The nitrogen content of graphitized petroleum coke is a critical quality indicator in high-end casting, especially in the production of high-value-added iron castings (such as engine blocks and cylinder heads).
High-quality graphitized petroleum coke recarburizers typically have a nitrogen content of ≤100 ppm (0.010%), with top-tier products achieving ≤50 ppm (0.005%).
The following details and their importance are explained in detail:
Nitrogen is a gaseous element in cast iron. While trace amounts of nitrogen contribute to its strength and hardness, excessive amounts can cause a series of serious casting defects:
Nitrogen porosity (pinholes): When molten iron solidifies, the solubility of nitrogen drops sharply. Oversaturated nitrogen precipitates and forms tiny pores, severely affecting the density and mechanical properties of the casting.
Increased brittleness: Nitrogen raises the brittle transition temperature of cast iron, reducing the impact toughness of the casting at room or lower temperatures and making it more susceptible to brittle fracture. Defect formation with other elements: Nitrogen forms high-melting-point nitride inclusions with elements such as titanium (Ti), zirconium (Zr), and aluminum (Al). These hard inclusions can impair the machining performance of castings (increasing tool wear) and mechanical properties.
Promoting fissured graphite: In ductile iron, excessive nitrogen content is one of the main causes of the formation of harmful "fissured graphite," which significantly reduces the material's elongation and fatigue strength.
Graphitized petroleum coke is a high-end recarburizer. Its nitrogen content can be roughly divided into the following grades:
Ordinary grade: 100-300 ppm (0.01% - 0.03%)
Premium grade: 50-100 ppm (0.005% - 0.01%)
Premium/low nitrogen grade: ≤ 50 ppm (≤ 0.005%)
So how can low nitrogen content be achieved? The low nitrogen content of graphitized petroleum coke is primarily due to its production process:
Raw material selection: High-quality petroleum coke with low impurity (including nitrogen) content is used.
High-temperature graphitization: This is the most critical step. During prolonged heat treatment in an electric arc furnace at temperatures of up to 2500°C to 3000°C, most volatile impurities (including sulfur, nitrogen, and hydrogen) in the raw material are "burned off" at high temperatures. Nitrogen escapes as nitrogen gas (N₂), significantly reducing the nitrogen content of the final product.
Summary:
Nitrogen content is one of the key indicators for evaluating the quality of graphitized petroleum coke recarburizers. Selecting graphitized petroleum coke with low nitrogen content (≤100ppm, preferably ≤50ppm) is a key measure for preventing nitrogen porosity in cast iron and ensuring excellent mechanical and processability of castings.
