Final answer:
The leak test should be conducted before the weld is subjected to Post Weld Heat Treatment (PWHT) to identify and resolve any leaks or defects prior to the heat treatment process.
Step-by-step explanation:
The leak test should be conducted before the weld is subjected to Post Weld Heat Treatment (PWHT). This is because the purpose of PWHT is to relieve residual stresses and improve the weld's mechanical properties, but it could potentially cause the formation of leaks if there are any defects in the weld.
Conducting the leak test before PWHT allows any leaks or defects to be identified and resolved prior to subjecting the weld to the heat treatment process.
By conducting the leak test after the weld is PWHT, any leaks or defects that were present before PWHT may not be as easily detectable due to the changes in the material properties caused by the heat treatment.