During the sintering process of graphite crucibles, the organizational structure of the green body has a series of changes, the changing part is mainly the plastic refractory clay. The basic process of the crucible sintering can be divided into the following four stages to illustrate:
1).Preheating and baking stage. The temperature at 100~300 degrees, it is the process of graphite crucibles exclude residual moisture.The heating speed should slow and the kiln roof should be open, it should control the temperature strictly especially when the graphite crucibles in the residual moisture content exceeds 3% , otherwise the crucibles will have the crack and it maybe explode.
2).Low temperature firing stage: 400-600℃ is the Low temperature stage of sintering of the graphite crucible. With the increase of the temperature of the kiln, the combination water in the crucible begins to decompose and remove. the main composition in clay are Al2O3 and SiO2, once formed,they are in the free state,the glaze layer of crucible surface has not yet melted, heating speed should be still slow, if the heating speed is too fast and uneven, it also prone to explode and the collapse of the crucible bing .
3).Mesothermal sintering stage:700-900℃ is the mesothermal sintering stage of graphite crucible. At this time, the amorphous Al2O3 in clay may be partially converted into y crystalline Al2O3 when the kiln temperature gradually increased, the Al2O3 and Si0'exist in the free state form hard silicate cured material , it connect the graphite and clinker in crucible and then solidified into a whole. In the medium temperature sintering process, the surface glaze layer in graphite crucible melt with the increasing of temperature gradually and form a layer of glass like glaze film, enclosed or covered on the surface of the crucible, it protect the crucible from oxidation. During this period, it is better to heat up quikly and The original flame should be good in the kiln .
4) High temperature firing stage:The temperature of this stage temperature is iooo-1200cc. The clay in graphite crucible after sintering in the medium temperature form curing, gradually transformed into mullite, such as 3A1203+2Si07:3Al203'2Si0. (mullite). During this period, the heating rate should be faster than in the middle temperature (commonly known as the fire process), the furnace flame should be controlled in the neutral flame or reducing flame. the glaze layer has been completely melted during the crucible in high temperature sintering process, , forming a glass like glaze film,close to the crucible surface. If the temperature exceeds the maximum temperature limit of sintering glaze, it easily cause the oxidation of the crucible. Therefore, in the high temperature sintering process, we should observe the glaze layer melting condition of graphite crucible and avoid the burnt and not up to the sintering. Mullite (3A1203'2Si02), a senior Department of physical and chemical properties of refractory minerals and a stable. The density, hardness, refractoriness and softening point are very high, low coefficient of thermal expansion. In the structure of the crucible, it is an important role of consolidation and hardening. It can be known that, in the crucible sintering process, due to the thermal effect of plastic refractory clay, the consolidation of graphite and clinker has a certain mechanical strength. Therefore, high and low of the sintering temperature, temperature control in the process of operation, it will has an important influence to the quality and service life of graphite crucible products ,.