Vacuum heat treating furnaces which employ electrical resistance heating elements are well known. Popular designs are presented in U.S. Pat. Nos. 4,559,631 and 4,259,538.
A typical vacuum furnace has a furnace wall and a hot zone chamber of a circular cross-section which houses a series of banks of axial-spaced electrical resistance heating elements suspended from an inner wall of the hot zone chamber by a series of support rods. A heating element is generally made from graphite or molybdenum alloy, and generates radiant heat in response to electrical current passing therethrough.
Over the life of an average furnace the heating elements are subjected to many expansions and contractions as a result of hundreds of heating and cooling cycles. Since only the ends of each of the elements is fixed, these heating and cooling cycles can cause the elements to undergo deformation. As a result of this deformation, the heating elements tend to bow. Stress caused by such deformation can also result in fractures which in turn necessitate replacement of the heating elements.