A2 vs O1 Tool Steel

Article - June 11, 2009

An excellent question from Brian:

My question relates to tool steel. I am about to purchase a Veritas shoulder plane and for the first time am faced with the choice between A2 and O1 steel. I understand that A2 is the harder steel, which has the benefit of holding an edge longer and the detriment of being more difficult to sharpen. I understand that O1 steel is softer, allowing for easier sharpening to a keener edge, but one that will dull more quickly. Can you please confirm, to the degree you are able, that I am correct in my understanding here.

Secondly, can you let me know which steel you would choose and why? I am leaning toward the softer steel (O1 by my understanding) because I understand it is easier to sharpen to a keen edge. I am not going to put a lot of miles on this plane, so more frequent sharpening will not be a big burden to me. Thank you for your input. I continue to enjoy the site and The Guild.

Hey Brian. You are absolutely correct in your understanding of the difference between A2 and O1 steels. In general, O1 holds a slightly keener edge and is easier to sharpen. But it also dulls faster and require more frequent sharpening. A2 steel takes a little more effort to sharpen, but you are rewarded with a more durable edge that lasts longer.

As a person who uses a lot of exotic hardwoods and doesn’t enjoy sharpening any more than I have to, I prefer A2. When I am sharpening, it usually turns into an all day affair. So whether I’m using A2 or O1, I’m still getting messy and still spending hours getting my tools all prepped and ready to go. The A2 just ensures that I won’t be doing that as frequently. What you choose really does come down to personal preference.

Now I am by no means an expert in tool steel, so I thought I would go to someone who is for a more thorough answer. Here’s a reply from Ron Hock of Hock Tools:

For our high carbon steel blades, HOCK TOOLS uses AISI (American Iron and Steel Institute) O1, a simple high carbon tool steel with very little added to the iron/steel alloy other than 1.1% manganese. That pinch of Mn allows the steel to harden with an oil quench (the “O” in O1 stands for Oil.) Oil removes heat more slowly than water, reducing the thermal shock that occurs when orange-hot steel is plunged into water. Reducing that thermal shock minimizes the risk of cracking or distortion in the hardened piece. With an alloy as simple as O1, containing so few alloying elements, the hardened grain structure is as fine as possible which allows honing to the sharpest possible edge.

AISI A2 differs from O1 with the addition of 5% chromium and 1.1% molybdenum, allowing it to quench in still air (“A” for Air.) While “stainless” amounts of chromium (12% or more) make tool steel gummy and hard to sharpen, the modest amount of chromium in A2 improves its toughness and abrasion resistance, but imparts only a slight measure of corrosion resistance (like high carbon steel, it will rust and appropriate preventative care must be taken.) But there is a trade off. During heat treatment the chromium addition combines with some of the carbon in the alloy to form chromium carbides: tough, hard particles dispersed through the steel. These carbides are the primary contributors to A2’s celebrated edge retention. However, during heat treatment, the chromium carbides can grow quite large. Large enough to affect your ability to hone the edge as close to zero-radius as you may want. And these carbides are held in place with less strength than the rest of the steel matrix which can allow them to pop out under the stress of honing or cutting leaving a small gap in the edge.

A2 is one of the steels that respond well to cryogenic treatment. This extreme cold treatment (-320°F) essentially finishes the original quench, increasing the steel’s toughness without any decrease in hardness. You get increased wear resistance without any increase in brittleness so a cryogenically treated blade will hold its edge longer. You can keep working instead of sharpening. A2 is a great steel that offers a real improvement in edge retention. O1, on the other hand, is still preferred by many for its relative ease of sharpening and its ability to get sharper.

Want to learn more from Ron Hock? Check out his book: The Perfect Edge: The Ultimate Guide to Sharpening for Woodworkers

Discussion

Johnny No-Name June 11, 2009 7:28 am

Brian – I have the Veritas shoulder plane with the A2 blade, and it holds a “keen” edge for a very long time. I don’t sharpen by hand, so difficulty in re-sharpening isn’t very important to me. It’s a great plane by the way. Very happy with it.

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Marty June 11, 2009 9:57 am

I hope this example helps. I have 4 friends who woodwork, one of them is what we call the “purist”. The only power machine he has is a tablesaw. Every other job is accomplished with planes and handsaws. (He has about $10,000 invested in planes, seriously). He swears by O1 steel. The rest of us “Normites” use A2 steel. I have a L-N shoulder plane and a Veritas block plane that I’ve used on and off for a month and the edge is still really sharp. The Purist has sharpening sessions every 10 days or so.
It just depends on how much you like to sharpen. For me personally, it’s the part of woodworking I hate the most.

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Jason Weaver June 11, 2009 12:00 pm

As a newbie, I am having trouble gauging the difference in the examples of O1 and A2, because there is a lot of difference in how much each person uses their tools. One person mentioned still having a good edge on A2 used on and off for a for a month while his friend sharpens every 10 days, but one person might use the blade more in 10 days than another might use it in an on-and-off month.

If someone used an O1 and A2 blade in equal ways — how much longer might the A2 last over the O1?

Also, many say it is harder to get a keen edge on an A2 blade, but the way Ron Hock described it you can’t get an A2 edge as keen as an O1 no matter how hard you try. Am I reading that right?

As a newbie, should I stick with O1 because it sharpens easier? It would mean more time sharpening, but as a newbie that could be a good thing. Practice makes (closer to) perfect, right?

Thank you all in advance for your further input,
Jason

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- Marty June 11, 2009 11:25 pm
  
  It all comes down to how you feel about sharpening. If you loathe it as much as main man Marc and I do, go with the A2. If you don’t mind or actually enjoy(?) it, get the O1. I happen to agree with BarryO, I can get an A2 blade sharp enough to damn near cut my thumb off (don’t ask). My friend the purist probably would say his O1 blade would have sliced my thumb off.
  
  Reply
Peik June 11, 2009 2:04 pm

The solution is to get a damascus steel blade? ;)

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BarryO June 11, 2009 6:19 pm

Wow, I love the info from Ron Hock! Just about the clearest-stated, most-informative few paragraphs on tool metallurgy I’ve ever read.

FWIW, I think if you can sharpen A2 to a keen enough edge to shave arm hair, why do you need anything sharper? Anything I can do to spend less time sharpening is a good thing. ;)

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Dan June 11, 2009 8:39 pm

A2 will definitely get shaving sharp if the angle of the grind is correct. O1 has less abrasion resistance. That’s why it dulls faster, and that’s why it sharpens more easily. But whether you want to sharpen quickly on a frequent basis, or sharpen longer and less frequently is a wash in my opinion. And the A2 is actually easy to sharpen if you do a quick touch-up on a regular basis and don’t let it get really dull between sharpenings.
For my money, the longer lasting edge, and slightly improved rust resistance are important factors. One other thing–A2 is more resistant to annealing, which means if you ever have to use a grinder to fix a chipped edge, you will be less likely to ruin the temper of your blade, should it get too hot.
My choice would be A2–or until someone comes out with plane blades made of ATS-34 or 154CM. :->
DD

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JohnBrien June 12, 2009 6:27 am

I actually use both A2 and O1 blades in my planes, now the biggest difference is I make my own wooden planes so changing and / or pulling a blade takes about 30 seconds for me. (With a bit of practice you can replace and tune a blade in a wooden plane before they other guy gets his blade out of a western plane) I keep a few Hock blades of various widths around and swap them out as needed. I also made a cart to holds my waterstones and grinder that I can be wheeled beside me when I’m working on a project to touch up my blades and chisels when need be.

So why both types, well It’s pretty simple to me, the A2 does keep it’s edge longer but is not as fine without great effort, and I can’t charge a client for sharpening time. The A2 to me is good for really digging into stock and when I am away from my shop and sharpening station on a client site. I use my O1 blades for finishing and fine tuning stock and project components. You can get really fine shavings with the O1 really quickly if you are a fair sharpener. I also feel that the O1 steel is more forgiving to give a decent edge if you are not the best at it.

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Mark June 12, 2009 12:24 pm

I make all my shapper blades from worn out sugger beet cutters, they are made of a multy layer steel composite, I can make about a 3/4 mile of molding between sharpenings! I think it is to some extent self sharpening.

The ony problem is you need friends at the suger beet factory, and they dont wear out vey often (the blades not the friends).

Is their a trade name for this mls, also I assume It would come in many variations, what would be the best for shaper blades, would it be any good for hand planes? as currently I am all out of used beet blades and need to buy some.

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Chris June 12, 2009 11:29 pm

Does anyone have a good reference book/article on quenching techniques for various hardness/sharpness/brittleness/ductility results with ‘common’ available steels? (old files etc.)

Roy Underhill had a few examples on a Woodwright’s show where he used an old file for making a cutting tool for wooden threads (which I stupidly did not record) – but other than that – I can’t remember much from our Woodworker magazines ever.

Thanks!
Chris

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Vic June 13, 2009 4:15 pm

Great info Marc! Very cool going to a respected expert for a full explanation. I wonder what Tom Fidgen uses? He’s all handtool.

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Tom June 15, 2009 4:56 am

What a great answer from Ron Hock! He gives detail down to the near-molecular level :)

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Tom June 15, 2009 5:13 am

A little Wiki is a dangerous thing: The Wikipedia article on “Tool Steel” points to a table by Paxton & Vierling Steel (a supplier), which recommends M2 steel for woodworking chisels. It doesn’t state the case for M2 as clearly as Ron Hock did in this blog.

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Ben H June 17, 2009 4:49 am

I’m telling you guys, buy a worksharp and you won’t care about sharpening any longer. I can take a plane plade from, dull, chipped, and down right nasty, to leather honed RAZOR sharp in about 5 min. All without water. No mess, just ulrta fine sharpening at it’s best.

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Christian February 16, 2011 8:21 am

I have two planes that I use about the same amount. One has a Hock A2 blade and one has a Hock O1 blade.

My experience: The A2 does hold an edge longer, but not so much longer as to make much difference to me – in fact I end up sharpening them pretty much at the same interval.

The O1 leaves a very fine finished surface. The A2 surface is noticeably rougher (but still fine, if that makes sense).

I’ll qualify here: “Rough” and “fine,” as I am using them, is visually measured by light reflection. I can barely feel a difference with my fingers.

Of note: I also have a Millers Falls blade and a Groz blade in two other planes. The MF blade compares well to my Hock blades (sorry Ron). The Groz blade is well suited for warm butter and not much else.

All this being said, I’m OCD anal and finish sand to 1000 or 2000 grit, depending on what coating I plan on applying (2000 for BLO, 1000 for shellac & copal varnish).

Also, I’m very possibly the worst woodworker ever, so my observations are probably be idiotic. Consume with many grains of salt.

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- Meryl May 28, 2021 11:35 pm
  
  That was awfully hard on yourself. How are you doing regarding your skills now, a decade on?
  
  Reply
Christian February 16, 2011 8:21 am

I’m also 2 years late for this conversation…

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henry June 23, 2011 1:19 am

d2 is awesome if you can get it

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Howard September 30, 2011 8:14 pm

Any discussion of steel hardness and edge behavior is incomplete without consideration for the heat treatment given to the steel. Heat treatment can vary a finished steel’s hardness considerably! Who does the heat treatment and what their process is and whether or not they follow the recipe will all have influence on the final edge and its performance. Variables such as type of oil, brine or fresh water, temperature of the quench, how close the heat comes to the optimal temperature, how soon the hardened steel is tempered, etc. are all influencing variables. It’s as much an art as a science and even commercial heat treatment doesn’t always yield consistent results. Another variable is the steel as not all 01 steels are the same formulation. They can vary relative to carbon and alloying elements. My favorite is from Bohler-Uddeholm. Soooo, all of this is to say that you may end up with quite different edge performance from other folks who are using a tool made from a particular steel even by the same purveyor.

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Nathan January 23, 2015 8:46 pm

What is the average carbide size of O1? Is it smaller than 13c26? Which one can get sharper?(carbide,grain size)

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Eddie May 30, 2017 5:50 pm

Buy an 02 with a replacement 01 blade and you can switch on different woods depending on hardness

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Gary Wright August 15, 2021 2:37 pm

All of my hand planes are older Stanley/Bailey planes. While I have seen a lot of comparisons between O1 and A2 steels, what I don’t know is this: What steels did Stanley typically use for their OEM blades? That is, how does O1 steel compare to the OEM Stanley blades? Will I notice any difference in edge retention?

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- Paul McCuish February 22, 2024 11:03 am
  
  I would like to know this, too. D
  I’d you ever get an answer?
  
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Brian McCue April 23, 2022 2:07 pm

“In general, O1 holds a slightly keener edge and is easier to sharpen. But it also dulls faster and require more frequent sharpening. A2 steel takes a little more effort to sharpen, but you are rewarded with a more durable edge that lasts longer.”

WHAT???

What does “slightly keener edge” mean???

If A2 steel provides a more durable dege then how can O1 steel provide a “keener edge???”

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- Marc Spagnuolo April 24, 2022 6:16 am
  
  Durability and sharpness are two different things.
  
  Reply
- David Weaver March 7, 2023 10:34 am
  
  The edge should be the same from both, but only if both are equally refined during sharpening, and similar in hardness. If that sounds foreign to some, try sharpening with micron diamond powder (found on ebay for about $10 for 1 micron diamond).
  
  If you’re using something like oilstones, O1 will seem like it’s more keen as oilstones are softer than the carbides in A2. Anything else (when a stone can cut all of the parts in both) is due to differences in hardness, failing to finish the job or not dealing with the more persistent A2 burr, even when it’s really small.
  
  Reply
David Weaver March 7, 2023 10:41 am

Cryogenic treatment does compact things in micrographs a little bit, but it’s more important to note that the primary function is to convert austenite remaining after a quench to martensite – we think of steel as tempered martensite. The more that’s converted, the harder the steel will be.

Steel loses a little bit of toughness (it’s actually more brittle) after a session in LN, but gains hardness. If there’s some errant work in the heating (especially minor overheating), Liquid Nitrogen can bail out the heat treater by not creating a situation where a bunch of austenite remain. Austenite stays soft.

Hardness creates strength (resistance to being deformed) but in most cases reduces toughness (the force needed to break a steel sample, not just deform it). This is the reason it’s harder to bend a really hard knife, but less hard to bend it enough to break it.

It’s safe to say that for woodworkers – we value strength over toughness in most cases as additional hardness makes it easier to deal with a burr and supports a finer apex, or feels more keen. If we get into the territory where we’re deforming steel and worrying about toughness instead of strength, something is wrong.

This toughness vs. strength thing can be thought of as trying to bend old doug fir vs. new. the old will stiffen over time, but it will tolerate less bending and could break under less force.

In my opinion, liquid nitrogen is probably an improvement for woodworking steels in general – it’s just less impactful in steels that convert with less remaining austenite. Even O1 is more crisp with nitrogen, but colder finishes invite more warping and we have somehow gotten to the point that a lot of woodworking manufacturers and heat treaters think O1 moves/warps too much.

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A2 vs O1 Tool Steel

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