A must-read for casting beginners: How to safely and efficiently control carbon and silicon content during cast iron smelting?

Part I: Safety Is Paramount

Any operation involving molten metal carries extremely high risks. As a Cast Iron Casting beginner, you must place safety above all else and strictly adhere to the following operational standards:

1. Strict Implementation of Personal Protective Equipment (PPE)

• Full Body Protection: You must wear standard-compliant flame-retardant suits/leather work clothes to shield against radiant heat and prevent splashes of molten metal.
• Eye and Face Protection: Wear specialized full-face shields (with tinted filters to protect against intense light and infrared radiation) and safety glasses.
• Hand Protection: Use long-handled, heat-resistant gloves.
• Foot Protection: Wear safety boots (they must be designed to be impact-resistant, puncture-resistant, and prevent molten metal from flowing in).
• Tool Usage: Ensure all tools that come into contact with molten metal are long-handled and have been confirmed dry and preheated before use.

2. Prevention of Explosive Hazards

• Strict Dehumidification: This is the most critical safety measure. If molten metal contacts any moisture (even trace amounts in tools, charge materials, or alloys), it will immediately generate a large volume of steam, causing a violent steam explosion that splashes molten metal everywhere.
• Charge Preheat: Before adding any revert scrap, steel scrap, or even alloying additives (like ferrosilicon, carburizers) to the furnace, they must be thoroughly baked and preheated to drive off adsorbed surface and internal moisture. Preheating temperatures should ensure complete drying, e.g., reaching $\ge 200^{\circ}C$.

3. Toxic Gases and Environmental Control

• Ventilation: A large amount of fumes will be generated during smelting, particularly carbon monoxide ($\text{CO}$) from oxidation and combustion on the melt surface. The melting shop must have highly efficient local exhaust ventilation and good overall shop ventilation.
• Air Monitoring: It is recommended to be equipped with $\text{CO}$ monitors to ensure the air quality in the working area meets safety standards.

Part II: Efficient Control of Carbon and Silicon Content

The properties of cast iron are primarily determined by the carbon content ($%C$) and silicon content ($%Si$), which together govern the solidification behavior and final microstructure. This is often assessed using the Carbon Equivalent (CE):

$$CE = %C + \frac{%Si + %P}{3}$$

1. Control and Adjustment of Carbon Content ($%C$) (Carburization)

Carbon is the core element for forming graphite and determining the fluidity of cast iron. When carbon is lost during melting or insufficient in the charge, a carburizer must be added.

• Carburizer Selection: The purity and particle size of the carburizer directly affect its dissolution rate and carburization efficiency (the percentage of added carbon absorbed by the molten iron).
  • High-Purity Graphite/Artificial Graphite: High purity, fixed carbon $>98%$, highest absorption rate (usually $>90%$), suitable for high-requirement melts or induction furnace smelting.
  • Petroleum Coke: Cost-effective, fixed carbon between $90%-95%$, a commonly used carburizer.
• Optimal Addition Method: Carburization efficiency is closely related to the position and timing of addition.
  • Mid-Charge Addition (Electric Furnace): This is the most recommended method. Mix the carburizer with part of the charge (like revert scrap or steel scrap) and place it in the middle and lower part of the melting pool. During melting, the carburizer has a longer contact time in the superheated zone to dissolve, achieving higher absorption rates.
  • Surface Addition Before Tapping: Suitable for minor adjustments. The slag must be thoroughly skimmed first, then the carburizer is evenly sprinkled on the surface, using electromagnetic stirring (in induction furnaces) or manual stirring to promote dissolution. This method is relatively less efficient but easier to operate.

2. Control and Adjustment of Silicon Content ($%Si$) (Siliconizing)

Silicon is a strong graphitization promoter, crucial for preventing the formation of white cast iron.

• Main Silicon Source: Ferrosilicon ($\text{FeSi}$). $\text{FeSi}75$ (containing approx. $75%$ silicon) is commonly used.
  • Addition Method: Usually added to the liquid metal just before tapping. To ensure homogeneity, it should be added after skimming the slag, and held for a sufficient soaking time (about 5-10 minutes) to allow for complete melting and mixing.
  • Preheating Importance: Ferrosilicon lumps must be preheated to prevent steam explosions from moisture.
• Combined Carbon-Silicon Additive: Silicon Carbide ($\text{SiC}$) is an excellent composite additive.
  • Principle: $\text{SiC}$ does not melt in the iron melt but dissociates via the reaction $\text{SiC} \rightarrow [\text{Si}] + [\text{C}]$, releasing both silicon and carbon into the iron.
  • Advantages: It simultaneously increases $%C$ and $%Si$ and has excellent pre-inoculation effects, contributing to the formation of fine graphite. It is usually added to the charge as a supplement or alternative to $\text{FeSi}$ and carburizers.

3. Avoiding and Compensating for Oxidation Losses

During the smelting process, especially in the later stages of melting and superheating, carbon and silicon can be lost due to reaction with the atmosphere or oxides in the slag:

• Burn-off Reactions:
  • $2[\text{C}] + \text{O}_2 \rightarrow 2\text{CO} \uparrow$
  • $[\text{Si}] + \text{O}_2 \rightarrow \text{SiO}_2$ (enters the slag)
• Countermeasures:
  • Slag Control: Timely removal of slag containing high iron oxide ($\text{FeO}$). $\text{FeO}$ in the slag will continuously oxidize the $\text{C}$ and $\text{Si}$ in the molten iron.
  • Compensation: When determining the final composition, a certain burn-off allowance should be factored in, meaning the amount added should slightly exceed the target value to compensate for normal losses during smelting.

Part III: Comparison of Common Carbon and Silicon Control Materials

To help you make an informed choice, the table below lists common carbon and silicon adjustment materials and their key characteristics: