Heat Treating O1 Steel

Roubo-Moulding-Planes

The cover story for the April 2016 issue (#224) is Caleb James’s article on making matched sets of Roubo-style moulding planes (the three sizes for which he reaches the most: Nos. 4, 6 and 8). To whet your appetite (and so I can show off the gorgeous opening image shot by Daniel Dubois, whom you really ought to follow on Instagram), below is a preview: a “sidebar” Caleb wrote about how to heat treat O1 steel using a simple setup and one clever jig (that is not a click-bait line – promise).

The issue mails to subscribers in mid-February, and will be on newsstands at the beginning of March.

— Megan Fitzpatrick

Heat Treating O1 Steel

heat treat steel

A firebrick enclosure helps to evenly distribute heat on the tool steel.

Heat treating O1 tool steel is simple. In short, bring it to critical temperature, quench it in vegetable oil, then temper it in an toaster oven or regular kitchen oven for one hour at 400˚.

Hardening steel is the easy part; minimizing warpage is another. The road to success is to evenly heat the metal. Creating an enclosure with something such as fire brick will help you achieve this.
Heat the steel slowly using a simple MAPP gas torch. Heat more slowly than you think. If one side is hotter than the other when you quench it in the oil it will warp. So heat slowly.

There are a few methods to determine when you have reached critical temperature. Avoid relying on color because ambient light will affect the color you see. Just like a flashlight appears brighter at night than in daylight, so too color alone in changing light conditions can be deceptive. The simplest way to check that you have reached critical temperature is to use a magnet, because O1 tool steel loses its magnetism at approximately this temperature. The stick of maple shown above has a rare-earth magnet glued to the end; it’s a simple tool for safely checking the steel as it reaches critical temperature. Just periodically remove the steel briefly from the flame and check with the magnet.

Another option is to watch the surface quality of the steel. It will change when critical temperature is reached because the carbon begins to flow within the steel and some decarburization takes place at the surface, thus changing how it looks. It can be described as the steel “sweating” or having a “flushed” appearance. Accompanying this change will be a noticeable uptick in the “brilliance” of the steel. In other words, rather than seeing a particular color change you would simply see the color become more radiant.

Once critical temperature is reached, quench the steel in oil by plunging the blade straight down vertically, not leaning to one side or the other. Now immediately move to your oven to temper for one hour at 400˚. — Caleb James

24 thoughts on “Heat Treating O1 Steel

    1. Megan FitzpatrickMegan Fitzpatrick Post author

      Caleb covers that in the main article, along with the plane build; the issue uploads to the printer next week.

      1. amoscalie

        Just curious, will Caleb also be covering annealing (softening) of steel? When I have gotten old irons on-line, I have to anneal them to reshape them to fit moulding planes that I have made.

  1. IrritableBadger

    I really hate to What benefits is this treatment of O1 steel supposed to endow on the material? I ask because the information in the article will not imbue the material with any functional benefit.

    That 400 temperature is the maximum operation temperature of the material. It has absolutely nothing to do with altering the chemistry of the material.

    Heat treated hardening of O1 steel requires heating temperatures of 1450 – 1600 F to induce a phase change and become austenite (the steel will remain magnetic until the phase change and not at any point before that). Heat at those temps is maintained until the phase change has penetrated to the depth you require.

    Following the heating cycle the austenite is quenched in water or brine which cools the steel and creates another state change. If the steel is cooled quickly is becomes martensite which is very hard but brittle. If cooled slowly the steel will be annealed (pearlite). Afterward the steel is tempered is ready to be made into something!

    The information above simplifies the process, but heat treating outside those temperature ranges or post heat processes simply isn’t a thing. Can’t be done.

    Instead of wasting your MAPP and probably your steel (quench at 400 F will not harden even the outermost layer of the steel but is almost guaranteed to destroy the temper) there are other ways to actually harden O2. Find someone into ceramics or stained glass and ask them to heat your steel and quench it there. If your local vocational school has a machine shop or welding program they’ll likely have at least one kiln,furnace,oven and may not even want to charge you as it’s a useful lesson for the students.

    Before everyone gets upset, the information above is taken directly from Machinery’s Handbook 29th Edition. If you’re going to do anything remotely serious with metal spend the $90 and buy yourself a copy of Machinery’s Handbook (it has materials data for woods too).

    1. amoscalie

      I have had great success with the methods mentioned both in the article and from readers blogs.
      1. Heat the metal until it is non-magnetic
      2. Quench the “O type” in oil (not water) to roughly 150 deg, moving the metal vigorously back and forth to dispel any air bubbles.
      3. Temper in an oven at 400 deg for an hour.

  2. amoscalie

    Tempering in your kitchen oven can be hazardous. Not from the standpoint of burning the house down, but from your wife finding out what you are doing in her oven. Since my wife has retired, I had to get myself a toaster oven to temper my irons.

  3. NuritheTurk

    Back when I was making knives, I quenched O1 in olive oil, the cheapest gallon size I could buy. The advantage is that it smells like baking oatmeal cookies. Learned that from Bob Loveless. I quenched O1 in water (by mistake) once. The blade cracked in a dozen places. Never tried it again.

  4. blake

    two things i would like to point out, the oil used for oil hardening is recommended to be above ambient, roughly 100-150F is the recommended temp. and the part should go in when its “cool enough to touch but too hot to hold” as they say. the other thing i would like to add is imho a2 is not that much different to harden and normalize as oil hardening or water hardening steels, or 1040 for that matter, a2 requires a higher heat (i think it’s 1600F vs 1400F of o1, i forget offhand you can look it up however in any good metallurgy or heat treat man) and longer soak times.

    1. blake

      oops! when i say “go in” i mean into the oven, it goes in the oil immediately when bright, sorry. b

  5. Barquester

    I agree with gilgaron to start out with 1095 simply because it uses water to quench and the 2 steels have nearly identical properties once finished. the O in O-1 is for oil quench, the W in W-2 is for water quench.

  6. sqmorgan

    I would (wood, for aperitif?) love to make a set, and look forward to this issue. One question – If I buy the set of plane blades (For example the tapered Molding Plane Iron Blanks from Lie-Nielsen), will I still have to do this process? Thanks.

  7. pmac

    May I ask a question regarding: “Once critical temperature is reached, quench the steel in oil by plunging the blade straight down vertically, not leaning to one side or the other. Now immediately move to your oven to temper for one hour at 400˚.”

    I think the word “immediately” is throwing me off. Does he mean plunge it down once for a quick quench and then move to the oven, or plunge it until it cools considerably, (below 400 degrees? )and then move to the preheated oven?
    Thanks and Happy New Year.

    1. jweisgram

      Read this from that (at Ron Hock’s web site):

      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.)

      http://www.hocktools.com/tech-info/diyht.html

      Ron prefers peanut oil for the quench and warns to safe from starting a fire, wear gloves, use tongs to hold the metal, etc.

    2. gtrboy77

      Yes, plunge and quench until it is cool enough to hold with a shop rag (it will likely be still too hot to hold with bare hands. Also, one thing Caleb didn’t mention that is equally important is that while quenching is to move the iron vigorously in the oil back and forth (side to side). The reason being is you will want to dispel any bubbles that will want to attach to the iron after plunging into the oil. Simply plunging straight into the oil and allowing bubbles to hang onto the tool can compromise the consistency of the hardness of the tool (softer spots where bubbles are). Another thing is to keep a small lid handy in case of any fire flare-ups in the oil. Tempering the tool should be done within an hour; I used to work in a heat-treating area of my company.

  8. JMAW Works

    I think the benefit comes from shaping the steel in it’s annealed (soft) state. since hack sawing hardened a2 isn’t going to be an option, that presents challenging ways to remove material. Also, I believe a2 is not as friendly for DIY hardening.

  9. Pleasanton

    Cool – looking forward to the Roubo-based planes article. I’m trying to build a bookstand from the Book of Plates.

    whet vs wet…. I learned something today

  10. Christopher Hawkins

    Please help me understand the advantages of heat treating O1 steel vs buying the harder A2 steel. Buying A2 steel would give you a harder edge without the risk of warping the tool. Better edge durability vs. A2? Or is this article aimed at folks who already own O1 tools?

    1. Jim Dee

      If you buy new steel from a metals dealer, it comes in an annealed or “dead soft” state, which is convenient so that you can forge and grind it into shape most easily. This would apply to both O1 and A2. The “A” in A2 stands for “air hardening,” (like O stands for “oil hardening”). I am not sure how the quench is done with air hardening alloys, maybe another reader of this blog has done it? Either you just pull the work out of the heat and let it cool at room temperature, or maybe you need to give it a good blast of compressed air through an air gun? By the way, for knife-sized and chisel-sized pieces of steel, I’ve had perfectly fine luck quenching O1 in water.

      1. IrritableBadger

        Air hardening doesn’t mean regular ‘air’ like the stuff we’re all (presumably) breathing or comes from a compressor. It simply means the material doesn’t require submerged quenching.

        At its simplest the material in the air hardening process is left in the oven and cooled by controlled lowering of the temperature over a period of time. The temperature and time curve for the cooling/hardening process is a function of the material itself, its size and the properties you want in the finished material.

        Depending on the scenario, air hardening can take anywhere from hours to days or weeks. The process minimizes heat induced distortion in the material and typically results in a harder steel. It is an expensive process that requires considerable experience to get the best results. As someone else stated, air hardening it is not a good place to start learning how to harden steel.

    2. gilgaron

      You wouldn’t want to make the tool out of hardened steel, but harden it once you’ve got it shaped. I’ve only done a little forging, but especially for beginners air hardening steel is not generally recommended. Some even say that O1 is tricky for novices to get the most of and they should use 1084 or 1095, but I went with O1 for the tools I’ve been making.

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