P91 to P22

Date 09-15-2007 03:47 Edited 09-15-2007 03:55

We use the P91 matching filler, which is ER90S-B9 or E9018-B9 for welding P91 to P22. 400-500 F minimum preheat and 1380-1420 F postweld heat treatment is mandatory, regardless of thickness or weld type. VM Tubes has a good P91/T91 welding guide that can be ordered from their website. Bohler-Thyssen has some good info on their website also (http://www.btwusa.com/html/tips_page_1.html#HRP91). Here's another good paper: http://www.sperkoengineering.com/html/Grade%2091%20R%2010-05.pdf

E9018M definitely is not an option.

By MBSims

Date 09-15-2007 04:25 Edited 09-15-2007 04:48

I should also add that after welding, the joint must be slow cooled to below 200 F to allow complete transformation of the martensite. And the minimum soak time for postweld heat treatment is 1 hour per inch, but no less than 2 hours. Postweld heat treatment should also not be performed more than a few days (< 1 week) after welding to avoid cracking. There are a lot of problems being dealt with regarding improperly welded P91 materials in power plant piping and headers. You would be wise not to weld it at all unless you have fully educated yourself on proper welding procedures and consumables, and then qualified the welding procedures. Otherwise some unhappy plant owner will be contacting you later about inservice cracking.

http://www.psimedia.info/Industry%20Alert.pdf

By Jim Hughes (***)

Date 09-16-2007 03:33

Marty,

have you heard of the HIDA reveiw? Some of the stuff I'm reading coming out of EU is starting to say that maybe 90sB3 is better suited for at least 9Cr to 2 1/4 Cr.

Thanks
Jim

By MBSims

Date 09-16-2007 20:59

Not familiar with the HIDA review. Is this from the ETD?

What I do know is that carbon migration from the lower Cr material to the higher Cr material will occur at the operating temperatures that P22 and P91 are typically used. This would put the decarburized zone in the weld metal instead of the P22 HAZ.

By chall

Date 09-17-2007 13:19

Hi Marty,

Which code of construction are you referring to with this response? My read of B31.1 limits the upper temperature to 1400F; while Section I only specifies a minimum temperature.

My curiosity stems from Spreko's commentary (whcih appeared in the April, '07 Welding Journal) about the risk of over heat treating "lesser chromium" materials; particularly when welded to P5B materials. In short, he offers that there is a risk of "softening" the lower chromium containing material if it is either heat treated at too high a temperature, or at temperature for too long a duration (due to carbon migration from the lower chrome material to the higher chrom material). The temperature he was referring to is not a transformation temperature; it is the upper limit for the P5B material. He discusses two situations (only one of which applies to this topic), which you can read about here:

http://files.aws.org/wj/2007/04/wj200704/wj0407-68.pdf

Thanks.
Charles Hall

By js55

Date 09-17-2007 15:11

If memory serves, the upper temps in B31.1 are not mandatory limitations. Though concerns for transformation is real and should be considered. Knowledge of this alloy is critical to successful welding.
As for fillers, the current popular recommendation is B9 (though I have been out of the Grade 91 loop awhile), which is perfectly acceptable, but consider that when using a B9 you will, with PWHT, have carbon migration from the P22 HAZ into the B9 (carbon migrates to the higher alloy side of the fusion zone) weld metal weakening the P22 HAZ. This loss of carbon being primarily responsible for the infamous Type IV cracking. Whereas when using a B3 filler the carbon migration will be from the B3 filler to the P91 HAZ. And even though the B3 weld metal will be weaker than a B3 used for a similar P22 to P22 weld (where carbon migration doesn't exist), the B3 weld metal will still be stronger than the P22 base metal.
Therefore, the strength gradient using a B3 filler is more linear across the entire weldment with B3.
And the other issue is, where would you rather have the high carbon zone? In the BM HAZ or the weld metal? With B9 the high carbon zone is in the B9 WM. With B3 it is in the P91 BM HAZ.
Add to this the fact that B3 is not as crack sensitive or heat regime sensitive as B9, and is cheaper, my opinion is that B3 is acceptable for this dissimilar application as well.

By chall

Date 09-17-2007 15:44

I just reread the PWHT section, and true to form, it is confusing if you don't read all of it.

Para 132.1 says: "Except as otherwise permitted in 132.2 & 132.3...shall be given a PWHT within the temperature range specified in Table 132."

132.2(A) says: "The upper limit of the PWHT temperature range in Table 132 is the recommended value, which may be exceeded provided that the actual temperature does not exceed the lower transformation temperature of either material."

Case closed.

BTW - thanks for the insight on the various alloy options.

Charles

By Jim Hughes (***)

Date 09-29-2007 15:46

Sorry, I was off the radar for a while. Thanks for the replys

Jim