Dissimilar weld between P9 (9Cr 1Mo) and SS 304

[starflex]

Fellow members,

I am in the process of writing a pWPS for a dissimilar joint (high temperature piping for a power plant).

Base Material 1: P9 (9Cr 1Mm - Ferritic Steel)
Base Material 2: SS 304 (Austenitic SS)
Consumable: specifically requested to be EPRI87 (48% Ni, 37 Iron if I remember correctly)

Joint has to be welded under Australian Standard (AS3992 for the qualification and 1210 as Pressure Equipment Standard, plus other to be applied for pre-heat, etc). AS far as I remember from my never-applied studies, I have to do the buttering first on the ferritic side, PWHT the ferritic and then I can perform the welding on the joint (without final PWHT because of the austenitic SS).

Pre-heat, interpass and PWHT temperatures for the buttering are not a problem. I can find them, not an issue.

My concern is, however, when I perform the joint. Should I pre-heat, perhaps from the ferritic side? Austenitic SS would recommend to avoid it, and I am not welding directly on the ferritic steel too (as I am welding on the buttering layers). Should I just avoid the pre-heat and keep a maximum interrun of 150 Celsius?

 

[metengr]

Your DMW should be performed as follows;

1. Deposit a butter layer over the 9Cr-1Mo side of the weld prep. Use a preheat of 400 deg F for this butter layer.
2. Subject the 9Cr-1Mo base material after buttering to PWHT.
3. Once this is completed, you join the buttered and PWHT 9Cr-1Mo end to the 304 ss using P87 filler metal with no preheat or PWHT.

 

[starflex]

Metengr,

That makes a lot of sense to me. Thank you for that.

Only concern (perhaps misinterpretation of your words, as I am not a native English speaker) is that one butter layer may not be enough; I was prone to think about two or three consecutive layers and then do the PWHT on the 9Cr-1Mo (TIG process, under the hypothesis of worst case scenario of 40% dilution).

 

[metengr]

The butter layer thickness is up to you because you are using the P87 filler metal developed by EPRI.

 

[HakiNoir]

Are you using 304L or dual-certified 304? If yes,the preheat shouldn't be that deleterious! We did various tests up to about 300 C inter-pass temp for 304L piping and there is no carbide formation due to low carbon content. However, I am not familiar with AS3992 and 1210 requirements.

By the way, I totally agree that the safest way is to use butter passes.

 

[starflex]

Gents,

Thanks a lot for your help.

The material is actually Super 304H - in my understanding, it contains a certain amount of copper (3% ?) .

Does it change ?
Sorry, I should have it specified before.

 

[metengr]

Yes, in that case you need to use a Thermanite 617 (ENiCrCoMo-1) filler metal in lieu of EPRI87. The rest of the method remains the same.