What you propose is a code violation, but not one I'd lose any sleep over.
What you're trying to do is make a 240v, 30-A "convenience outlet." The convenience outlet is the only instance in the NEC where we do not size a circuit to a known load, setting the breaker as close as we can to the minimum necessary to make things work.
Alas, the 'convenience circuit' is defined as a 15 0r 20 amp, 120v circuit. Since your circuit does not fit this definition, you cannot apply convenience outlet rules.
FWIW, the idea of circumventing AFCI rules by simply using a 10-A breaker runs afoul of this same issue.
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Now ... for some general 'welder' comments:
I cannot stress enough that every welder is different, and you need to use ALL of the nameplate information to run a proper circuit. "Just give me 240 for the welder" just won't do. Welder circuits are sized using different rules than are circuits for clothes dryers. You're trying to put wheels on a submarine.
Do it right.
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Issue #3: the "240 convenience outlet circuit" concept turns up again, like a bad penny. I've lost count of the homes I've seen with 4x2 fuseboxes- and that had the dryer, water heater, and kitchen range all cobbled onto the sole 240v circuit. Heck, some even had a 240v air conditioner added to the mix!
Quit adding patches to fixes to improvisations to bandages! At some point you need to recognize reality and admit that you've seriously outgrown the original electrical service. Bite the bullet, rip it out, do it right.