Summary of knowledge on precision casting smelting and casting degassing

Summary of knowledge on precision casting smelting and casting degassing

The permeability of precision casting shell is an important performance indicator of the shell. The permeability of the mold shell has a significant impact on precision casting castings. For example, it is often found that insufficient pouring of castings (including incomplete shape and rounded corners at right angles) and suffocation may be caused by poor permeability of the mold shell. Another point is that the breathability of the mold shell is also an important factor affecting the quality of the mold shell dewaxing. Today, precision casting manufacturer Yufeng Metal will talk in detail about the breathability of precision casting molds.

1、 The source of gas

The gases that can dissolve in metals are mainly hydrogen and oxygen. During the smelting process, the main sources of gas are:

1. Furnace gas: In non vacuum melting, furnace gas is the main source of gas in the metal. In addition to oxygen and nitrogen, the furnace gas also contains water vapor, hydrogen, carbon monoxide, carbon dioxide, sulfur dioxide, and hydrocarbons. The composition of furnace gas varies with the use of fuel and combustion conditions; In reflective furnaces or crucible furnaces that use heavy oil or gas as fuel, there is often 5-10% water vapor and a significant amount of hydrogen, carbon monoxide, etc.

2. Furnace charge: most of the electrolytic metal surface has residual electrolyte, and most of the returned materials from the processing workshop contain oil, water, lotion, etc. Most foreign waste materials include water corrosion, rust, etc. Especially during outdoor stacking and humid seasons, there is moisture adsorbed on the surface of the furnace material. These will cause the metal to absorb more hydrogen during the melting process.

3. Refractory materials: The moisture contained in refractory materials can also promote metal suction, especially when new furnaces begin production.

4. Fluxes: Many fluxes contain moisture, some of which (such as charcoal, rice bran, etc.) contain adsorbed moisture, while others (such as borax) themselves contain crystalline water. To reduce the source of gas during the smelting process, the flux should be dried or dehydrated.

5. Operating tools: Insufficient preheating of operating tools can also increase the gas content of the metal.

2、 The dissolution process and solubility of gases

Gas solubility in metals: When metals are in solids, the solubility of gases is very small. As the temperature rises, the solubility slowly increases, and when it reaches the melting point temperature, the solubility sharply increases. Continuing to increase the temperature of the molten metal, the solubility of gases continues to increase and begins to decrease after reaching the limit point. When it reaches the boiling point temperature of the metal, the solubility of gases is almost zero.

The influence of different alloying elements on the solubility of gases in alloys is also different. Some elements such as nickel have a strong binding force with gases, which increases the solubility of gases in alloys. Other elements such as aluminum and tin can reduce the solubility of gases in alloys. The influence of alloying elements on hydrogen solubility in copper alloys is as follows: Cu+Ni>Cu+Pb>Cu+Ag>Cu+Au>Cu+Sn>Cu+Al

3、 Degassing method

3.1. Gas degassing method: one is inert gas (such as N2), and the other is active gas (such as Cl2). The smaller and more bubbles there are, the more beneficial it is for degassing. However, due to the high speed of bubble ascent, the short time it takes to pass through the melt, and the fact that bubbles cannot be uniformly distributed throughout the entire melt, it is not easy to completely remove gas using this method; As the hydrogen content in the melt decreases, degassing significantly decreases.

3.2. Flux degassing method: Flux degassing is the use of thermal decomposition of molten salt or displacement reaction with metals to produce volatile bubbles that are insoluble in the melt and remove hydrogen. Aluminum bronze is commonly degassed with cryolite flux, while white copper and nickel alloys are commonly degassed with fluxes such as fluorite, borax, and calcium carbonate. When refining flux, dry flux is generally pressed into the melt pool with a perforated cover. To improve degassing, dry nitrogen gas can also be used to blow powdered flux into the melt. The flux can also remove slag while degassing.

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