In the smelting and production of ductile iron, the purity of the molten iron and the stability of carbon directly determine the graphite spheroidization effect, the mechanical properties of the castings, and the finished product qualification rate. Graphite petroleum coke (GPC), with its advantages of high fixed carbon, high carbon absorption rate, and low impurity content, has become the preferred carbon-enhancing raw material for an increasing number of foundries. However, many companies often encounter problems such as unstable carbon absorption rate, poor spheroidization effect, and casting defects during actual feeding. Below, we detail the key practical points for using graphite petroleum coke as a carbon enhancer in ductile iron from six aspects: raw material selection, particle size control, feeding process, temperature control, molten iron management, and storage, helping foundries stabilize product quality and optimize production costs.
I. Raw Material Selection: Prioritize and Strictly Control Sulfur and Nitrogen Impurities
Ductile iron production is highly sensitive to the sulfur content in the molten iron. Excessive sulfur will consume the spheroidizing agent, resulting in poor graphite spheroidization, insufficient spheroid count, and shrinkage defects in the castings. Therefore, low sulfur and low nitrogen content are the primary selection principles when purchasing graphite petroleum coke.
It is recommended to choose fully graphitized petroleum coke products with a sulfur content ≤0.05% and nitrogen content controlled within a suitable range. High fixed carbon content ensures efficient carbonization and reduces the introduction of harmful impurities such as ash and volatile matter into the molten iron. Do not solely rely on product price as the purchasing standard; low-priced, high-sulfur graphite petroleum coke is highly likely to cause spheroidization failure and increased scrap rates later on, ultimately raising overall production costs. When purchasing for export, it is essential to require suppliers to provide quality inspection reports for each batch, verifying a complete set of indicators for fixed carbon, sulfur, nitrogen, ash, and volatile matter to ensure consistent batch quality.
II. Appropriate Particle Size Selection to Avoid Floating or Burn-off
Inappropriate particle size selection is a common cause of low absorption rates. For ductile iron smelting in medium-frequency furnaces, the recommended particle size range for graphite petroleum coke is 0.5-2mm, which can be fine-tuned according to the electric furnace capacity.
• Excessively large particle size: Slows down the dissolution rate, making it difficult to fully absorb in the later stages of smelting, resulting in insufficient carbon content.
• Excessive powder: Fine powder is easily carried away by the flue gas in the furnace, causing burn-off and significantly reducing the carbon-adding effect.
For minor carbon adjustments, a slightly finer particle size can be used; for large-scale basic carbon addition, granular materials should be prioritized.
III. Mastering the Optimal Timing and Method of Feeding
It is not recommended to add all of the graphite petroleum coke to the furnace bottom at the very beginning of smelting. The recommended feeding time is in the middle to late stages of furnace melting, adding it in batches after a certain amount of molten iron has remained in the furnace.
1. Place the graphite petroleum coke in the lower part of the electric furnace;
2. After charging, cover with scrap steel and recycled material to weigh down the recarburizing agent and reduce particles floating on the surface of the molten iron;
3. Avoid premature slag removal during the complete melting stage of the molten iron to prevent the graphite petroleum coke from being trapped in the slag and unable to be absorbed by the molten iron, significantly reducing the absorption rate;
4. If fine-tuning of the carbon content is required after smelting, after adding a small amount of graphite petroleum coke, be sure to start thorough stirring and let it stand for 3-5 minutes to ensure uniform dissolution and diffusion of carbon elements.
IV. Controlling the molten iron temperature to ensure carbon absorption efficiency
The temperature of the molten iron directly affects the dissolution rate of the graphite petroleum coke. It is recommended to carry out the recarburization operation when the molten iron temperature reaches the range of 1450℃-1500℃.
If the temperature is too low, the graphite petroleum coke dissolves slowly, and the absorption rate decreases; prolonged exposure to an ultra-high temperature oxidizing environment can easily cause carbon elements to be oxidized and burned off. After completing the recarburization, raise the temperature to the temperature required for spheroidization treatment and carry out the spheroidization and inoculation processes.
V. Monitor Iron Composition and Match it to the Spheroidizing Process
After adding graphite petroleum coke, it is essential to sample and analyze the carbon equivalent of the molten iron to avoid excessive or insufficient carbon content.
After carbon addition, the residual sulfur content of the molten iron needs close monitoring. If the sulfur content is too high, the amount of spheroidizing agent added should be adjusted promptly to prevent graphite distortion. Graphite petroleum coke is only responsible for adjusting the carbon element; the simultaneous control of other elements such as silicon, manganese, and phosphorus should not be neglected to ensure the overall balance of the molten iron composition, laying the foundation for the subsequent formation of high-quality spheroidal graphite.
VI. Warehousing and Moisture Control to Protect Raw Material Quality
Graphite petroleum coke particles have a certain degree of hygroscopicity. Moist raw materials entering the furnace can easily introduce moisture, increasing the risk of porosity in castings.
Raw materials should be stored in a dry and ventilated warehouse, protected from rain and moisture. Damaged packaging bags should be dealt with promptly. It is recommended to dry damp graphite petroleum coke before entering the furnace. Different batches of goods should be stacked separately, and the first-in, first-out principle should be followed during production. Samples of each batch should be retained for traceability and troubleshooting in case of quality problems.
VII. Long-Term Production Recommendations: Stabilize the Supply Chain and Reduce Quality Fluctuations
Ductile iron production involves continuous processes, and fluctuations in the quality of the carburizing agent can directly lead to the scrapping of the entire furnace of molten iron. It is recommended that foundries prioritize suppliers that integrate manufacturing and trading, and possess stable graphitization production lines. Long-term, fixed-point procurement can maximize the stability of various indicators of graphite petroleum coke, reducing the production risks associated with frequent changes in suppliers.
In conclusion, graphite petroleum coke is a high-performance carburizing material, but to fully leverage its advantages in ductile iron production, it is crucial not only to consider product parameters but also to implement comprehensive operational management, from procurement, warehousing, material feeding, temperature control to molten iron testing. Standardized use can improve carbon absorption rate, reduce spheroidizing agent consumption, and effectively increase the yield of castings, creating long-term economic benefits for the enterprise.
