The required NDE would be based on the code of construction, joint efficiency, and contract requirements. More details of the project will be required before accurate advice can be given.
I would consider X-Ray. You are more worried about the migration of carbon out of the heat affected zone as the result of temperature during the melt to surrounding adjacent surfaces. This carbon depletion zone will lead to brittleness and often associate itself with various forms of corrosion leading to cracking. i.e. intergranular corrosion.
1. Mechanical design (strength calculation) : If it is applicable to ASME Sec.VIII (or any other pressure vessel code), the required thickness of shell section (including the weld joint of 2:1 ellip. head to shell) shall be strength-calculated with the joint efficiency per the applied RT Level (none, spot, or full).
2. Mechanical design (metallurgy requirements) : The weld joint shall be designed/examined with RT (or other NDE) according to the requirements for service. e.g., 100% RT for cryogenic service and/or environment cracking service.
