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Sometimes brand-new chisels and planes (even from the best manufacturers) don’t hold an edge well. I’ve seen some edges crumple like tin foil after two whacks with a mallet or two strokes on a board.
Weak edges aren’t as common a problem as weak chin lines, but they do happen. When I teach a class of 18 people, for example, there’s always one person with a spanking new tool that would crumble if you chopped a Moon Pie.
My solution to this problem has always been to take the tool to the grinder and create a new primary bevel. Then I grind off just a tad more. I take the tool back to the stones for honing and then (by magic) the tool holds its edge.
The strategy almost always works, but I’ve never known exactly why.
So I went to tool steel guru Ron Hock of Hock Tools looking for answers. As always, Ron set me straight. There could be two culprits: too much heat or too much oxygen during the manufacturing process.
“Should the blade be subjected to temperatures in excess of the steel’s critical temperature (the temperature at which the iron crystals transform from ferrite to austenite) the steel will tend to form large grains, which don’t stick to each other as well as we’d like,” Hock writes. “This will cause the resulting steel to be very brittle and crumbly, though it will test as properly hard with a Rockwell hardness test.”
If a tool breaks, you can see evidence of this problem, according to Hock. In a well-treated tool the fracture should look a very fine-grained gray (almost like gray primer paint).
“If you see sparklyness instead, it’s been overheated,” Hock writes, “Which is probably why it broke and you’re looking at it.”
Because the cutting edge of a tool is typically the thinnest part of the tool, it’s the easiest part to overheat, even if the overheating is brief.
The other culprit is oxygen. As steel approaches its critical temperature, the carbon is released and is free to migrate about the steel. If there is air present when it reaches the surface (such as when heat-treating in air with a torch or forge) the carbon atom will run off with the oxygen atom to become carbon monoxide or carbon dioxide and the carbon is lost from the steel, according to Hock.
Most professional heat treaters use furnaces with atmosphere control (vacuum or inert or carbonaceous gas) to minimize this problem, which is called “decarburization.”
“This creates a low-carbon skin on the steel,” Hock writes. “This would not be a big deal except for the fact that the flat back of the tool is the cutting edge, and any loss of carbon results in a loss of hardness. Here again, the edge takes it in the shorts with the most to lose and the least to lose it from.”
Both of these problems can completely ruin a piece of steel through-and-through. But usually the damage is localized, and you can get to the good stuff by grinding away some of the bad stuff.
Just tell your spouse you’re exfoliating.
– Christopher Schwarz
