Enamel Ground Coat Reacts During Firing
During the firing process of the enamel ground coat, complex physical and chemical reactions occur between the surface of the iron blank and the ground coat. In the oxidizing atmosphere, an oxide film is formed on the surface of the iron billet, so that the molten ground coat can infiltrate the iron billet. Experiments have shown that when enamel is fired when the oxidizing atmosphere is insufficient, even if there is a sufficient amount of high-quality adhesive, it is difficult or impossible to form an adhesive layer.
The firing process of enamel ground coat can be roughly divided into five stages, and the physical and chemical reactions of each stage are as follows:
1. The first stage: room temperature~150℃
The adsorbed water in the layer is excluded, the adsorbed water in the powder layer evaporates in large quantities, and some grinding additives begin to decompose.
2. The second stage: 150~400℃
Structural water is removed, organic matter is decomposed, and oxidation reaction occurs on the surface of iron billet. Structural water in the clay is removed, tiny cracks appear in the powder layer, iron is decomposed, and the iron billet is oxidized to form iron oxide.
3. The third stage: 400~600℃
The powder layer is sintered, and the iron blank and the ground coat undergo a redox reaction. Continuing to remove structural water, the porcelain layer is completely sintered and begins to soften, and part of the gas channel is closed.
4. Fourth stage: 600°C ~ firing temperature
Adhesion layer is generated. The powder layer is completely melted, the adhesive is replaced by iron and deposited on the surface of the iron billet, and electrochemical erosion is carried out.
5. The fifth stage: continue for a period of time after reaching the firing temperature
The small holes are gradually bridged, and the adhesion is good. This stage is the key to the quality of the product. The ground coat is melted and has a certain fluidity, the residual gas is further removed, the redox reaction between the adhesive and iron in the ground coat continues, the melting of iron oxide, the mutual penetration of the ground coat and the iron blank is accelerated, and the dense intermediate layer is thickened. The holes are closed and the firing is over.