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Participant

Join Date: Feb 2012
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PWHT

02/10/2012 5:46 AM

Why PWHT Required for pressure vessels ?

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Guru

Join Date: Jan 2010
Location: Mineral wells Tx
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#1

Re: PWHT

02/10/2012 6:05 AM

We can consider three main reasons:
1. By type of material
2. By wall thickness
3. By type of service

WP

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Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

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#2

Re: PWHT

02/10/2012 6:33 AM

Equalisation of stresses and prevention of catastrophic self-disassembly under operating pressure is as good as any reason.

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Guru

Join Date: Mar 2007
Location: by the beach in Florida
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#3

Re: PWHT

02/10/2012 11:34 AM

Post weld heat treatment(sic) is generally used to relieve stresses built up in the structure during fabrication, and to reduce hydrogen cracking...This process is used to insure the integrity of the pressure vessel the same as the pressure test performed prior to certification...These practices have been initiated over time as safety measures and quality control practices, adopted after catastrophic failures and deaths in the past...

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Power-User

Join Date: Dec 2010
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#4

Re: PWHT

02/10/2012 11:51 AM

Theory you can study from the books. Summary is as below:

When vessels are fabricated by welding, internal structure which is affected due to welding sets in stresses along the grain boundaries. Also due to un even cooling different sizes of grains are in the weldment which again induces stresses in the heat affected area along the weld length. Further, some unwanted gases are absorbed in the solidifying weld metal, which intends to escape as the metal srinks causing another form of stress.

To remove all these stresses, PWHT is performed to gain the uniform grain structure in the welded metal and heat affected zone.

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Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

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#5
In reply to #4

Re: PWHT

02/10/2012 4:41 PM

Very good answer. (The voting system is saturated just now.)

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Guru
Engineering Fields - Mechanical Engineering - New Member India - Member - New Member

Join Date: Aug 2008
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#6
In reply to #4

Re: PWHT

02/14/2012 3:32 AM

GA.

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Anonymous Poster #1
#7
In reply to #4

Re: PWHT

11/18/2013 9:04 AM

if different sizes of grain formation due to uncontrolled heating and cooling cycle of welding are the main reason for PWHT, then why ASME B31.3 is not insist welding contractor to do PWHT up to 20mm thickness on P1 material (Ex. ASTM A 106 Gr.B) ?

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