Welding Super 304H with P91 1

[tamal1475]

We are going to weld Super 304H with P91 for boiler application during site erection. I propose to butter the P91 edge with E NiCrFe3 and complete PWHT at 760 deg C in shop. Site weld with Super 304H at site will be done with E NiCrFe3. I understand that buttering the P91 edge with E9018 B3 before E NiCrFe3 layer is recommended for better life. Does anybody have any experience to share?

 

[stanweld]

We have used your initial approach on three boilers. As of now, we have not had any reports of failures after over over 6 years of operation. I would not recommend the E9018 B3 approach. This becomes the weak link in the chain at creep temperatures.

 

[pandithan]

For pipe we have used as u said( Buttering with NiCrFe3, heat treated and welded with Super 304H
for tube we have directly welded with Super 304H and heat treated. we have not faced any problems sofar
Pandithan, AWS-CWI, CSWIP 3.1
Welding Consultant

 

[tamal1475]

Since Super 304H is likely to be sensitized during PWHT at 760 deg C, I think it is not a good idea. A sensitized tube material will remain vulnerable to Stress Corrosion Cracking or other corrosion damage during acid cleaning or due to chloride contamination, all through the life of a high pressure boiler.

 

[metengr]

Since Super 304H is likely to be sensitized during PWHT at 760 deg C, I think it is not a good idea.

This is for a boiler application carrying elevated temperature steam, and is typical. 304H is alloyed with higher carbon for creep strength and sensitization of field welds is unavoidable. Even if there is sensitization, so what. There are trade-offs in design.

 

[stanweld]

Super 304H is somewhat a misnomer. It's governed by Code Case 2328 and is stabilized with Niobium/Columbium with added copper. Super 347H would seem more apt.

 

[metengr]

Also, the 304H will not be subjected to PWHT exposure. The Grade 91 material will have been buttered and subsequently PWHT'd before being joined to the Super 304H tube material.

 

[tamal1475]

Sensitized Super 304H, as such, may not be a problem in a steam boiler under normal conditions. But a boiler goes through chemical and acid cleanings during its long life time. There could be accidental chloride contamination of the feed water also. In such conditions, a sensitized Super 304H would be vulnerable to SCC and other corrosion damage. Hence, it is advisable to avoid PWHT of Super 304H.

 

[metengr]

If you have accidental exposure to chlorides and boiler clean chemicals you have serious water chemistry issues that will threaten any austenitic stainless steel, regardless if it is sensitized.

The Super 304H as stanweld pointed out is a hybrid stabilized austenitic stainless steel, unlike traditional 304H. Second and last point, there will be welding required during fabrication of the assembly that will not have a solution treatment performed, meaning exposure to elevated temperature is unavoidable. Also, elevated temperature exposure will result in eventual sensitization of the bulk tube material over time.