I made a set of 8 from mild steel many years ago, and I can't imagine going back to the old methods. I never thought of hardening them but, if you wipe the mating surfaces every time before fitting them, they should last for ever. One word of advice: removing the outer skin from the lump of bright drawn mild steel, then cutting a large slot across it for the tool to fit into, will cause it to distort slightly, so rough them out all over before finally finishing to size. This doesn't matter too much, of course, except for the two angled surfaces.
If I had the larger machinery to made another dozen, I would.