Most Deltech furnaces are designed for maximum operating temperatures in the 1500-1800 deg C range and for operation in air, so molydisilicide heating elements are used because of their ability to quickly attain and sustain high temperatures and their affinity for oxidizing atmospheres. Despite their robust appearance they are a fragile, glasslike material with low mechanical shock resistance. Basically, the molybdenum core is coated with quartz glass. The elements become nearly liquid as they approach the maximum temperature, and the glass surface reacts with the oxygen in the air atmosphere to form a renewed coating of protective glass on the element surface. The maximum element temperature is always 100 deg C higher than the maximum rated furnace temperature, so for example we use 1800 deg C elements in our 1700 deg C furnaces. Operation in inert atmospheres requires either the use of a higher temperature element (for example 1800 deg C elements for 1600 deg C operation in argon), or a restriction on the maximum furnace operating temperature to accommodate element limitations.
Element manufacturers warrant only against manufacturing defects because many variables affect element life. For example, operation in a reducing atmosphere or rapid heating and cooling cycles are hazardous to element health.
Here are some tips for preserving the life of the molydisilicide heating elements in your furnace:
—Condition new heating elements by ramping the empty (no product) furnace up to 1550 deg C and hold for one hour. Then ramp down or simply turn the furnace off. This applies a thin, protective glass layer to the elements.
—If you must operate your furnace for a prolonged period at temperatures below 800 deg C, use the initial firing procedure mentioned above to recondition the surfaces.
— Reduce the ramp rate for temperatures 100-200 deg C below the maximum rated furnace temperature to 60-100 deg C per hour.
Contact Us for information on obtaining silicon carbide, molydisilicide, and zirconia heating elements from Deltech. We have many sizes in stock and we offer discounts.