The heater case or housing can be aluminized steel, stainless steel, or the heater can be supplied, in some instances, without a case.
Aluminized steel is by far the most common case material.
This assures a heater case that will outlive the heater itself. The housing is typically riveted together. The rivets, along with slots in the heater case, allow for controlled expansion and contraction during heat up and cool down. Some applications require the slots to be removed, which can be achieved for many designs. Stainless steel cases are used in both food and semiconductor applications.