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Making and Using Tools - Miscellaneous Tools

  About Steel by Ron Hock  

The three qualities that most effect the selection of a steel for a hand-tool application are edge-holding, sharpenability, and corrosion-resistance.  For metallurgical reasons, you can only have two of the three.  We at HOCKTOOLS feel that in woodworking, corrosion-resistance is the least important of the three, and prefer an edge that is easily sharpened and long lasting.

A steel's carbon content determines its ability to harden with heat treatment.  That hardness determines a tool's ability to hold a sharp cutting edge under abrasive pressure (wear).  Generally, the harder the metal the better its edge holding, but it will be more brittle.  Tempering reduces that brittleness, although it also reduces the tool's hardness and wear resistance. So a balance must be struck to decide how hard a blade should be.  Our blades are hardened to Rc62 for long edge life.  This is harder than most available replacement blades yet not as hard or brittle as most Japanese blades.

"Tool Steel" refers to a class of steels that are metallurgically very "clean" and fall within strict limits for alloy proportions. Vanadium, tungsten, and molybdenum are often added to tool steels to make the steel resist annealing (softening) when used in "high-speed" (high heat) applications. Chromium is added in very large quantities for corrosion resistance ("stainless").

High-speed steels are essential in metal-working tools (drills, milling cutters, etc.) and "stainless" steels can be cost effective by resisting rust during the manufacture, shipping, and storage of the tool itself. Correctly heat-treated, tools made from high-speed, stainless, and "chrome-vanadium" steels may hold an edge well in woodworking applications, but, due to the large, hard carbide particles that form during hardening, they are difficult to sharpen and cannot be honed as sharply as a blade of plain high-carbon steel.

Our choice of High-Carbon Tool-Steel (.95% Carbon) offers the finest, sharpest edge possible. Its chromium and vanadium additions amount to only 1/2% each allowing quick, clean honing with traditional techniques. High-carbon steel holds and takes an edge better than anything else. We guarantee it.

Some thoughts on Do-It-Yourself Heat Treating of Tool Steel

I posted this some time ago when the group project was the St. James Bay plane "kits" and some were (bravely) doing their own blades for them. It's doable; get some extra pieces of the same steel to practice on...

The only addition this time around has to do with the great question of which quenchant to use with which steel. 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 - 1500F. Got a good pyrometer? No problem. For some reason (let it be a mystery; there are so few left) 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.

When it's hit critical temp, remove it from the heat and quickly dunk it into a sufficient quantity of oil (preheated to about 150F.) Swish it around a bit until it's cooled then let it cool to ambient in the air. 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 temp. If you have a very accurate oven in the kitchen, just heat it to 325F and you're done. An accurate deep-fryer will do the same. But without the accurate temp 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.

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. 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.

Good luck!

Ron Hock

Ron Hock 2002 

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