Channel overlayed Tp 317L PWHT

[Peter113]

Hi,
we are fabricating a heat exchanger in CS + SSTp 317L weld overlay due to nafthenic acid corrosion. Our design has more than six passes and according to API 660, carbon steel channels request PWHT in order to reduce residual stresses by welding. But also in API 660 para.6.6.5 says textually "Weld-overlaid carbon steel channels and bonnets shall be postweld heat treated when specified by the purchaser". We are discussing if it is requested the PWHT on the following basis:
1) As the partition plate are SS Tp 317L, these weldments are Inox to Inox and no effect, i.e, residual stresses are not expected to be induced in the base metal.
2) According to UCL of ASME VIII, with a thickness of 29+3 mm, the PWHT is not required for CS.

Our opinion is that PWHT could be avoided in this situation. What´s your opinion?

 

[metengr]

I agree.

 

[stanweld]

While I agree with your opinion, another option is to PWHT the carbon steel pressure welds in the channel prior to making the 317L weld overlay. I would not recommend PWHT the channel after the making overlay.