I fail to see how two circumferential groove welds placed adjacent to each other is terribly different than a multi-bead weld where there are multiple weld beds, one on top of the other.
In the case of a heat treatable aluminum alloy, time at temperature is concern with regards to the degradation of the mechanical properties. To minimize a reduction of the mechanical properties, close control of the interpass temperature is a must. Even then, the various welding standards recognize the problem and reduce the mechanical properties. The problem is that some designers do not take that into consideration and use the as wrought properties in their calculations. Oops!
In the case of carbon steels, the HAZ can be softer than the weld if proper preheat/interpass temperature is maintained during the welding cycle. If the maximum interpass temperature is controlled,the probability of low CVN values is mitigate, but not eliminated, since there is more to controlling CVN than limiting interpass temperature. The residual stress adjacent to a multipass groove weld isn't going to be worse or better than two groove welds placed adjacent to each other. The residual stress is on the same order of magnitude as the yield strength of the base metal for any of carbon steel typically used in a piping system that experienced temperature gradients greater than about 220 degrees F. I would be more concerned where the welds are perpendicular to each other such as where longitudinal welds and circumferential welds intersect. The residual stresses can be handled as vectors. The problem is more severe as the yield strengths increase.
If high hardness is a concern or if fatigue is a concern, use PWHT and eliminate geometric stress risers.
When designers are asked why certain restrictions are included in their specifications, all too often the response is its company policy or they saw it in another specification. Hardly a rational justification to include restrictions that are based on a old wives tail. A better approach would be to consult with a welding engineer that is familiar with welding and the particular governing code.
Best regards - Al
