Karl Townsend wrote:

If you hardened it so that the file is skating, this may be where it breaks one of these
days. Better make sure you have stock on hand to fab a new shaft.

Wes

Na, its just a surface phenomena. The torch is set slightly
carbonizing when using a rose bud so the flame don't pop out. When you
get something red hot, you get a bit of carbon in the top couple thou.
Just a light shot with a grinder takes it off. I've done this many
times.

So it was low carbon steel. Glad you have it under control.

Wes