Welding P91 to SA-106 Gr. C

[Mr168]

Aside from the issues with carbon migration and PWHT, what are some of the other negative considerations when joining P91 and carbon steel (SA-106c) materials?

While we are forced into a current situation in which we are required to make a series of these welds, I am looking for further justification as to why we would prohibit joining these two materials without a transition piece on future projects.

 

[stanweld]

The high PWHT temp required for P91 will adversely affect mechanical properties of the A-106 C. If you use E7018 to make the joint, the deposited weld metal tensile properties will be lowered below that of the specified minimums.

 

[metengr]

The main reason to prohibit joining carbon steel to P91 is simply the PWHT issue. You cannot effectively PWHT Grade 91 joined directly to CS using a PWHT temperature that would be required for Grade 91 because you would exceed the lower critical transformation temperature for carbon steel.

Second reason, there is such a strength mismatch between these two grades of materials at ambient and elevated temperature this would be a poorly design weld joint.

 

[Mr168]

Unfortunately, we're looking at a number of these welds where some short CS spool pieces ties into some P91 valves. CS in on one side, P91 on the other.

The manufacturer's justification behind it was that the CS was more economical (thinner wall thickness than P22 because of the allowables), is acceptable per code, has been done in the past, and is "just fine."

 

[stanweld]

Code does not prohibit the dissimilar joint. Your contract with the supplier obviously did not as well. Review the manufacturer's WPS and suporting PQR with the actual PWHT charts and determine Code compliance (see metengr's reply); if not in compliance, the production welds are to be rejected and you may then require appropriate corrective action.

You may also remediate but at additional cost.

 

[Mr168]

Stanweld: They did in fact leave that out of the contract. These welds had not yet been made, as the dissimilar joints were brought to my attention prior to final release of the ISO's by the vendor.

In discussing matters with the client, they agreed with my stance on the matter and will be sending the valves back for weld buildups/remachining and having the supplier provide suitably sized P22 replacement spools.