Obviously, the full penetration joint (above) is far stronger than the cheapo decorative fillet joint ...
ASME B31.8 is the wrong place to look !!! .....Check out ASME B31.1 and ASME B31.3 Codes for details of these types of joints
There is a discussion regarding the acceptability requirements for these fabricated joints there.
Did you buy the fillet welded joint component from China ???? ... Where was it sourced from ?????
You may have to evaluate your spherical component as a Pressure Vessel in accordance with the ASME B&PV Code Section VIII
What are your dimensions, materials and design temperatures/pressures ?
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MJCronin
Sr. Process Engineer