But a good modern file will skate across a 63-64 RC hardness blade. I've been running into some newer files including Nicholson that are not as hard as the older ones though. I can cut a little with a 40+ year old Black Diamond into a small 6" Nicholson I bought last year. But if a file skates against the blade you should be fine.
I have some Nicholson Swiss needle files in a set that are just as good as they ever were and a few mill files as well. A flat mill file tested at RC 66.+ when I had access to a Harness tester I tested it just to see. All from the 70's.
I looked into the changes in files just a while ago. All those files from the 70's were basically high carbon like 1.5% or higher. If you have an old Black Diamond or a Grobet and the file skates you're probably 64+ hard. I use my half round to check hardness and use the round side. It's a Nicholson as most of my files are, I have a couple of Grobet files too and they held up pretty well too when I was a welder. I wore the heck out of files on the edges of laser cut steel and the Grobet and Nicholson files held up the best, had some Heller files that didn't last nearly as long.
Now you know more than you wanted to know about files.
