Magnesium melt behaves differently from aluminum melt when it comes to reactions with iron. Unlike aluminum, magnesium does not react significantly with iron, allowing the use of iron crucibles for melting magnesium alloys. However, the crucible must be made from high-quality materials such as welded or cast steel, preferably low-carbon or nickel-free low-alloy steel. Graphite crucibles are not recommended because they tend to absorb solvents and become brittle over time. Similarly, cast iron crucibles should be avoided due to their susceptibility to defects like blowholes and their poor resistance to high-temperature creep, which shortens their service life.
To protect the crucible from excessive heat, heat-resistant materials are often applied to its outer surface. Using heat-resistant stainless steel as an outer layer can also help prevent the formation of metal scale buildup, extending the crucible’s lifespan and reducing contamination risks in the molten metal. Low-carbon steel crucibles are commonly used for melting magnesium alloys, especially in large casting operations.
The typical capacity of a magnesium melting crucible ranges from 35 to 350 kg. Smaller crucibles are usually fabricated from low-carbon steel with a carbon content below 0.12% by mass. The presence of elements like nickel and copper can severely affect the corrosion resistance of the crucible, so their concentration in the steel should be kept at 0.10% or less.
During the smelting process, especially in flux-based methods, a layer of residue tends to form at the bottom of the crucible. These residues, often composed of oxides like magnesia, can accumulate over time and lead to contamination of the molten metal. If not removed regularly, they can reduce melting efficiency and even cause issues during die casting. Metal compounds like aluminum, magnesium, and manganese that build up on the crucible bottom may harden and resist melting, further reducing thermal conductivity and potentially causing localized overheating. This buildup can also react with iron, degrading the quality of the alloy. To avoid these problems, crucibles should be cleaned regularly by soaking them in water to remove all deposits. Fluxless melting processes generally produce less residue.
Regular inspections are essential, typically conducted once every quarter. A clear standard for retiring a crucible should be established, such as when the wall thickness is reduced to less than half its original value. Visual inspections should check for cracks on both internal and external surfaces, and all metal scales must be removed before inspection. Each crucible should have a usage record card documenting its service history, wall thickness, and maintenance activities.
Flaws in the crucible can be repaired using nitrogen arc welding. The process involves first welding the deepest layers, then polishing and applying additional welds until the damage is fully repaired. The recommended welding wire is EMK63. Due to the reactive nature of magnesium alloys, special precautions are necessary during melting. Unlike aluminum, which forms a dense oxide layer that prevents further oxidation, magnesium forms a loose oxide film that allows oxygen to penetrate, leading to oxidation and even combustion. Additionally, molten magnesium reacts violently with water, producing hydrogen gas and potentially causing explosions. Therefore, protective measures such as flux or an inert atmosphere are essential.
In industrial settings, SF6 is frequently mixed with other gases, such as air, nitrogen, or CO2, and introduced into the furnace as a protective atmosphere. The gas mixing system ensures precise proportions are delivered to the furnace. Experiments have shown that even a 0.01% concentration of SF6 can provide effective protection, but in practice, higher concentrations are often required due to reaction losses. While increasing the input concentration raises the SF6 level above the melt, it also leads to higher consumption. Therefore, the furnace must be well-sealed to maintain optimal SF6 levels, typically not exceeding 1%, as higher concentrations can corrode equipment. Optimizing the protective gas supply is crucial for both safety and efficiency, requiring accurate gas mixing and proper control to minimize waste and environmental impact.
Stainless Steel 2.4mm Series Blind Rivets
TIANCHANG FASTENER SYSTEM CO..LTD , https://www.toprivet.com