Practical Implications with PWHT

[qaisarabbas]

I want to know about some technicalities related to PWHT. Please guide on followings:

a) What is the max. cycles of PWHT repetition on a specific weld joint (if there comes repeated repairs in a same weld)
b) What if we increase or decrease the recommended Soaking Time during PWHT
c) What if we increase or decrease the recommended Soaking Temperature during PWHT

Request for knowledge & experience sharing (w.r.t. code & industrial practices)

Regards -Qaisar

[ashfaqanwer]

This is a tricky question but considering the code requirements first, I would say that the essential variables must be considered for your specific welding process. If lets say, the welding process is SMAW, its has clearly been listed in QW-253 that time and temperature range is a supplementary essential variable.
And a supplementary essential variable is one that have an affect on the impact properties of a joint. They are classed as Non-Essential if impact testing is not required. So first of all see for yourself that the specific material you are dealing with requires impact testing or not.
Pipe welds to be operated at lower temperatures require notch toughness which would get affected if repeated cycles or change in time or temperature range occurs.

Now some to the industrial practices and effect of time / temperature change during PWHT. It is always recommended to follow the PWHT temperature, heating rate, holding time and the cooling rate. But if situation asks for it you may decrease or increase the holding temperature with correspondence increase or decrease in time respectively. You can follow the Larsen-Miller parameter in order to evaluate the same.

Also remember, continuous PWHT cylcles result in decrease of tensile strength and toughness of a certain material. Clients sometimes require a re-qualification of PQR if PWHT cycles are to be repeated repair after repair as it impairs the mechanical properties of steel.

Hope this would help.

[arcpro]

1. Maximum number of cycles shall be considered more closely if notch toughness of the material is a concern. With every repeating cycle you would be having a decrease in tensile strength along with notch toughness. For the cases of alloy steel, metallurgical properties may also get affected.
2 & 3. You have to see ASME Sec. IX for this considering your specific case / type of welding.