Molybdenum and Tungsten Heat Shields
Introduction of Heat ShieldsAccording to the different requirements of various operating environments, the number of layers of metallic heat shield will be different. And a certain space is reserved between layers for heat preservation. The key of all-metal heat shield’s thermal insulation effect lies in the number of layers of metal plate and the space between the plates. The function of these intervals is to prevent the heat conduction from radiating out from the central area of the heat field. Molybdenum plate has a good performance in thermal reflection, it can reflect the heat to the target parts. All-metal hot zone has the characteristics of zero pollution, so it is suitable for the environment with high vacuum and cleaning requirements. Attentions need to be paid during operation to prevent metal recrystallization, because the metal will be embrittled after recrystallization, and the metal on the heat shield will fall on the fixture or workpiece once embrittled. Therefore, it is necessary to select the corresponding number of shielding layers in combination with the use temperature when designing the furnace body. Molybdenum heat shield is most commonly used in vacuum furnace. Because Molybdenum has the following irreplaceable characteristics: linear thermal expansion coefficient is small, production and processing are relatively easy, thermal conductivity is low and specific heat is small.
Types of Heat ShieldsHeat Shields can be divided into the following types: metal reflective heat shields, interlayer heat shields and graphite felt heat shields. Metallic heat shield is composed of multiple layers of metal plates, spacer rings and support rods. It can insulate and reduce heat loss. Metallic heat shield has relatively high fire resistance, good thermal insulation properties, and thermal shock resistance and corrosion resistance as well. The inner and outer screens of the interlayer heat shield are made of multi-layer metal plates. The middle of the interlayer is filled with ceramic fibers. Although the thermal insulation effect is good, the degree of vacuum can be affected due to the large hygroscopicity of ceramic fibers. Interlayer heat shield cannot be used in demanding vacuum furnaces. Graphite felt heat shield is made of graphite rope or molybdenum wire on the net made of steel sheet. However, graphite is easy to pollute the environment, part of the furnace body is not suitable for this kind of heat shield.
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