Heat Treating O1

What is A2 steel

“A” stands for air hardening which means you don’t quench in any liquids but set it aside and allow it to cool down on its own. It contains .95% High Carbon a 1% Molybdenum, 1% Manganese, .3% Silicon, 5% Chromium, .15% Vanadium, .03% Phosphorous and the same for Sulphur.  Excellent edge retention is possible thanks to the Chromium Carbides that are mixed during the heat treating process that makes it the most preferred cutting tool steel by tool makers.  However, the trade-off is that honing A2 steel takes longer and more effort than O1, it won’t hone an edge as sharp as O1 and the edge fractures quicker if the bevel angle is honed at 25 degrees.  To prevent the edge from fracturing Lie Nielsen recommends to hone the bevel at minimum 30 degrees but preferably to 35 degrees.  So, if you notice your blade isn’t cutting as well as it should be hone a steeper bevel.

O1 Tool Steel

O1 is a high carbon medium alloyed cold work tool steel with 1.1% manganese, 0.6 chromium, 0.6% Tungsten AND 0.10% Vanadium added to it with good hardening capacity.

The O stands for oil quenching.  Quenching in oil is recommended over water because it cools slower reducing the chance of cracking.  O1 also takes an edge better than A2 but will not stay sharp as long as A2.  So, the main difference between the two is; O1 sharpens relatively quicker than A2 and hones an edge sharper than A2 but the edge retention in A2 is better than O1.  Obviously thinner O1 blades that come with old Stanley planes will sharpen very quickly than the thicker modern A2 blades because their less steel to hone which is why they’re still a preferred choice for many old-time woodworkers who know the difference between truth from fiction.  Thicker blades do not reduce chatter as advertised, instead they are a pain in the backside to sharpen but for more information on that refer to Paul Sellers blog.  I believe he’s done a video on that.

 

Heat Treating Process

So now that we know all the technical jargon and we don’t want to spend an eternity on sharpening lets go with the heat treating process of O1.  Remember this is the tool steel I’m going to use on my moulding planes and the small router plane that’s been holding me back from the true build.  Maybe I should fill you in before I continue.

The small router plane I currently have which is the Veritas, the blade is ¼” wide which is too big for the mouth opening on the moulding plane which should be no bigger and smaller than 1/16” and I’m referring to the gap between the edge iron and the breast.  The mouth on the plane is 3/16” (5mm) and I’m using an iron that is 1/8” (3.175mm) thick so that should leave a precise gap of 1/16” to allow shavings to go through.  The thicker the iron the larger the mouth opening should be, so make your adjustments according to the thickness of the iron, all in all the gap should equal to 1/16”.  This I learned from Charles Hayward may he rest in peace.

I actually thought that a gap opening of 1/32” was ideal but that would only allow very thin wispy shavings through and that isn’t ideal for a moulding plane.  So, this router plane once I actually get it done will have a longer blade to do 2” or more depth and I’m yet to find out if it can handle that depth and also have a 1/8” width blade.  This will be the only tool of its kind in the world as far as I know and I’ve looked everywhere for one.  What’s holding me back you might ask, I’m trying to make it look pretty.

Ok now with process of heat treating O1.  O1 comes annealed so we can skip this part.  Grind your bevel and shape first, then whatever process you like to use as heat go with that, I used 2 blow torches.  You heat the cutting iron to about 1500 degrees Fahrenheit (815deg. Celsius) you will know you’ve reached that temperature when it reaches a bright cherry red colour.  In my opinion, I think that a bright orange colour is a more correct description of it but that’s the word that’s been in use for a few hundred years now so I won’t rock the boat.

Once that critical temperature has been reached you plunge the iron into an oil bath.  I used peanut oil over motor oil as it has less tendency to flame up and apparently, it smells better but I couldn’t smell squat.  I even stuck my nose into it and still couldn’t smell any peanuts.  You can use a metal container or a glass jar, I used a glass jar that was fairly thick.  I think the thickness is important due to the heat build-up of the oil, you don’t want the glass shattering and spilling oil all over your bench so a Nescafe jar is ideal.

When you plunge the iron into the oil, plunge it in vertically and keep it upright vertically while you continue to plunge.  If you angle it in and stir it you could induce warpage, I’ve seen Tod Herrli stir it gently but it was still held vertically.

Now take it out and let it sit for ½-1 hr to cool down, be careful though as it is still quite hot so don’t touch it and don’t ask me how I know that.   This is the confusing part though, some technical websites say cool it in the oil until you can touch it with your bare hands and temper it immediately, others say let it sit depending on its thickness, so 1 hr for 1” thickness and then temper it.   Up to you on this one, I don’t know who is right or wrong here but as for me personally on the next one I will choose to let it cool in the oil and then temper it.

Tempering

Tempering is the process of reducing the steels brittleness, if you didn’t temper it, the steel would shatter like glass if dropped on the ground.  You also wouldn’t be able to shape the edge nor sharpen it, your file would just skate over it.

Ever wonder how those martial arts experts were able to karate chop an iron in two, well you too can do that in its brittle state and that’s why we need to temper.

The temperature may vary according to the desired hardness and the hardness scale we are working to is the Rockwell C scale.  You see in every tool sellers’ description hardened to Rockwell C 60 or 62.  To reach that Rockwell C scale we need to heat up the iron in an oven to about 325 deg. F (162deg. C) for about an hour.  The iron will reach a light straw colour, you don’t want any other colour but that.  If you were tempering a knife then your Rockwell C should be about 55 -57 which is about 500-600 deg. F (260deg. C).  My oven only goes up to 260 degrees and there is no guarantee your oven is accurate.

If you can’t use your oven here’s the way I did it.  I held the flame back from the cutting edge and observed the colour change.  I withdrew the iron from the flame and watched the heat travel up the iron until it reached the edge during which a colour change was occurring.  Once the light straw colour was reached I immediately plunged it into the oil and then left it to cool in air.

I did skip an important part, after hardening clean and flatten the back to take out any potential warpage and clean the black oxidation around the cutting area up to the beginning of the tang.  It’s important to do that so you can observe the colour change during the tempering process.  Also get yourself a good magnet so you know you’ve reached the correct hardness and either glue it on wood or get one big enough you can hold by hand.  If you your burn fingers in the process you’ll live, don’t be pussies about it you won’t burn them second time round.

 

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2 thoughts on “Heat Treating O1

  1. Nice explination Salko. I’ve only done this a few times and only with O1 steel. The whole process always seemed very mysterious until I actually did it. The most difficult part is generating enough heat for the initial hardening. Tempering is a little nerve wracking. Once the color changes begin they progress quickly. Wait for it…wait for it…wait for it…PLUNGE IT!

    Liked by 1 person

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