An industrial heat treatment furnace has a cylindrical chamber which is sealed by an inner door which is mounted on an outer door. The inner door is made of thermally insulating material and has a number of radially arranged leaf springs fixed to its outer surface. These leaf springs have their outer ends fixed to the inner door and their inner ends attached to the flange of a central boss. This boss has a screw thread to receive a bolt attached to the outer door. The bolt is attached to an external knob which enables the inner door to be pressed against the furnace wall to form a gas tight seal.
