A Nicholson file is quite hard, probably upper 60's. Valtitan files are Rc72 and are harder than Nicholson files.
The situation is that a file won't "bite" anything much higher than about Rc 54. Above that it will slide. Above Rc 60+ it might as well be glass you are trying to file.
The following is a rough guide (not an accurate Rc reading):
When checking by file it is more of the sound and feel than actual removal of material. A file is a good ,quick check to see it the HT worked, but a poor determination of Rc value.
If a blade is filing any at all, it is not fully hardened, or is greatly over tempered, probably below Rc54.
If it slides with moderate pressure, but has some resistance it is in the mid to upper Rc50s.
If it slips without any resistance, it is probably above Rc 58.
The presence of decarb (rind) and overheating of the edge can skew file readings a lot. If you get a low reading (lots of "bite"), don't despair yet. Sand the edge down a bit on the belt grinder and test again. Once past the bad steel it will often skate along just fine.
I answered part of this question off the forum for Pohan. Here is some info that may also be of use to those getting into HT.
..........The other thing is the temperature of the tempering oven. If you can check it with an ACCURATE thermometer, make sure that 400F is really 400F. Many kitchen ovens are off by as much as 10% (and they can swing as much as 50 degrees total between on and off), which could mean a temper at 360F or one at 440F. Even a kiln is usually much less accurate at the bottom of the range (they are designed for high temperatures). The thermocouple is designed for a temperature maximum, and does not read well at either extreme, especially the lower limit. A 2500F type K thermocouple is most accurate between 1500F and 2000F. I have a 0-600F unit with a Pt100 thermocouple that is extremely accurate at the temper readings. I use it to check the temper temps...........
Stacy
