The design and research of heating elements for vacuum furnaces are crucial aspects of vacuum heat treatment technology.

I. Selection of Heating Element Materials

1. Graphite Materials
• Graphite has several advantages, such as high-temperature resistance, low thermal expansion coefficient, and excellent thermal shock resistance. It maintains good mechanical strength at high temperatures (1700°C to 2500°C).
• Graphite also has a high melting point and low vapor pressure, which allows it to create a purifying effect in a vacuum environment. This results in bright surfaces on the processed workpieces.
• The electrical resistivity of graphite changes little with temperature, making it suitable for low-voltage, high-current power supplies. Additionally, graphite is cost-effective.
2. Metal Materials
• Metal heating elements (such as tungsten, molybdenum, and tantalum) are typically used in vacuum furnaces operating at medium and low temperatures (<1500°C). Their design must include measures to prevent deformation at high temperatures.
• Metal heating elements have the advantage of good machinability but are more expensive and prone to oxidation at high temperatures.

II. Structural Forms of Heating Elements

1. Single Rod Heating Elements
• Current is directly introduced from both ends of the graphite rod. This structure is simple and suitable for small vacuum furnaces.
• It can be designed with a hollow structure to improve thermal efficiency.
2. Assembled Rod Heating Elements
• These consist of multiple graphite rods and insulating components assembled together. They are not limited by the size of raw materials, making them easy to process and maintain.
• They are suitable for large vacuum furnaces and can reduce manufacturing and operating costs.
3. Flat Plate Heating Elements
• This is a newer structural form that has been adopted in recent years. It is suitable for applications requiring large-area heating.

III. Design Considerations

1. Temperature and Power Matching
• The heating power should be reasonably selected based on the working temperature of the vacuum furnace and the size of the effective working area.
• For graphite elements, the voltage should be below 200V, typically in the range of 170-30V.
2. Temperature Uniformity
• The layout of the heating elements must consider the temperature uniformity inside the furnace chamber. This can be achieved by optimizing the position and number of elements.
3. Service Life
• The main causes of graphite heating element degradation are oxidation, collisions, and aging. The service life can be extended by optimizing the structure and operating environment.

IV. Research Directions

1. Material Optimization
• Research on new composite materials or coating technologies to enhance the oxidation and corrosion resistance of heating elements.
2. Structural Optimization
• Using computer simulations and numerical analysis to optimize the structure of heating elements, improving thermal efficiency and temperature uniformity.
3. Energy Efficiency and Environmental Protection
• Developing energy-efficient heating elements to reduce energy consumption and minimize environmental impact.