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Re: [MW:35723] PWHT after Hydro

By Materials & Welding
 For main pressure welds on materials like P11, P91, and LTCS, PWHT is essential when prescribed, and the integrity test (hydro) 
must follow all heat treatments unless the code or a client-specific deviation allows. The code does not permit skipping a repeat hydro. 
The logic is that since PWHT can alter the microstructure and relieve stresses, it might reveal or create discontinuities that could 
compromise pressure integrity—meaning a follow-up hydro is generally required.

For P11 (1.25Cr-0.5Mo)
PWHT is mandatory for most thicknesses per ASME.
PWHT after the hydro test is typically acceptable, if permitted by project specs.
For P91 (9Cr-1Mo-V):
Highly sensitive to heat treatment cycles.
Incorrect PWHT can alter microstructure and reduce creep strength.
Often, clients require a hydro test after PWHT, especially in critical service.
LTCS:
Generally, more lenient.
PWHT after hydro test may be acceptable depending on the joint type ,thickness, and low temp. impact.

According to ASME B31.3, Clause 345.4.2, PWHT after hydro testing is allowed, and a repeat hydro test is NOT required if:

The system has not been opened or altered after the hydro test.

PWHT does not weaken or alter the pressure-retaining capacity of the system.

The PWHT is part of the planned fabrication sequence, and the test pressure already applied exceeds the pressure that the 

system would experience at the elevated PWHT temperature.

However, some client specifications override this and may demand a re-hydrotest after PWHT, especially for P91, which is 

sensitive to microstructural changes. PWHT is not required at all for LTCS unless impact testing is required at a low temperature.

C Sridhar.

  


On Thu, 17 Jul 2025 at 16:10, Kannayeram Gnanapandithan <kgpandithan@gmail.com> wrote:
Dears
     Materials P11,P91 & LTCS.
  Is there any provision to perform PWHT after Hydrotest without having to repeat the Hydrotest again ?

THANKS & BEST REGARDS,
KG.PANDITHAN, BE, IWE,  CSWIP 3.1,
ISO 9712 Level 2 in VT,
ASNT-Level II in PT,MT,RT & UT,
LA ISO 9001-2015, ISO 3834, EN 15085, 
International Welding Engineer. 
CONSULTANT-WELDING & QUALITY
Mobile no: +919940739349

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