Graphitized recarburizer vs. coal-based recarburizer in induction furnaces

In induction furnace melting, the choice of recarburizer directly dictates the metallurgical quality of the final casting.

In short, Graphitized Recarburizer is superior to Coal-based Recarburizer primarily because it undergoes a physical phase change during high-temperature treatment, forming a more ordered microstructure that shares thermodynamic characteristics with the graphite flakes found in molten iron.

The following is an in-depth analysis focusing on crystal structure, kinetics in an induction furnace environment, and the metallurgical impact of impurities.

1. Microscopic Crystal Structure: Atomic Arrangement Defines Solubility

The dissolution of a recarburizer is not merely a physical mixing process, but a process of mass transfer and diffusion. This depends entirely on the microscopic arrangement of carbon atoms.

A. Coal-based Recarburizer (Amorphous/Turbostratic Structure)

• Structural Characteristics: Coal-based recarburizers (such as calcined anthracite) are processed at temperatures typically between $1200\text{–}1300^\circ\text{C}$. At this stage, carbon atoms exist in an amorphous or turbostratic (disordered layer) state.
• Atomic Bonding: The carbon layers are arranged haphazardly with irregular interlayer spacing. Strong cross-linking bonds exist between atoms, making the structure dense and rigid.
• Dissolution Barrier: In molten iron, “pulling out” individual carbon atoms to diffuse into the liquid requires significant energy to break these chaotic and strong chemical bonds.

B. Graphitized Recarburizer (Hexagonal Lattice Structure)

• Structural Characteristics: True graphitization requires heating the raw material (usually petroleum coke) to 2500 – 3000 ℃. At these extremes, carbon atoms rearrange into a perfect hexagonal layered crystal structure.
• Atomic Bonding:
  • Intralayer: Atoms are bound by extremely strong covalent bonds.
  • Interlayer: The layers are held together only by weak Van der Waals forces.
• Dissolution Advantage (Peeling Mechanism): In molten iron, this layered structure allows carbon atoms to detach rapidly in “sheets” via a peeling mechanism. It is much like pushing a deck of cards across a table compared to trying to shred a solid block of wood.

2. Dissolution Kinetics Under Induction Furnace Conditions

Induction furnaces have specific melting characteristics that demand high “wettability” from the recarburizer.

A. Limitations of Electromagnetic Stirring

Induction furnaces use electromagnetic induction for heating. While this creates internal stirring, it is relatively gentle compared to the physical impact of a Cupola or the intense convection in an Electric Arc Furnace (EAF).

• Coal-based: Due to its dense amorphous structure, it has a large wetting angle with molten iron. Without intense agitation, it tends to float on the slag layer, making it difficult for the iron to “grab” the carbon, leading to low and inconsistent absorption rates (usually 60%–70%).
• Graphitized: Its layered structure is easily wetted. Upon contact, iron atoms quickly penetrate the graphite layers, vastly increasing the contact area. Combined with the electromagnetic stirring, it is easily sucked into the depths of the melt, achieving absorption rates of 90%–95% or higher.

B. The Core Difference: Atomic Diffusion Velocity

Dissolution is essentially the diffusion of carbon from a high-concentration solid phase to a low-concentration liquid phase.

Once in the melt, graphitized carbon acts as a nucleation core. Its structural similarity to the graphite that will eventually precipitate out of the iron reduces the energy barrier for dissolution.

3. Sulfur (S) and Nitrogen (N): Impurities and Casting Quality

Beyond structural differences, the purity levels resulting from different processing temperatures directly impact the metallurgical integrity of the iron.

A. Nitrogen (N): The Culprit of Porosity and Brittleness

• Coal-based (High Nitrogen): Anthracite naturally contains high nitrogen, and low-temperature calcination cannot effectively remove it. N content is typically 5000–8000 ppm.
• Graphitized (Low Nitrogen): During graphitization at $3000^\circ\text{C}$, nitrogen atoms escape the lattice due to intense thermal vibration. High-quality graphitized recarburizers can have N levels below 100 ppm.

Nitrogen Hazards:

1. Gas Porosity: When dissolved nitrogen exceeds solubility limits, it precipitates during solidification, forming subcutaneous pinholes or fissure-type gas holes.
2. Nitrogen-induced Brittleness: Nitrogen stabilizes pearlite and hinders ferrite formation, leading to abnormally high hardness, poor machinability, and age embrittlement.

B. Sulfur (S): Interference with Nodularization

• Coal-based: Sulfur content is usually 0.3% – 0.5% or higher.
• Graphitized: Sulfur volatilizes at high temperatures, typically resulting in levels below 0.05%.

Sulfur Hazards:

1. Consumption of Nodulizers: In Ductile Iron production, sulfur reacts preferentially with Magnesium (Mg) to form Magnesium Sulfide (MgS), directly consuming the nodulizer and causing poor nodularization.
2. Interface Interference: Sulfur is a surface-active element. It can form a thin film around recarburizer particles, hindering the diffusion of carbon atoms and further lowering the absorption rate.

Summary: Performance Comparison Table

Conclusion

In induction furnace melting, using a graphitized recarburizer is not just about adding carbon; it is a pre-treatment of the melt. Its ordered crystal structure ensures efficient dissolution kinetics, while its low nitrogen and sulfur profile eliminates the root causes of gas defects and nodularization failure.