Refractory materials are used in furnaces to withstand high temperatures. These materials are able to withstand extreme heat without melting or breaking down. They are used to make the inner lining of furnaces, which helps to insulate the furnace and protect it from damage.
In high temperature furnaces, such as crystal growth furnaces and sapphire growth furnaces, all-metal hot zones are used. These hot zones are made of metals like molybdenum, molybdenum-lanthanum, TZM, tungsten, and tantalum. Molybdenum is the most commonly used metal for temperatures ranging from 1000-2500°C (1800-4532°F). Tungsten is used for temperatures higher than 2500°C, while metals with ordinary heat resistance, such as steel, can be used for temperatures lower than 1000°C.
Heating elements in high temperature furnaces can be made of metal or graphite. The choice of material depends on the operating temperature. For low-temperature operations, nickel-chromium alloys are commonly used. For higher-temperature applications like hardening or brazing, molybdenum or graphite are popular choices. For temperatures above about 1482°C (2700°F), a refractory metal like tantalum is often used, although graphite can also be used.
In box furnaces, which are the most common and basic type of heat treat furnace, the furnace chamber is made of highly insulated steel. The insulation can be fibrous (blankets or mats) or rigid (boards, slabs, or bricks). The temperature control in these furnaces is important, and better units will promote tighter temperature control.
In non-atmosphere controlled furnaces, burners fire directly into the furnace chamber. In controlled atmosphere furnaces, the burners are designed to isolate the firing from the load in the furnace. Box furnaces are versatile and can be used for various heat treatments such as annealing, pack-carburizing, and hardening of low alloy steels.
Overall, the choice of materials used in furnaces to withstand high temperatures depends on the specific application and operating temperature range. Refractory materials, all-metal hot zones, and appropriate heating elements are all utilized to ensure the furnace can withstand and operate effectively at high temperatures.
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