Case hardening

Only after I had done the differential tempering process on my blade and spring, did I find out about case hardening. I wasn't sure what steel I had used for knife blade and spring, so I searched"How to harden and temper mild steel?" I came across this video and many articles on case hardening. I learned that gunsmiths use mild steel to machine parts, I thought to myself "Isn't mild steel to week for all the wear and tear a gun goes through?"then I learned what case hardening is.