That'd give you a carburizing atmosphere, perhaps useful for case hardening, but more likely to produce a large BOOM if any air remains in the furnace.
Not much use for an oxidizing atmosphere in a heat treating furnace. You'd get *lots* of scale, decarburization of the steel, etc.
Now you're talking. An inert atmosphere will prevent either carburization or oxidation of the steel. The part will come out nice and bright and clean with exactly the chemical composition it had when it went in, but with a different grain structure due to the heat treating. That's exactly what you want.
Gary