Ouenching will not provide case hardening. Case hardening is a hardened layer 1/16" or less in thickness. If You quench a steel part with enough carbon and alloys, it will get hard, parts under 3/4 thick will get hard all the way through, depending on the material even thicker parts might. If there is much carbon it will be brittle, welds included if there is enough carbon to harden. To case harden mild steel a compound called KaseNit can be used to add carbon to the surface of steel, and when quenched the surface will be hard. You can get it from MSC industrial, follow the directions on the can.