Performance requirements of refractories for electric furnaces

What are the performance requirements of refractories for electric furnaces

1. Refractoriness refers to the resistance of refractories to high temperature. It refers to the temperature when refractories soften to a certain extent after being heated, but it is not its melting point. According to the refractoriness, refractories can be divided into: ordinary refractories, with a refractoriness of 1580-1770 ℃; advanced refractories, with a refractoriness of 1770-2000 ℃; super refractories, with a refractoriness of more than 2000 ℃. Refractoriness is an important performance index of refractories, but it does not represent the highest temperature of refractories. In practical use, refractories also have to bear certain pressure. Therefore, it is necessary to consider the high temperature structural strength of refractories, that is to say, they bear certain pressure without deformation under high temperature. For example, the fire resistance of nz-40 refractories is as high as 1730 ℃, but the maximum service temperature is only 1350 ℃.

2. High temperature structural strength high temperature structural strength is evaluated by load softening point. The load softening point refers to the temperature at which the sample begins to deform (the deformation is 0.6% of the original sample) under a certain pressure (2kg / cm 2 ≈ 0.2MPa) and heating at a certain heating rate. This temperature is the starting point of load softening of the refractory. The sample size is φ 36 × 50mm. The fire resistance of nz-40 clay brick is 1730 ℃, while the start temperature of load softening is only about 1350 ℃. Therefore, its maximum operating temperature is not more than 1350 ℃.

3. When the high temperature chemical stability is used to manufacture the heat treatment furnace without tank carburizing atmosphere, because the high carbon atmosphere has a destructive effect on the common clay brick, the refractory material inside the furnace wall needs the refractory brick with iron oxide less than 1% (i.e. anti carburizing brick). In the manufacture of electrode salt bath furnace, because of the scouring effect of molten salt on refractories, the refractories of crucible must use heavy refractory bricks or refractory concrete. The brick laying of electric heating elements shall not have chemical effect with the electric heating body, and the high aluminum brick shall be used as the brick laying for the iron chromium aluminum electric heating body.

4. In the working process of heat treatment furnace, the working temperature of refractory often changes rapidly. For example, when the furnace is heated and the trolley type furnace is normalizing, the working temperature fluctuates greatly. If the refractory is not able to resist the rapid change of temperature, it will be damaged prematurely. At present, the standard method is to heat one end of the standard brick in the electric furnace to 850 ℃, and then put it into the flowing cold water for cooling. This is repeated until the part of the brick falls to 20% of the original weight due to the rupture. The number of cold and hot alternation experienced is taken as the index of the resistance to quench and heat.

5. The volume stability of refractory products at high temperature will expand or contract due to the change of its structure when they are used for a long time at high temperature. This change of volume is different from the general thermal expansion and cold contraction, and it is irreversible, which is called residual expansion or residual contraction. If the volume of refractory changes too much, it will affect the strength of masonry and even cause the collapse of masonry. Generally, the volume change is not more than 0.5-1%. In addition, there are other performance requirements, such as smaller volume density, thermal capacity and thermal conductivity. Good electrical insulation performance is also required for resistance furnace.

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