Having responding as I did, I will go one step further and say that I always recommend the welders be required to qualify on carbon steels, nickel alloys, and (austenitic) stainless steels separately.
The code sets the minimum requirements, but most welders will admit there are differences in the way the metal flows when comparing carbon steels, stainless steels, and nickel alloys. The cost of qualifying the welder on each base metal family is inexpensive compared to repairing unacceptable welds.
The welder qualification test is the easiest way to ensure the welder can handle the different base metals and follow the WPS.
There is a saying about welders; "You can always tell a welder, but you can't tell him much!"
An engineer said that to me many years ago while I still under the welding helmet earned my living burning rod. I now understand what he meant.
There is no better time to teach a welder something new or tweak his welding skills than right after he fails a qualification test. Other than that brief moment, all hopes of teaching the old dog a new trick or two is lost.
Best regards - Al