I think we've got a really serious problem regarding PWHT of dissimilar
materials.
In some of our in-house applications, we have to weld hydro test caps on
open ends' of pipe-spools in order to hydro test them.
After welding of caps are completed, we heat treat the spools together with
(most of the time caps made of dissimilar material, mostly CS Caps because
of cheapness) Hydro-Test caps. Even though, thickness of these hydro test
caps are calculated as per related code, one of our concern starts because
of the effects of excessive heat treatment on hydro test caps. Because; as
per codes, allowable stress values should be chosen in accordance with the
hydro test temperature which should be lower because of excessive PWHT, but
how much ? Are there any codes explaining this situation ? Does any member
has experienced same kind of problem ?
Other concerning point is the service life of that work piece. Even though
the caps work properly after PWHT, then the question should be how many
times we can use that caps in our other hydro tests, of course safely.
The main problem indeed is, are there any approach which can assume/predict
the material properties (Especially reduction in durability.) after PWHT
depending of PWHT parameters ?
It would be highly appreciated if some of you can address a code with a
working example of solution.
In order to visualize the problem pls see below example of the concerning
situation;
- Material of spool is P91 and the cap's is CS.
- PWHT temp : 730 C
- PWHT duration : 1 hr.
How can we calculate the reduction in durability ?
Thanks for your time,
Regards,
Safa
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