How to select graphite grade for different metal casting

The surface quality, dimensional accuracy, and production continuity of the finished bars in the horizontal continuous casting production of brass, copper, zinc bars, and ductile iron largely depend on the graphite crystallizer.

Melting and Actual Casting Temperatures for Various Materials

• Copper (Pure Copper): Melting point 1083℃, actual continuous casting temperature 1150–1220℃
• Brass (Copper-Zinc Alloy): Melting range 900–950℃, actual continuous casting temperature 980–1040℃
• Zinc: Melting point 420℃, actual continuous casting temperature 480–530℃
• Ductile Iron: Melting range 1150–1250℃, actual continuous casting temperature 1280–1350℃

The temperature differences between different metals are very large. Ductile iron has the highest casting temperature, placing the greatest pressure on the graphite crystallizer to resist thermal shock and oxidation; zinc casting has the lowest operating temperature.

Different metals require different substrate processes

Different non-ferrous metal processing conditions necessitate specific substrate matching; a single graphite substrate cannot be used for all products:

• Zinc rods: 480–530℃, molding, one dip, two bakes, ≥1.75 g/cm³, controls porosity, prevents zinc penetration, coating as needed.
• Brass: 980–1040℃, molding, two dips, three bakes, ≥1.80 g/cm³, prevents zinc penetration, wear-resistant, with pyrolytic carbon coating.
• Copper: 1150–1220℃, molding, three dips, four bakes, ≥1.85 g/cm³, extremely high density, strictly prevents copper penetration, with pyrolytic carbon coating.
• Ductile cast iron: 1280–1350℃, multiple dips and bakes, high-density substrate, ≥1.85 g/cm³ Resistant to thermal shock and high-temperature oxidation, but the coating wears out quickly.

If the matrix is not dense enough, molten metal can easily penetrate the graphite pores, causing material sticking, grooving, and spalling. Scratches appear on the surface of the rods, necessitating premature production and mold replacement. Simply pursuing low-priced graphite may seem to reduce procurement costs, but in reality, downtime, scrap, and labor replacements significantly increase overall costs.

The core function of impregnation and calcination is to seal the open pores inside the graphite matrix, improving overall density.

• One impregnation and two calcinations: Basic densification, suitable for low-temperature zinc rod applications.
• Two impregnations and three calcinations: Medium densification, suitable for medium-temperature brass applications.
• Three impregnations and four calcinations: High densification, suitable for high-temperature, high-penetration copper applications.

Graphite formed solely by primary molding has high porosity, making it highly susceptible to penetration of molten copper and zinc into the pores, eroding and damaging the matrix.

The pyrolytic carbon coating forms a dense, airtight protective layer within the workpiece's inner cavity, further isolating it from molten metal corrosion, preventing metal carburization, improving the wear resistance of the inner wall, extending continuous production cycles, and reducing mold change frequency.

Note: The pyrolytic carbon coating is only applied to the inner working surface; the adhesion between the coating and the substrate is crucial. If the substrate has excessively large pores, the coating is prone to peeling and flaking. Therefore, it is essential to first densify the substrate through impregnation and calcination before applying the CVD coating. The substrate material, impregnation and calcination process, and pyrolytic carbon coating must work in tandem; performing only one step will not achieve the desired performance.

Semicorex supplies high-quality customized graphite crystallizer for different metal casing. For customized solutions or additional technical information, please feel free to contact our engineering team.

Phone: +86-13567891907

Email: sales@semicorex.com