Another P91 bake out question

[ageometry]

Hi Everyone,

I have read a lot about P91 on this website and have benefited tremendously. I do have just one more area I like to get some clarification:

For field welding (following the recommendations from ASME B31.1), when I insert the bake out process in between welding and PWHT, do I cool down to 200F before or after the bake out? There seems to be 2 school of thoughts on this: those who thinks we need to get rid of the hydrogen first, or those who thinks we need to let the martensite form completely first. I have not been able to locate an answer for this question.

Since this is field welding, I could not eliminate bake out. We will try our best to do PWHT asap, but again, due to field condition, there is no guarantee on it as well. So i think the bake out process is very important.

If anyone can help clarify the cool down process to be before or after bake out or both, it will be much appreciated. Thanks.

A

 

[metengr]

Typically, cool down is done to 200 deg F to allow for complete transformation to martensite for Grade 91, creep strength enhanced ferritic steel. Once the 200 deg F is reached, the post weld bake would be used if you intend to delay PWHT. Otherwise, if low hydrogen consumables are used, the post weld bake step could be eliminated if PWHT is immediately performed after 200 deg F is reached.