craigd, mild steel can be hardened, even though you won't be able to get it to the same Rockwell hardness as the steel you might use to make a knife or spring.
You can even work harden mild steel. Just take a soft piece of mild low carbon steel wire and repeatedly bend it, and it will become hard and brittle, and eventually break. Bending the wire has changed the microstructure and made it much harder. But you can take that work hardened wire, and anneal it to return it to a soft ductile state... if done correctly. Steel mills routinely anneal mild steel because the cold rolling process can make it too hard for the customer requirements, such as steel that is intended for stamping and forming operations.
