Hi friends.i m looking for answer. I has seen the tube to tubesheet has been welded without second cycle of PWHT. Tubesheet already been done with PWHT. But later on, tubes inserted through tube hole,welder did their thing and after tube to tubesheet welding is completed, the HEX was examined NDE, Final dimension and hydrotest. Is it any specific clause in code mentioned no need to do PWHT or preheat at 90celcius for such design.
PWHT requirements are imposed by either service restrictions or thickness of weld metal deposited.(ASME SECTION 8 DIV1 UW 40(f) &UCS 56 for carbon steels)
In your case if the deposited weld metal thickness is within the limit then you dont need to conduct any pwht(If there is no service requirements for pwht).
This is the general case and i am not familiar with heat exchangers.
Please specify the code that you follow and the material that you used.