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lwllms

544 posts in 1933 days


#1 posted 11-17-2012 03:19 AM

Sorry for the delay here. I do paperwork for my business in the evenings and last night I had a lot that had to be done.

Let’s talk about A-2 steel.

Does A-2 steel have greater wear resistance than oil or water hardening steels? Yes, in some ways and no in other ways. A-2 has a greater resistance to abrasive wear than oil or water hardening steels when using ASTM testing methods. There are a lot of testing machines that meet these standards and they all test a flat surface rubbing against a flat surface. These tests aren’t designed to test acute cutting edges like one finds in woodworking tools.

An other thing is there are more wear mechanisms than abrasive wear. Most American steel producers seem to use abrasive wear as the indication of wear resistance. What is the wear mechanism that causes the dulling wear to cutting edges? I’m not an expert in tribology so I can’t say for sure. My best guess is that it’s either temperature induced corrosive wear, adhesive wear or a combination of the two. This chart from Uddeholm, a European based steel company shows that both A-2 steel and O-1 have nearly identical resistance to adhesive wear:

Abrasive wear is interesting. Abrasives are just a collection of very small cutting edges. For abrasion to work the abrasive needs to be harder than the material being abraded. For instance, I use Arkansas stones for my fine stones. The abrasive in novaculite, which is what true Arkansas stones are made of, is slightly softer than the carbide inclusions in hardened A-2 steel. Arkansas stones won’t abrade A-2 steel because of this.

This leads me to a question. If abrasion is the wear mechanism that causes dulling wear in woodworking edge tools, what in the wood is harder than hardened tool steel. For the most part, the answer is nothing. Yes some tropical woods and a few domestic woods like bois d’arc and western cedar have some silica in them. Silica is hard enough to abrade tool steel but domestic woods normally used for woodworking have, at most, trace amounts of silica or anything else hard enough to abrade hardened tool steel. If abrasion was the dulling wear mechanism, edge tools working woods like white pine, poplar or cherry should last indefinitely. They don’t. So when do you, as a woodworker, encounter the abrasion resistance of A-2 steel? When you sharpen or grind it. I does leave you thinking, “Boy, that was hard as h*ll to sharpen. That edge will really last.” It’s a misconception, though.

What are inclusions, anyway? Inclusions are impurities that aren’t structurally part of the steel. Think pebbles in a big sheet of ice. If you could make a big cutting edge out of that ice, a lot of the pebbles that wind up on the cutting edge will just fall out. The carbide inclusions in A-2 steel do the same. They leave you an unrefined ragged edge. This is why some people suggest relatively obtuse bevels on A-2 steel. They’re trying to encapsulate as many of those inclusions as they can. Inclusions, of any type, were traditionally considered a flaw in steel for edge tools. I don’t see why that should change.

Were do these inclusions come from? The carbide inclusions in A-2 steel are formed during heat treating, mostly during a relatively long high temperature soak. That high temperature soak is above critical temperature. One of the rules of heat treating is for a fine grained steel bring the steel temperature only to critical temperature and hold it there only long enough to get a uniform change from ferrite to austenite. The longer you keep it at critical temperature or the higher the temperature the more coarse the grain will be. With highly alloyed steels this can be mitigated to a degree with either a stainless foil wrap or by using an inert atmosphere furnace but only somewhat mitigated. With A-2 the steel surrounding the carbide inclusions will have a more coarse grain than properly heat treated oil or water hardening steel. There’s no way around this if you want to form the carbides during heat treating. So even if all the carbides fall out of the cutting edge of an A-2 steel tool, the remaining steel is relatively coarse grained and won’t take as good an edge as properly heat treated oil or water hardening steel.

I don’t use Western cedar, bois d’arc, or other woods with a high silica content. I don’t make bamboo fishing rods of cane with a high silica content. I don’t know what to tell someone who does. I use the domestic woods normally used in woodworking and there’s no gain for me with A-2 steel but A-2 has a lot of down sides for me.


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