The refractoriness of mullite sand is a key index to measure its physical and chemical stability at high temperature, which is usually determined by a series of standardized test methods. The following is the method for determining the refractoriness of mullite sand:

I. Test criteria

The refractoriness test of mullite sand usually follows national standards or industry standards. For example, according to the group standard "Sintered Ceramic Sand for Foundry" (T/CFA 0202042-2022), the test method of fire resistance is carried out according to GB/T 7322.

II. Testing methods

Cone test: This is one of the most commonly used fire resistance test methods. Mullite sand is made into a cone-shaped sample with standard size, which is heated in a high-temperature furnace to observe the deformation of the sample at high temperature. When the sample reaches a certain temperature, it begins to deform, and recording the temperature at this time is the refractoriness.

Load softening temperature test: By applying a certain pressure to mullite sand at high temperature, observe its softening deformation temperature. The load softening point of sintered mullite is usually above 1600℃.

High temperature microscope method: the microstructure changes of mullite sand at high temperature are observed by high temperature microscope, so as to determine its refractoriness.

III. Factors affecting fire resistance

Chemical composition: The main components of mullite sand are alumina (Al₂O₃) and silica (SiO₂). The higher the alumina content, the higher the refractoriness. The fire resistance of sintered mullite is usually between 1750 and 1800℃.

Sintering process: Sintering temperature and time have significant influence on the refractoriness of mullite sand. High temperature sintering can improve the crystallinity of mullite sand, thus improving its fire resistance.

Impurity content: low iron content and low alkali metal oxide content (such as Na₂O and ko) are helpful to improve refractoriness.

IV. Fire resistance in practical application

In practical application, the refractoriness of mullite sand is usually between 1750℃ and 1800℃. For example, the fire resistance of some mullite sands with high aluminum content can reach 1800℃. The refractoriness of mullite sand depends not only on its chemical composition and sintering process, but also on its stability at high temperature.

Through the above methods and standards, the refractoriness of mullite sand can be accurately determined, thus ensuring its performance in high temperature applications such as precision casting.