There are 4 classes of welding, A, B, C, & O. Welders shall be qualified to weld with each procedure used for production. A welder shall requalify to weld with an appropriately qualified procedure when there are changes made in the following essential variables:
(1) Change in welding process (and Wet Welding and Dry Hyperbaric Welding)
(2) Change in welding method (manual, semi-automatic, automatic)
(3) Omission of backing, but not vice versa
(4) Change in pipe diameter or wall thickness beyond the limits
(5) Change in electrode classification (for example: AWS EXX10, to AWS EXX11) or proprietary type, including waterproofing
(6) An increase in electrode diameter for wet welding
(7) Change in position from that qualified
(8) Change in depth beyond the limits specified
(9) Change from a mixed breathing gas to air (Dry Hyperbaric Welding)
(10) Change in shielding gas beyond the limits
(11) Change in direction of vertical travel from upward to downward or vice versa
(12) Change from stringer bead to weave technique
(13) Where production welding visibility is less than 150 mm (6 in.), unless the welder has already qualified with visibility less than 150 mm (6 in.)
Welder qualification test results for Class A groove welds shall meet the following requirements as applicable as applicable: Visual Examination, Radiographic or Macroetch, Ultrasonic Examination, Bend Tests, Four samples shall be tested.
Welder qualification test results for Class B groove welds shall meet the following requirements: Visual Examination, Radiographic or Macroetch, Bend Tests.
At this time any CWI (OKed by the owner) may witness tests (this will change in the future)
There are specific electrodes used when wet welding check your PQR/WPS (I assume you have one)
The above is just a common list of items that effect a weldor certification there is a lot more.
Hope this helps