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Up Topic Welding Industry / Technical Discussions / Which type heating method used for PWHT P91 joints in U.S.A?
- - By dragon (**) Date 05-08-2009 05:56 Edited 05-08-2009 06:02
electric resistance heating or induction heating? the ID of the pipe is 431.8mm, and pipe thickness is 88mm.
Parent - - By js55 (*****) Date 05-08-2009 13:07 Edited 05-08-2009 13:10
Either one is fine but you face different problems with each. ER heats from the surface so you have to allow enough soak time to penetrate through thickness. Induction heats volumetrically but it is generally performed by the fabricator himself (as opposed to ER being subcontracted from time to time) and training is a real issue. It takes some knowledge to understand how to place the coils to get the control needed.
A bigger concern with Grade 91 is making sure you heat treat it properly. 1400F is the industry consensus ideal temp. Make sure you allow enough time to homgenize the soak through thickness, and ramp it up and down very slowly to avoid any through thickness stress variances.
Do not do a lot of handling until stress relief is performed. In situ is best, right after the weldment is completed. And avoid storing it in areas where it may be exposed to moisture.
Parent - - By 3.2 Inspector (***) Date 05-08-2009 15:13 Edited 05-08-2009 15:44
In situ is best, right after the weldment is completed.

No middle cooling?
What does "In situ" means?

3.2

EDIT: Got the "in situ" part now :)
Parent - - By jrw159 (*****) Date 05-08-2009 15:44
In situ=In place. I believe this is what is being referred to.

http://en.wikipedia.org/wiki/In_situ

jrw159 :-)
Parent - By js55 (*****) Date 05-08-2009 16:02
The weld metal especially, is really brittle as welded. An advantage to local stress relief is the ability to cook it where it sits. The disadvantage of course is the weld station is occupied.
Parent - - By dragon (**) Date 05-12-2009 02:33 Edited 05-12-2009 03:10
Thank you js55, we use ER heating commonly. The soaking time is 4 hours at least. We do PWHT just after finishing welding. We don,t permit moving the pipe until stress relief is performed.
I have ever read an article <guideline for welding creep strength-enhanced ferritic alloys> of EPRI, in this article "induction heating technology does experience limitations due to the curie point on heavy wall(2-inch wall )P91 welding joints."
Parent - - By 3.2 Inspector (***) Date 05-12-2009 05:19
You need to let the weld cool below Mf temp before PWHT.

3.2
Parent - - By dragon (**) Date 05-14-2009 03:52 Edited 05-14-2009 03:55
yes ,we cool the joint to 100-120 degree celsius for 2 hours, then heat the joint to soaking temperature.
Parent - By 3.2 Inspector (***) Date 05-14-2009 07:31
To the best of my understanding the Mf temp is 80 degree celsius for P91

3.2
Parent - By js55 (*****) Date 05-15-2009 15:26
The CCT diagrams currently available on P91 that I have seen have Mf around 200C. But it is very cooling rate sensitive, (dropping off to lower temps at faster cooling rates), and CCT diagrams are seldom based upon welding regimes, which means that consideration of grain size variance is generally not considered. To be safe IMO its best to let it cool to RT.
Parent - By js55 (*****) Date 05-15-2009 15:28
The curie point seems to be accomodated OK. You just need more power. In all the induction locals I've been a party to I've never seen a lack of power as an unsolvable problem for PWHT.
Up Topic Welding Industry / Technical Discussions / Which type heating method used for PWHT P91 joints in U.S.A?

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