I thought the teeth on those things were induction hardened to a point where a steel file would not cut, or does that vary from brand to brand?
Induction hardening is about the only way to keep the teeth hard while keeping the blade flexible. It doesn't confer magical properties to the metal, though. Files cut steel, even when it is hardened, just not as well, according to how good a file you are holding and how much the steel has been hardened.
Consider the results with your round file on a chainsaw tooth that has been burned hard with a chain sharpener. Those teeth get burned blue sometimes, losing all their flexibility while becoming so hard they are almost impervious to files. I have dulled many a file on a machine-ground chain, although I never found one so hard that the file couldn't sharpen it. It just took a LOT longer. If you have done much hand filing, you should have noticed your file skating off the chain teeth every now and then without removing much steel.
The feather file from Silky does a nice job. When you begin filing each tooth, the file skates over the top for a little bit, and then begins to cut smoothly as some of the metal is worn away. Each tooth has previously undergone what is called "work hardening" during its use. This causes the file a bit more trouble at the beginning of each tooth, but that work hardening doesn't extend deep into the steel, so the file slowly cuts through the extra-hardened area, then moves right on through the induction hardened teeth without any trouble.
I cannot say whether the file is especially hard, but I don't think so. Induction hardening isn't the same toughness as the hardening done to a high-carbon file. But they will snap in two if hit very hard.
As I recall, the Silky blades are induction hardened and then they add another electroplate layer of nickle, for just a bit better tooth-life.
New information:
