You are probably right. With case hardening (Kasenit from Dixie Gun Works) a file bounces off the surface. Without it a file cuts. this is true of a drill bit. I was taught a procedure, the result was a reliable part that worked well and did not break. I was told heating a firing pin made from a drill shank to a dull glow and then tossing it in oil wasn't quite enough as the tip shank would be too brittle and would break.
The gunsmith I learned this from was apprenticed in Germany in the 1920's. He called the procedure "carbonizing".
The fellow made most of his gunsmithing tools , including headspace gauges that had a 40 T.P.I. adjustable plug that could be either preset or adjusted to exact headspace with a long screwdriver down the bore. What was remarkable was that the plug would turn out and he could read the exact headspace because the head of the plug plug was stamped with index marks like a micrometer. One full turn was. 0.025" , never seen that since.