They are most certainly hardened. Usually not thru hardened but most likely surface hardened. You will be hard pressed to find a metal gear that isn't hardened in a gearbox. The method of hardening will also vary quite a bit depending on the conditions the gearbox will encounter in service, i.e. loads, speeds, etc...The realm of heat treatment is a pretty broad one. If you post your question in the gear and pulley section, they might be able to give you a better idea of what state of the art is in terms of processes.