I'm not involved in material selection for the design, I'm just helping them with some welding details. Everyone I know of that is involved in the design is aware of the effects of welding and the PWHT on the mechanicals. This material wouldn't have been my first choice (or the designers I think); however, there is an issue with material size &/or supplies. The design has accounted for the limitations of the 6061 (wrt to its welding and the effect of the PWHT). All welding procedures are supposed to be qualified and there is a coupon being done during production to verify the PQR results and validate the mechanicals of the components after the PWHT.
As for the UT acceptance criteria, we are trying to go back to the customer to obtain UT acceptance criteria. We have also in our internet search for acceptance criteria found the following Norwegian standard (M-102) that we feel will be useful (
http://www.side2.webkonsepter.no/dok/tekniskestandarder/M-102.pdf).
Sean