Case hardening is a term that is used to describe a process in which changes the composition of the surface layers of ferrous alloys a hardened surface layer can be produced to made the surface harder and used on many bridges such as Telecasters, Precision Bass, Jazz Bass and others such as neck and control plates. To get more technical carburizing is a form of case hardening which results in a hard surface layer being manufactured with ferrous alloys. You heat the alloy to the austenitic state in a carbon rich atmosphere so that carbon diffuses into the surface layers, then quenching to convert the surface layers to martensite. This information is from the Engineering Materials Pocket book, CRC Press.

Many repairman have noticed when trying to add a switch to a Telecaster lever plate how hard it was to drill through. Fender neck plates are case hardened too. It keeps the materials strong and especially when height adjust screws are used, it helps prevents deep pits and surface dents from the pressure. Early lap steels had the bridge units case hardened to keep the bent steel firmer and to keep the strings from cutting into the bridge saddles.