Some Thoughts on DIY Heat Treating
First, know your steel. If you bought commercially available tool steel you should know precisely what it is. But if you are using something found, scavenged or of otherwise uncertain provenance you may have problems hardening it. The steel used in any given blade is not an easy thing to determine. A metallurgical lab charges a fair amount to test for alloy and there is no home test kit that I know of ("Look, Honey, it turned blue!") And there is some risk in quenching, say, an oil hardening steel in water. It could fracture at worst or warp like crazy at least. The old-timers "sparked" steels to tell what was in them. The sparks generated from a grinder will burn with different visual characteristics depending on the alloying elements. (Like the different colorants in fireworks.) So you can grind a corner, observe the sparks, then grind a known steel and try to compare the little spark-flares for shape, brightness, complexity, etc. and attempt a match.
Mostly we're talking oil vs. water hardening steels. The air hardening ones are the Cr-V and stuff that us Galoots don't use too much and that weren't used in old tools at all. It is safer to quench an unknown, perhaps water-hardening steel in oil than vice versa. The water-hardening steel may not harden in the oil and if that is the case, you can try again in water. I don't mean to muddy the water with all this but, hey, if it were easy, everybody'd be doing it.
The first step is to get the metal to its critical temperature, which with good old O-1 (the oil hardening stuff) is 1450° - 1500°F. Got a good pyrometer? No problem. During the crystal transormation from ferrite to austenite steel ceases to be magnetic at that temp. This phenomenon is called the "Curie Point" after the discoverer, Pierre. So one can simply heat the metal till the magnet is no longer attracted to it then quench in oil. I like to use peanut oil because the flash point is very high which minimizes the risk of fire (the risk is still there, though; be prepared: use long tongs to handle the work to keep your hand out of the way, wear gloves and keep the fire extinguisher handy) and it smells nice(r) when it smokes. How to get the blade to the Curie point is probably the biggest problem for the DIYer. When the metal is glowing red, the carbon behaves as if it's in a liquid and can therefore migrate around as it pleases. This is necessary for the hardening to occur but near the surface of the metal those unfaithful little carbon atoms would just as soon run off with any available oxygen-sluts it runs into (oxygen is soooo seductive) and they're lost then forever. We hate that. We attempt to prevent this by: heating the metal in an inert (oxygen free atmosphere) and/or limit the time at red-heat (in air) to as little as possible. A torch makes both of those very difficult. It's very hard to heat something as large as a Norris-type blade evenly with a small torch-generated spot of heat. A forge fire is better because of its uniformity and it can be starved for air a bit to decrease the oxygen in its immediate vicinity. A small lab-type test oven works quite well. (Also used for ceramic glaze tests.) Toss in a charcoal briquette to scavenge some of the oxygen.
Update: There are coatings that prevent oxidation and carbon loss at www.rosemill.com that promise to make home heat treating a more successful endeavor.
When it's hit critical temp, remove it from the heat and quickly dunk it into a sufficient quantity of room temperature oil. Swish it around a bit until it's cooled throughout to below 150°F. It should now be very hard and too brittle to use. (If you attempt to file it, the file should skid on the blade.)
Two ways to temper to a useable hardness/toughness: by colors or by temperature. If you have a very accurate oven in the kitchen, just heat it to 325°F and you're done. An accurate deep-fryer will do the same but use a good thermometer to double check on the oven or deep fryer's thermostat. Without accurate temperature control, you'll have to use the surface oxide colors to know when enough is enough. First, clean some part of the blade (probably the flat area back from the bevel) till it's bright metal again. When heated, that spot will change colors (you've seen the rainbow of colors on any overheated steel) starting with a very faint yellow (called light straw). Since we like our blades Good-n-Hard(tm), stop there (remove from the heat, quench if necessary to stop any further increase.) Any color beyond the faintest straw is too much. (The blade will still work, it just won't hold the edge you want.) Be overly cautious with tempering. You can always re-temper a too-hard blade, but if you go too far and soften it too much, you have to re-harden it all over again. So if a blade seems too hard, just toss it back in the oven and go a little higher. The oven/deep fryer method is preferred, however because you can leave the part at tempering temperature long enough for true tempering to occur. The torch method, using the surface colors, may leave some of the transformation undone.
You're done. If the blade looks awful, you can sandblast or grind it pretty but it should work well regardless. Before honing, be sure to grind back the bevel a bit . That thin section probably took more than its fair share of carbon burn-out abuse and you need to get to the good stuff. (it could take as much as .025" to get through the de-carbed layer.) Same for the back. Doing a good job on the back is at least if not more important than the work on the bevel. A little extra elbow grease will remove the de-carbed layer and get to good metal. Don't forget: the back IS the Cutting Edge. Think about it. If the back hasn't been honed deeply enough, the blade will never work well.