Circumferential welds to hemispherical heads are considered a Category A joint due to the acting stress over that weld. In the case of semieliptical 2:1 heads, the longitudinal stress (half the governing stress) would be acting over a circumferential weld. This is not the case for hemispherical heads, which is the reason why it becomes a Category A weld detail when the head is hemispherical in lieu of Category B when it is semielliptical 2:1.
Cat "C" or "A" ? It is "C" which is considered as an integral forging/machined piece welded to plate. and which can have less stringent NDE requirements than A or B as indicated in many vessel specifications from oil companies or engineering firms.
Even the forging piece will be tapered and machined to match the head for welding purpose, it is still an integral forging piece and the weld is "C" per Fig UW-3.
