They are almost certainly mild steel and as such can not be hardened/tempered. Probably something like one of the very common 1018 or 1026 steels and heat treating them would be a waste of time. You could use some "Kasenit" or "Cherry red" hardening compounds to case harden them which would harden them to a few thousandths deep but leave the inside soft and less likely to break. Simply heating them red hot and plunging in oil or water will harden tool steels and other alloy steels but then they are brittle and very easy to break. Hardening must be followed by tempering at a lower temperature for a specified period to achieve toughness without losing the hardness. Hardening is a two step process and the tempering step can not be skipped. All this however unfortunately does not work, as already mentioned, for mild steels and that is almost certainly what those screwdrivers are.
That suggests tempering at 325 deg, not likely to accomplish much at that low of a temperature and for a "one size fits all" tempering temperature 400 deg to 425 deg in an oven for a hour would be better. If the type of steel is not known, or close to it, then using ball-park figures for the heat to either harden or temper is going to a hit-or-miss proposition at best.
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