How many times PWHT can be done

[demsha]

Hello All;

A pressure vessel was constructed with SA 516 Gr.70 has undergone PWHT initially in 1975.
How many times the vessel could undergo repair & PWHT in its lifetime? Are there any maximum times can anyone do PWHT?
Will metallurgical changes occur during subsequent PWHT?
Where I can find supportive answers in terms of metallurgy and codes?

Appreciate all your comments

 

[Dhurjati Sen]

Check API 510 if you are re-rating the vessel.

DHURJATI SEN

 

If it exists and can be located, the history docket from original construction will be helpful.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[weldstan]

With the exception of pressure vessels subject to impact testing per ASME VIII, Div.1, ASME IX and the NBIC, there are no limits imposed. I have practical experience of 4 PWHT cycles with SA-516 up to 12 hrs total time at temperature below 1200F without adversely affecting mechanical properties.

At 1300 F I have seen reduction of mechanical properties below the specified minimums, especially the deposited weld metal, with as little as 2 hrs hold time.

 

[Dalinus]

You can look at ISO/TR 14745 Welding - Post-weld heat treatment parameters for steels.
There is some info about holding time and temperature, multiple heating cycles, Hollomon-Jaffe parameter and so on.