Reply To: Ashley Isles Chisels
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@ed — we can agree that any metal which requires both a different angle and attentiveness should be banned 🙂
It’s often said that O1 “can” get sharper, for example, Ron Hock’s summary of the differences between the two steels, but as with many things, the devil is in the details.
You know tool steels contain carbon, about 0.5% up to over 2%. The minimum level of about 0.5% is required to allow the steels to harden to a decent level, say 60 HRC level during heat treating. The excess carbon above 0.5% plays little role in the hardening of the steels, rather it is intended to combine with other elements in the steel to form hard particles called carbides. Tool steels like A2 contain elements such as chromium, molybdenum, tungsten, and vanadium to combine with the excess carbon to form chromium carbides, tungsten carbides, vanadium carbides, etc. These carbide particles constitute from less than 5% to over 20% of the total volume of the microstructure of the steel. The actual hardness of individual carbide particles depends on their chemical composition — chromium carbides are about 65/70 HRC, molybdenum and tungsten carbides are about 75 HRC, and vanadium carbides are 80/85 HRC.
Who cares about carbides? Well, people with concerns about durability and sharpness. Why? Because carbides are like granite rocks in cement — they are the hardest thing making up the mix, and take the most wear. The downside is that like rocks in cement, carbides are also the largest individual thing in the mix, sometimes dozens of microns.
This, btw, is Lee Valley’s not-so-secret sauce with their PMV-11 steel. It’s just a more expensive mfring process whereby the steel is atomized into tiny droplets rather than cooling in a huge vat. Keeping the droplet size small allows super-rapid cooling which minimizes the carbide size, and they also probably use tungsten instead of simple chromium and Mo, so their carbides are harder than those in A2.
OK, so carbides are hard but big. How does A2 ever get sharp if it’s these huge chunks of 50 micron carbide? Well, as I mentioned above, only between ~ 5-20% of the A2 contains any carbides at all, and just that small percentage is what makes A2 so much more durable, difficult to sharpen, etc, etc. So 80-95% of your A2 chisel can get every bit as sharp as your O1, and, depending on the carbide size and distribution, maybe even more.
The steeper angle is because the carbides are easier to break out of the surrounding matrix due to their size — you can get leverage on them. So if they break out, they leave a hole, therefore not a perfect edge anymore. By fattening that angle you put more metal behind the large carbide particles at the edge, leaving more volume in which the big iceberg of the particle can be safely submerged, and reducing any leverage available against the particle.
Lastly, O1 has carbides, too. That manganese in Hock’s O1? Yup, manganese carbide. So O1 has these issues, too, just to a lesser extent because yikes, too much coffee again, I’ll cut this off here with apologies!
TL DR it’s not quite so cut-and-dry