Thanks Omid, that prompted me to remember another option. A blacksmith by the name of Rob Gunter came up with a solution called SuperQuench some time ago. It's a mixture that will get the thinnest hard skin on a bar of mild steel. I don't have the recipe handy and I'm sure there are now a half dozen variations.

A good friend of mine who makes hatchets out of milder steel uses it. The hardened skin is thin enough that it will hone away much quicker than a better grade of steel. But it's something to use when you're in a pinch.