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 is 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.

Heat Treating Process

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 wimps about it you won’t burn them second time round.

4 thoughts on “Heat Treating O1

  1. Why are we as craftsmen performing heat treatments? Isn’t what we buy treated and tempered? If we do engage in these activities, shouldn’t we invest in remote temperature detectors?

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    1. When you buy metal it comes annealed so you can easily work it and then you heat treat to the temperature desired for whatever it is that your working it.

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  2. A few years back I made a few wooden planes including buying the O1 steel and heat treating the blades. The issue I had was minor warpage; maybe I didn’t keep the blade adequately vertical in the oil but if you have put a bevel on the iron already and the warp is convex across the flat side then a fair bit more flattening work ensues. I felt at the time it was probably better to grind the bevel later.

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  3. Another thought, I somewhat agree re A2 but for those of us with A2 blades that need to be sharpened I found the YouTube video by Brian Boggs an eye opener (but be patient to watch through it), this changed the way I sharpen for the better. I have adopted sharpening on the forward stroke on a waterstone and the use of end grain loaded with slurry or autosol for final burr removal instead of a strop with excellent results.

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