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General Information
High temperature electric furnaces, operating in dry gases including hydrogen, dissociated ammonia, nitrogen, argon, helium, mixtures, or in rich exothermic gas of 78°C dew point or less, or in a vacuum, can effectively utilize molybdenum-heating elements. Such elements can be formed, by experienced fabricators, and can be expected, under proper conditions, to maintain temperatures up to approximately 1900°C. These temperatures can be maintained almost indefinitely, if proper operating pre-operational phases of 1) heating, 2) cooling off and 3) atmosphere or vacuum maintenance.
Molybdenum may be brittle at room temperature, particularly during cold weather. After use at high temperatures, elements usually become very brittle when cooled to room temperature. Care must be taken not to mechanically shock or stress the material or to thermally shock a cold element by too great a change in electrical input.
Molybdenum elements can be welded and spliced, but must be handled with care because such joints may be quite brittle, especially at room temperature. Heat is frequently used to increase ductility during forming especially for thicker sections. Wherever possible, therefore, Molybdenum elements should be made of one continuous piece of wire, rod, or ribbon.
First Heat-Up
The first heat-up of a furnace is extremely important to the element's life as well as the furnace refractories' requirement to be well dried. After thoroughly purging the furnace and introducing the gas or obtaining the vacuum, an allowance at least 48 hours is recommended to bring a newly built production sized unit up to 1500°C. An additional (8) hours should be used for each 200°C increase above 1500°C. Further, it is suggested that, if practical, the new furnace be taken up to about 100°C higher on this initial heat-up than the temperature of maximum intended operation. This procedure tends to stabilize the physical characteristics of the element and refractories.
Cooling
The cooling of a furnace having heavy gauge Molybdenum elements should be handled as carefully as the heating with (6) hours being the bare minimum of gradual reduction time before turning the power completely off. This precaution prevents undue stressing of the refractories. Furthermore, to prevent oxidation of the molybdenum, a protective atmosphere flow, or continually pumped vacuum must be maintained until the heating element is below 400°C. A minimum temperature of 1000°C is recommended for idling a Molybdenum element furnace.
Construction Considerations
The basic three fundamental conditions are recognized as essential for practical success in building industrial Molybdenum element heated furnaces:
- The use of wire of large cross-section with low voltage across the elements.
- Avoiding, as far as possible, any contract of the heating element with ceramic furnace parts.
- Use of suitable, (often water-cooled), terminals for the heating elements.
If thin wires or ribbons are used as leads, corrosion, oxidation, carburization, etc may soon reduce their effective cross-section. This can lead to the early failure. Larger cross-sections can sustain more surface loss or degradation without serious effect. In the case of wire and rod, this problem can be minimized by doubling or tripling the material back and forth on itself or by welding a larger diameter section to the end of the element to form the terminal.
In large furnaces the design is similar to that used for furnaces wound with nickel chromium elements of coiled wire or over bent rod or ribbon elements. This construction allows for direct radiation of heat to the work.
Support of Molybdenum elements at the higher temperatures is a special problem because of reaction with many materials at the points of contact. Allowing the element to hang freely minimizes the number of contact points. Using certain materials (e.g. high purity Al2 O3 up to 1700°C, or Molybdenum hooks up to 2000°C), prevents this reaction damage. It is also essential to insure that other materials do not inadvertently contact the elements. This is a particular problem for floor elements that are subject to contact by falling pieces of refractory, insulation, or process parts. Furnace design should keep elements well clear of the floor, or other places where debris might accumulate.
Molybdenum and high purity alumina refractory are expensive. Therefore, a Molybdenum element for a high temperature furnace can be a very expensive item, when compared to the same size nickel-chromium or silicon carbide furnace. On the other hand, Molybdenum elements properly applied and cared for last very long. Barring accidents, they may last many years and, therefore, can have the lowest overall cost.
Disclaimer
To the best of our knowledge, the information contained in this site is accurate. However, National Element Inc. does not assume any liability for the accuracy or completeness of data. Determining the suitability of information or material for any use, or the manner of use. The furnace manufacturer is solely responsible for handling, installation, and control of heating elements, their cycling and furnace heat up time. Maintenance schedules for thermocouples is required for proper control of elements, we recommend they be replaced once a month minimum, depending on the type being used.
Procedures mentioned are merely recommendations to follow as a guideline and are not intended or implied as operating instructions regarding furnace equipment. Refer to furnace manufacturers operation manual.
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