An excerpt from The Perfect Edge: the Ultimate Guide to Sharpening for Woodworkers
You can successfully harden your own tools with a little knowledge, some heat and a known alloy of steel. That’s often the hardest part: what steel have I got here? And, which quenchant do I use? The steel used in any given tool is not an easy thing to determine. A metallurgical laboratory charges a fair amount of money to test for alloy and there is no sure-fire home-test that I know of. Also, there is some risk in quenching, say, an oil hardening steel in water. It could warp like crazy or, worse, it could fracture. 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 mineral colorants in fireworks.) So you can grind a corner of the piece in question, observe the sparks, then grind a known steel and try to compare the little spark-flares for shape, brightness, complexity, etc. and then attempt a match.
For the do-it-yourselfer the most common issue will be whether a piece of steel (car spring, old saw, whatever) is oil- or water-hardening. It is safer to quench an unknown, perhaps water-hardening steel in oil than vice versa. The water-hardening steel may not fully harden in the oil and if that is the case, the chemistry and physics let you try again in water. I know this sounds cautionary and that’s because it is. Although I am wary of giving false hope to do-it-yourselfers, I also know it is possible to harden steels on your own. When I was making knives from saw blades, I clobbered together a high-temperature oven and an oil bath (like a deep fryer) for hardening and tempering on a very tight budget and managed to do a respectable job of heat treating over a thousand knives. I learned a lot about the process through trial and error, building and re-building and fine tuning the gear. I still keep my hand in for simple, small jobs in the shop and for curiosity testing on a small scale, but I now let the pro’s handle our production heat treating. With our current batch sizes, they do a much more consistent, uniform job than I could with my shoe-string setup.
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 (790° to 815°C). Got a good pyrometer? No problem. Two events signal the transformation of pearlite (the low-temperature iron crystal) to austenite. One is a sudden leap in the color of the red-hot steel. As the piece approaches critical temperature, the red glow that starts at around 1200°F (650°C) will visibly brighten until the piece approaches 1400°F (760°C). It will then remain at that color while the pearlite converts to austenite. Once that transformation is complete the color suddenly jumps to a much brighter orange. Recognizing that color change can take some experience, though; it may not be that obvious to the novice. Lucky for you austenite is not magnetic. The point at which plain high-carbon steel ceases to be magnetic is its critical temperature. Because of this handy fact of physics, you can simply heat the metal until a magnet is no longer attracted to it then quench it in oil. There are commercially available quench oils but, for simplicity’s sake, I use peanut oil. Peanut oil has a very high flash point that reduces the risk of fire* and it smells nicer than a petroleum oil when it smokes.
*FIRE DANGER ALERT! Safety first: the risk of fire here is very real – Flames + Red-Hot Metal + Hot Oil = Danger! Be prepared: use long tongs to handle the work, wear gloves and eye/face protection and keep your fire extinguisher handy. If you’re doing this in the shop or garage, turn on a fan and don’t be surprised when the smoke alarm sounds off. If you’re doing this in the house, call a divorce attorney.
How to get the blade to the Curie point is probably the biggest problem for the do-it-yourselfer. As the metal passes 1300°F (700°C) or so, 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 carbon atoms become fickle and would just as soon run off with any available oxygen atoms they encounter, lost forever to their new relationship as atmospheric carbon dioxide. We attempt to prevent this decarburization of the steel by heating the metal in an inert (oxygen-free) atmosphere, or by severely limiting the time at red-heat. An oxy-acetylene torch, which is often the heat source of choice for the do-it-yourselfer, makes the former impossible and the latter very difficult. It’s a real challenge to heat something as large as a plane blade evenly with a small torch-generated spot of heat. A forge fire is better than a torch because of its uniformity, and it can be starved for air just a bit to decrease the oxygen in the immediate vicinity. A small lab-type test oven or a kiln used for ceramic glaze tests works quite well though you may not be able to observe the color changes as the steel is heating. Toss in a charcoal briquette to scavenge some of the oxygen from inside the oven while the steel is in it. The best solution to the carbon burn-out problem for the DIY heat treater is to coat the steel with an occlusive powder coating like those available from www.rosemill.com. The powder is applied to the part that’s been preheated to 450°F (230°C). The part is then returned to the heat to finish the process. The coating washes off with water after quenching.
Even though the official instructions say to allow a piece to soak at critical temperature for twenty minutes per inch of cross section, for most backyard work with a thin section, such as a knife, as soon as you’re sure it has reached critical temperature you can remove the piece from the heat and quickly dunk it into a sufficient quantity of room-temperature oil. Be prepared for the oil to catch fire – don’t hold the blade in such a way that your hand (or your face) is directly above it. For uniform cooling, move the piece up and down in the oil. If you swirl it around, there is a risk that it will cool faster on one side which could lead to warpage. The piece should be tempered as soon as it has cooled to about 150°F (65°C). Without tempering it will be very hard and too brittle to use. You can check for hardness with a file. There will probably be a thin, soft skin on the piece due to decarburization, so the file may grab that decarburized skin which is soft. Push a little harder on the file to get through that skin and you should find hard, non-fileable steel beneath that the file will simply skid across.
Temper immediately after quenching to avoid the risk of damage mentioned above. The goal is simple: heat the part to the desired tempering temperature (see the chart), hold it there for twenty minutes per inch of cross-section and you’re done. Knowing whether or not you’ve reached the right temperature may pose a problem. If you have a very accurate oven in the kitchen, and when you’re spouse is out of the house for the day, just dial in the target temperature and heat your blade in the oven for the time required. An accurate deep fryer will accomplish the same result and tempering in an oil bath* works well. Always use a reliable thermometer to double check the oven or deep fryer’s thermostat. There is no need to quench at this step (although you can), just be sure the blade has thoroughly reached the intended temperature without exceeding that temperature.
*Another FIRE DANGER ALERT here! Hot oil is flammable! Don’t heat the oil over an open flame! An electric fryer used outdoors is recommended. Wear appropriate clothing, protective eye/face gear, gloves, sleeves, etc. Bring your most alert brain with you along with a fire extinguisher when you’re heat treating. Be careful! And if you’re doing this indoors, the same divorce-lawyer warning applies.
Without accurate temperature control, you’ll have to use the surface oxide colors to know when enough is enough. The as-quenched piece will be black and scaly so first, if you’re tempering with a torch, use a piece of sandpaper to clean some part of the blade until it’s bright metal again. When heated, that spot will change colors (you’ve seen the rainbow of colors that appears when steel heats) starting with a very faint yellow, called light straw and progressing through a striking vermilion to teal blue and gray. HOCK TOOLS’ high-carbon plane blades are tempered to 325°F (163°C) for a hardness of Rc62. I recommend this hardness, it has worked well for us for decades, but it presents a conundrum for the DIY heat-treater because the first, faint hint of color will appear at a temperature just slightly higher than 325°F (163°C). It’s like telling a fellow passenger on the bus, “Get out at the stop right before mine”. So my best advice is to overheat slightly, to the first sign of color, the faintest of the light straw, and stop there. Your finished part may not be quite as hard as Rc62 but it will be very close and should perform well. Tempering “by the colors” may have a long romantic blacksmith’s tradition but it is not as effective as an oven or oil bath that allows the steel to soak for the optimal time to allow full transformation.
Start heating away from the cutting edge, apply the torch flame sparingly and allow the colors to run to the edge. You may have to quench the piece to arrest any further increase in temperature. Any color beyond the faintest straw is too much. (The blade will still work; it just won’t hold its edge as long as you may like.) 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. If a blade seems too hard, you can always toss it back in the oven and let it go 25°F (13°C) hotter, holding it there for a few minutes before removing it from the heat.
If you are using the oil-bath, deep-fryer method for tempering, the blade won’t present the tempering colors because the oil prevents oxygen from creating the colorful oxides on the steel surface. Trust the thermometer and the method, remove the blade from the oil or just turn off the heat and let it cool.
You’re done! If the blade looks awful, you can sandblast or grind it pretty but it should work well regardless. If you’ve made a plane iron or a chisel, be sure to grind back the bevel a bit before honing. Without protective coating or atmosphere control, that thin edge probably received more than its fair share of carbon burn-out abuse and you need to get down to the good steel (the de-carburized layer may be as thick as 0.025” (0.6mm)). The same goes for the back: careful honing of the back is at least, if not more, important than honing the bevel. A little extra elbow grease will remove the de-carbed layer and expose the hardened steel. 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.
That’s it for this excerpt from The Perfect Edge. I’ve included a photo that illustrates the section on cryogenic treatment just as a teaser.
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