicable to impact testing is because the Code committee recognizes that slight changes in base metal composition (that would require classification under a different group number) will affect notch toughness behavior after welding.

So to answer your question, you can indeed substitute base metals with different group numbers classified under the same P-No for applications that do not require impact testing of base metals.

 

[RNDguy]

metengr:
Thank you very much. I could not have imagined to receive a more lucid answer. Regards.

 

[metengr]

As a follow-up to your post, you referred to "when is it safe" and "without bringing into question the validity of a PQR and WPS”. This tells me you are looking to use the ASME Section IX qualified WPS/PQR for applications outside of Code Space - Correct? The welding guidelines in Section IX of the ASME Boiler and Pressure Vessel Code can be used for any application outside of the Code. In other words you can use Section IX welding guidelines to develop welding procedures for general use.

"When is it safe" could mean different things, and is highly dependent on application. If your application requires specific notch toughness requirements to assure reliable or safe operation in service, I would follow the guidelines in Section IX, specifically QW-256, and qualify the WPS for impact testing, period. Of course, if you are not bound by any Code requirements in your application you are free to follow the minimum requirements for procedure qualification.

 

[RNDguy]

metengr:
You are right, I am considering the qualification to be used beyond code application, however, no notch toughness requirement is there. I guess using the same qualification with a different group number is OK in this case. Thanks again for all the help.