Saturday, March 20, 2010

01 Steel

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I have been working on my knives this spring. I found a useful review on eBay from a seller that I have purchased steel in the past.

When a steel is fairly easy to machine, can be heat treated with a torch, tempered in the oven, and yields a very hard, fine grained end product, it's no wonder it continues to remain a popular choice for many applications. The AISI O1 steel group is an oil hardening, non-deforming tool steel that can be hardened at fairly low temperatures with minimum volume change. It typically has deep hardening qualities with a fine grained structure.

Machining: Keep in mind that traditional machining, such as sawing, drilling, milling, and filing, need to be done before heat treating. After heat treating the steel is ready for grinding and polishing. (We now have solid carbide and diamond coated tooling that can be used on the heat treated steel, but this is usually only done in emergency or special situations.)

Heat Treatment Data: While the different brands of O1 steel will vary slightly in composition, they will all conform to AISI standards and therefore will all react to heat treatment with similar results. The temperatures given here are the ranges of temperatures that were acquired from two O1 steel makers and one O1 steel supplier.

SAFETY: Please remember that we're talking about using an open flame and heating metal to a temperature that will damage most things if it makes accidental contact. The quenching oil can splatter or spill and it's flammable. Your work area should be prepared beforehand for the hazards, or this can be done outside. Safety glasses, protective clothing, fire extinguisher, good ventilation, and great care is recommended.

Hardening: Pieces with sections (thicknesses) 1/4 inch or smaller should be heated to 1450 to 1500 degrees F. If you use a torch, be sure to heat the piece (or area to be hardened) evenly. I found that it will glow a cherry-red color when ready to quench. Experience will help you recognize when the temperature is correct, but some people use a permanent magnet to determine when the right temperature has been reached. The hot metal will not be magnetic at the correct temperature. Be sure it's heated all the way through. Over-heating can make the steel brittle. Heat treating done properly will also give the nice, fine grain. (Thicker pieces should be pre-heated to 1200 degrees F. Raise the temperature to 1450 to 1500 degrees F and hold that temperaure for half an hour per inch of cross-section.) The easiest way to hang on to pieces while heating and quenching is to use a heavy wire through any hole. The wire should be heavy enough not to melt through during heating. I've used old (heavy) coat hanger wire with good success. This makes retrieval from the oil quench quick and easy instead of slow and messy. Trying to hold a piece by pliers or tongs can be a real adventure and potentially dangerous (if you drop the piece, etc.). If the piece doesn't have a hole, sometimes you can put a hole somewhere where it won't hurt anything, such as the tang of a knife blade, so that you can use a wire.

Quenching: The oil quenching should be done as soon as you're satisfied that you've reached the correct hardening temperature. (except for thick pieces as mentioned above that need longer hardening times - and possibly an interrupted quenching process - using oil and air) Any light-weight oil will work - such as motor oil. Recommended oil temperature is 120 to 150 degrees F. The temperature of the steel must be quenched to 150 degrees F or lower. I have succcessfully used room-temperature oil, but of course the closer you follow recommended procedures, the better your chances of attaining the best results. Safety tip: Make sure your oil container is metal so that there's no chance it will melt. You also need good ventilation - it will smoke! The metal is hot enough to flame, but that's OK and normal. If it still flames when you try to bring it out of the oil quench, then it's still too hot.

Tempering: Any piece that's going to be tempered should be tempered as soon as its quenching is done (and the excess oil is wiped off). The easiest way to temper is by using an oven. Our first manufacturer's information tells us that O1 hardened in the manner we just described should typically be 63 to 65 Rc as hardened. They claim that tempering 2 hours at 350/450 degrees F should result in a hardness of 60 to 64 Rc. Another manufacturer provides the following three tempering schemes: 1.) 300/350 degrees F for 1 hour for 62/64 Rc - 2.) 400/450 degrees F for 1 hour for 58/60 Rc - and 3.)800/850 degrees F for 1 hour for 48/52 Rc hardness.

After the final cooling, you need to clean off any oil, scale, or dirt. A wire brush or wheel, sand blaster, or whatever's convenient for you will prepare the piece so that it doesn't clog up your grinding wheel. Hardened pieces are then ready for grinding, polishing, and finishing. They are now too hard to work on with regular tool steel tools, such as: saws, drills, files, etc. (unless the tooling is solid carbide or diamond coated - and held down firmly).

Disclaimer: There are so many variables, that your results may very well vary from what we're writing here. As I said before, the more experience you get, the more you can modify things to get exactly what you're after. This is just a rough outline to try and help beginners get started. Professional heat treating facilities get the best and most consistent results because they have the correct equipment - ovens, with accurate temperature control, timers, and lots of experience. They start out by pre-heating correctly and can follow the manufacturer's instructions accurately to the well-timed conclusion.

Having said that, myself and many, many others have used O1 in our local tool & die shops - with torches, dirty room-temperature oil quenches, and looking for the right cherry-red color to great success for many years. Talking with my custom knife making friends, I know that many of them feel that they are pursuing an art of which heat treating is just one of the many components of the overall art. To whoever reads this guide, I wish you the best luck with your projects. (Please - think of safety first and always.)

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