The Influence of Fixed Carbon, Sulfur, Nitrogen, and Ash Content of Graphite Petroleum Coke on Castings

Graphite petroleum coke is a byproduct of petroleum refining. After high-temperature calcination or graphitization, it has become an indispensable auxiliary material in the foundry industry due to its extremely high fixed carbon content and low impurity content. In the cast iron smelting process, graphite petroleum coke is mainly used as a carburizing agent to adjust the carbon content in molten iron, thereby improving scrap steel utilization and reducing production costs. However, the various chemical indicators of graphite petroleum coke-fixed carbon, sulfur, nitrogen, and ash content-directly determine its carburizing effect in molten iron and its impact on casting quality. A deep understanding of the mechanisms of these indicators is of great significance for foundry enterprises to rationally select carburizing agents, control production costs, and improve casting quality.

Fixed carbon is the core indicator for evaluating the quality of carburizing agents and the most direct factor determining the carburizing effect. The higher the fixed carbon content, the greater the proportion of effective carbon in the carburizing agent and the fewer impurities. In cast iron smelting, the carbon in the molten iron mainly originates from carburizing agents, and these carbon elements play a crucial role in the quality of the castings.

Carbon is a fundamental element in cast iron, existing in two forms: free carbon (graphite) and compounded carbon (cementite). Carbon strongly promotes graphitization; increasing the carbon content helps graphite nucleation and growth, but results in coarse graphite particles; conversely, reducing the carbon content results in finer graphite particles.

The effect of a fixed carbon content on the microstructure and properties varies depending on the type of cast iron: In gray cast iron, the carbon content is typically controlled within the range of 2.7% to 3.8%, with carbon mainly existing in the form of flake graphite. High-carbon gray cast iron has a microstructure of ferrite and coarse flake graphite, exhibiting lower mechanical strength and hardness but better flexibility; low-carbon gray cast iron has a microstructure of pearlite and fine flake graphite, possessing higher mechanical strength and hardness but poorer flexibility.

In ductile iron, the carbon content is controlled within the range of 3.5% to 3.9%. Carbon exists in the form of spheroidal graphite; once the graphite is spheroidal, the influence of carbon content on mechanical properties is relatively weakened. However, carbon content still affects the casting process: above the eutectic composition, increasing the carbon content easily leads to graphite floating, reducing mechanical properties; below the eutectic composition, decreasing the carbon content easily leads to free cementite, causing a decrease in mechanical properties, increased brittleness, and increased casting defects such as shrinkage cavities and porosity.

From the perspective of carburizing agent selection, different scenarios have specific requirements for fixed carbon content: ductile iron, vermicular graphite cast iron, and precision gray cast iron parts should use high-purity graphitized petroleum coke with a fixed carbon content ≥98%; ordinary gray cast iron parts can use industrial-grade carburizing agents with a fixed carbon content of 95% to 97%; while low-value rough cast iron parts and steelmaking carburizing can use economical products with a fixed carbon content of 85% to 94%. Fixed carbon content is negatively correlated with ash and volatile matter content-the higher the fixed carbon content, the lower the proportion of impurities. Therefore, high-fixed-carbon recarburizers can minimize the introduction of harmful impurities.

Sulfur is an impurity element that needs strict control in cast iron smelting. Its impact on casting quality is dual, but in most cases it is considered a harmful element. Sulfur in graphite petroleum coke exists mainly in the form of organic sulfur (sulfides, thiols, etc.) and inorganic sulfur (iron sulfide, etc.). The former can be removed at lower temperatures, while the latter requires high-temperature graphitization treatment to volatilize.

The Influence of Sulfur on Gray Cast Iron: Sulfur can stabilize cementite, prevent excessive graphitization, and promote graphite flake bending and tip blunting. Within a certain range, it can even improve the tensile strength and hardness of castings. Therefore, gray cast iron has a relatively high tolerance for the sulfur content of recarburizers; generally, a sulfur content below 0.5% is sufficient.

The Severe Influence of Sulfur on Ductile Cast Iron: Sulfur is an element that reacts strongly with spheroidizing agents such as magnesium and rare earth elements. Sulfur reacts with spheroidizing agents to form sulfides, consuming the agent and forming inclusions, leading to graphite spheroid distortion, reduced spheroid quantity, and even the formation of flake graphite, severely damaging the spheroidizing effect. Therefore, in ductile iron production, the sulfur content of the molten iron must be strictly controlled to ≤0.015%, and the sulfur content of the carburizing agent should be as low as possible; high-quality carburizing agents should have a sulfur content ≤0.05%. Using low-sulfur carburizing agents not only ensures spheroidizing quality but also reduces the amount of spheroidizing agent used by 15%~20%, achieving cost savings.

Other negative effects of sulfur: High-sulfur carburizing agents (such as calcined petroleum coke, with a sulfur content of 0.5%~1%) easily form sulfide inclusions during smelting, isolating carbon particles and reducing carbon absorption rate. Studies have shown that when the sulfur content increases from 0.5% to 1%, the carbon absorption rate may decrease by 10%~15%. Furthermore, sulfur can react with furnace linings and other equipment at high temperatures, accelerating equipment corrosion. When the sulfur content exceeds the equilibrium concentration (approximately 0.14%), if the nitrogen content of the carburizer is also high, the casting is prone to developing fissure-like nitrogen porosity defects, leading to decreased toughness.

High-quality carburizers treated with high-temperature graphitization can reduce the sulfur content to ≤0.05%, and even better products to <0.03%. This is also an important indirect indicator of whether the carburizer has undergone sufficient graphitization treatment.

Nitrogen is a trace element that has received widespread attention in recent years. It has a "dual effect" on the properties of cast iron-beneficial in moderation, harmful in excess. The main sources of nitrogen in molten iron include carburizers, scrap steel, molds and sand cores, alloys, and inoculants. Especially with the popularization of "synthetic cast iron" processes, the amount of scrap steel used has increased significantly, and the nitrogen content in molten iron has approached the critical upper limit. At this point, the nitrogen content of the carburizer becomes crucial for control.

Positive effects of nitrogen: Appropriate amounts of nitrogen can stabilize the pearlitic structure of gray cast iron, refine graphite, and improve mechanical properties. When the nitrogen content in molten iron is controlled within the range of 70-120 ppm, good overall performance can be obtained. Compounds such as boron nitride promote graphite nucleation and accelerate the graphitization process.

Negative effects of nitrogen: When the nitrogen content exceeds a certain threshold (generally considered to be above 120-140 ppm), the risk of nitrogen porosity defects in castings increases significantly. Nitrogen porosity is commonly found inside, on, or near the surface of castings, appearing as round, rectangular, or irregular shapes of varying sizes, and is one of the common defects in casting production. When the nitrogen content increases further, fissure-like nitrogen porosity can appear-the pores lack graphite around them, have bright white edges, and exhibit a "carbon-deficient" phenomenon. In ductile iron, excessive nitrogen can tear apart graphite spheroids, causing a 30% drop in mechanical strength.

Nitrogen content control requirements for recarburizers: The nitrogen content of recarburizers varies greatly depending on their quality. Naturally calcined petroleum coke recarburizers have a nitrogen content of approximately 1000 ppm, while coal-based recarburizers can reach 2000-7000 ppm. High-quality high-temperature graphitized petroleum coke recarburizers can have a nitrogen content below 100 ppm. Recarburizers used in ductile iron generally require a nitrogen content ≤300 ppm, while high-end precision castings require ≤100 ppm. Foundry enterprises should be wary of granular recarburizers made by extruding graphite powder with binders. Although the carbon in these products appears in graphite form, the nitrogen content is often as high as around 2000 ppm, easily leading to nitrogen porosity defects.

Ash refers to the inorganic mineral residues in recarburizers that cannot burn at high temperatures, mainly including oxides such as SiO₂ and Al₂O₃. Ash content directly affects the purity and performance of the recarburizer and is an important indicator for evaluating its quality.

The impact of ash content on the recarburization process: Ash hinders the dissolution and diffusion of carbon particles in molten iron. Electron microscopy observation of partially dissolved coke and coal particle samples revealed the formation of a thin, sticky ash layer on the sample surface, which is the main factor affecting the diffusion and dissolution performance of carbon particles in molten iron. High-ash recarburizers have significantly lower carbon absorption rates than low-ash products, prolonging smelting time and increasing energy consumption.

The impact of ash content on casting quality: The large amount of non-metallic oxides contained in ash can be incorporated into castings as inclusions, contaminating the molten iron and reducing the mechanical properties and surface quality of the castings. Simultaneously, these oxides may adhere to the furnace lining surface, exacerbating erosion and damage, and shortening the furnace lining's service life.

Ash content control standards for high-quality recarburizers: High-quality graphitized petroleum coke recarburizers, after high-temperature treatment at 2500~3000℃, exhibit significantly reduced ash, sulfur, and gas content, with ash content controlled below 0.5%. When selecting carburizing agents in casting production, products with low ash content and high fixed carbon should be prioritized to ensure the purity of molten iron and the quality of castings.

Different types of castings have significantly different requirements for various indicators of graphite petroleum coke. Foundry enterprises should scientifically select carburizing agents based on specific product and process requirements:

Ductile Cast Iron: High-carbon, low-sulfur products that have undergone graphitization treatment must be selected. The basic requirements are fixed carbon ≥98.5%, sulfur ≤0.05%, and nitrogen ≤200ppm. This can improve the spheroidization rate and reduce the amount of spheroidizing agent used by 15%~20%.

Gray Cast Iron (Mid-to-High-End): Graphitized petroleum coke or high-quality calcined coke should be selected, with fixed carbon ≥95%~98%, sulfur ≤0.5%, and nitrogen ≤500ppm. This can achieve good mechanical properties and machinability.

Ordinary Castings/Cost-Sensitive Scenarios:Under the premise of ensuring the basic quality of castings, petroleum coke-based carburizing agents can be used (sulfur content must be controlled), but high-sulfur coal-based carburizing agents should be avoided unless a desulfurization process is used. For precision thin-walled parts/oversized parts: Fully graphitized recarburizing agents with high-temperature graphitization above 2600℃ should be selected, with fixed carbon ≥99%, sulfur ≤0.03%, and nitrogen ≤100ppm, to promote nucleation, reduce shrinkage porosity, and improve casting density.

It is worth noting that quality control of graphite petroleum coke involves a series of testing standards, mainly including GB/T 26310 series (calcined coke for primary aluminum), YS/T 587 series (calcined coke for carbon anodes), and international standards such as ASTM and ISO, covering testing methods for multiple items such as ash content, sulfur content, and trace elements. Foundry enterprises can refer to current industry standards such as JB/T 14236-2023 "Recarburizing Agents for Foundries" and YB/T 6261-2024 "Recarburizing Agents - Determination of Nitrogen Content - Formaldehyde Method" when purchasing and using graphite petroleum coke to ensure product quality traceability and stability.

In conclusion, the various chemical properties of graphite petroleum coke-fixed carbon, sulfur, nitrogen, and ash content-are crucial in every stage of cast iron smelting, profoundly impacting the microstructure, mechanical properties, surface quality, and production costs of castings. Fixed carbon content determines carbon enrichment efficiency, excessive introduction of sulfur and nitrogen can lead to serious casting defects, while ash content indirectly affects furnace stability and alloy quality. Modern foundry enterprises should establish a holistic quality control approach, rationally selecting high-quality graphite petroleum coke products with matching indicators based on their product positioning and process characteristics. Simultaneously, they should establish a comprehensive testing and monitoring mechanism during the smelting process to achieve the optimal balance between casting quality and production costs. With the increasing demand for high-quality castings in the foundry industry and the increasingly stringent requirements for energy conservation and emission reduction, high-quality graphitized petroleum coke with low sulfur, low nitrogen, and high fixed carbon content is becoming the mainstream direction for industry development.