Defects such as oxidized inclusions, surface pitting, shrinkage shrinkage, cracks, etc., are produced immediately after the molten metal is poured into the shell. The best way to avoid and reduce the defects of the casting is to It is necessary to ensure the purity and the most suitable temperature of the molten metal; the second is to try to make the characteristics of the shell adapt to and follow the different changes in the molten metal during the injection of different types of shells. For the latter, colleagues in the industry have successfully taken many external measures, such as the use of the lidding box method to cool the metal in a reducing atmosphere to prevent pitting of the surface of the oxidized steel castings; Local water spray before casting or partial casting water after casting, change the solidification sequence of castings to prevent local shrinkage of castings. There are many similar methods such as the above, but most of them are taken from the outside of the shell. The author of this article has changed from the material of the shell itself, the graphite material has been referenced into the shell material, from the search for suitable graphite materials and in different structures. Castings and different metals were tested repeatedly, and some practical experiences were obtained, which will play a role in promoting the future application of graphite sand and powder in the shell.
Graphite Characteristics and Feasibility of Application in Casting Shell Materials
Graphite is grayish black, soft, artificial graphite is hard, specific gravity of 1.9 ~ 2.3, melting point up to 3850 ± 50 °C. Its main features are: high temperature resistance, even in the ultra-high temperature arc burning, the weight loss is very small; linear expansion coefficient is smaller than 2 × 10-6, is 1/4 of the fused alumina,2/5 of zircon, therefore, the thermal shock resistance is high, and the volume change when the temperature changes abruptly is also small, so that the mold shell is not cracked due to the addition; the electrical conductivity is excellent, and the thermal conductivity exceeds that of a metal material such as steel, particularly the thermal conductivity. Decrease with increasing temperature, graphite is almost thermal insulator at high temperature; chemical stability is good, does not react with acid and alkali and organic solvent; in addition there is good plasticity, lubricity and high temperature strength.
In foundry production, graphite is not unfamiliar. For example, graphite is used as a recarburizer in metal smelting, electrodes for electric arc furnace steelmaking and crucibles for melting non-ferrous metals are all graphite products, Casting with graphite as a casting surface coating is also very common. According to many excellent characteristics of graphite, graphite materials are selectively used in the precision casting of shell materials, in order to achieve the following goals should be feasible:
1. Use of graphite reacts with oxygen at high temperatures to form a reducing atmosphere, protecting the high-temperature metals injected into the mold from oxidation. Graphite almost does not oxidize at low temperatures, but it starts to oxidize slightly above 400°C. As the temperature rises, the oxidation rate increases and CO2 or CO is formed. When the temperature is below 1000°C, C + O2 = CO2, and above 1000°C, 2C + O2 = 2CO, so to achieve this goal, high purity dense artificial graphite sand can be used as a sanding material in the transition layer. When the high temperature metal enters the cavity of the shell, the reducing atmosphere produced is closer to the shell surface layer. The contacted metal is better for preventing oxidation of metals, but at the same time, it is not necessary to worry that the graphite sand in the transition layer will be ablated and retreated in the shell-type roasting. On the other hand, the high-purity dense artificial graphite sand is very slow in oxidative weight loss. On the other hand, graphite sand has a certain protective effect when it is coated in the slurry of silica sol, and when the shell is calcined in a box-type roaster, it is in anoxic condition and the calcination time is short. Graphite grit Weight loss is limited, not enough to affect the strength of the shell.
2. The use of graphite heat insulation at high temperatures, can increase the metal liquid filling capacity of the shell thin-walled cavity, but also can change the local shell cooling capacity, to change the cooling solidification sequence of the casting. In order to achieve this goal, artificial graphite sand can be used instead of calcined kaolin sand in the third and subsequent layers of the shell. Due to the heat insulation effect of graphite in the high-temperature shell, the molten metal is injected into the shell to dissipate heat slowly and prolong the metal flow. The advantage of time is that, on the one hand, the casting is not easy to under-cast due to the extra-thin area, and on the other hand, due to the delay of solidification of metal, the feeding path of thick-walled castings will not be quickly blocked, thereby increasing the feeding capacity of the pouring riser to castings, and It can also be used in slow-cooling parts of the shell, so that the various parts of the shell have different cooling rates, make up for the lack of process design, to overcome the casting shrinkage tendency.
