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PFHT vs PWHT 1

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CrisDom

CrisDom

Mechanical
Apr 25, 2019
24
Hi folk, I'm having a discussion regarding PFHT/PWHT.
If a vessel is to undergo PWHT, is there the need to PFHT the heads before PWHT?
I think there is no need, but someone is asking "where it is written".
I replied asking where is written that "there is the need", but I got no answer.
So, I'm wondering: is there any Interpetation that CLEARLY allows to perform a stress releiving of a head after forming during final PWHT of the complete vessel?
Is there the need for an interpretation or it's obvious that PFHT of a formed componente can be performed during the final PWHT of the entire vessel?
If I'm righ, how to "convince" the skeptics, or how to demonstrate it with Code?

Thanks

Cristiana
 
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I found the Interpretation relevant to ASME VIII Div.1 (VIII-1-95-50):

Question (1): A cold formed head has to be heat treated to fulfill the requirements of UCS-79(d) in Section VIII, Division 1. Does the term "heat treated subsequently" mean just after forming and prior to any other operation?

Reply (1): No.

Question (2): If Reply (1) is no, then will PWHT in accordance with UCS-56 satisfy the requirements of UCS-79(d) following welding of the head to the vessel?

Reply (2): Yes.



Based on that, do you think it is possible to extend it to Div.2?

Thanks

Regards
 
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The reason that people do PFHT is that they don't know what will be done with the head, they don't know if it will eventually be PWHT or not. They need to certify it to the spec so the HT.
If you are fabing the head and entire unit and know that it will receive a full body PWHT then there is no need to do it.
Unless you have experience that tells you that the fab goes smoother if you HT the head first.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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