I. Physical and Chemical Indicators (This is an objective, quantifiable core standard)
1. Fixed Carbon Content
Core Indicator: Fixed carbon is the active ingredient in recarburizers and directly determines their efficiency.
How to Identify:
High-Quality: Fixed carbon ≥ 98.5%, sometimes even above 99.5%.
A higher content means fewer impurities, better recarburization, and less slag.
Low-Quality: Fixed carbon is typically between 90% and 97%, or even lower.
This indicates high levels of impurities such as ash and volatile matter.
2. Sulfur Content
Key Indicator: Sulfur is a harmful element in cast iron, causing brittleness and shrinkage in castings and affecting spheroidizing performance.
How to Identify:
High-Quality: Sulfur content ≤ 0.05%, with high-end products requiring ≤ 0.03%.
The lower the sulfur content, the better.
Low-Quality: Sulfur content is typically above 0.1%, sometimes as high as 0.5%.
Using such recarburizers will significantly deteriorate hot metal quality and increase spheroidizing agent consumption.
3. Ash Content
Important Indicator: Ash is non-combustible inorganic impurities in the recarburizer. It enters the slag, increases the slag volume, and may contaminate the molten iron.
How to Identify:
High-quality: Ash content ≤ 1.0%, with high-end products reaching below 0.5%.
Low-quality: Ash content is typically 2%-5% or higher, resulting in sticky slag, difficult slagging, and poor molten iron purity.
4. Nitrogen Content
Special Indicator: Nitrogen content is crucial for the production of high-end ductile iron (such as wind turbine castings and automotive parts). Excessive nitrogen content can cause nitrogen porosity.
How to Identify:
High-quality: Petroleum coke recarburizers using high-temperature purification typically have a low nitrogen content (< 100 ppm).
Low-quality: Pitch coke or under-calcined petroleum coke may have a high nitrogen content (> 500 ppm).
5. Moisture
Basic Indicator: Excessive moisture increases energy consumption and may create a risk of porosity in the molten iron.
How to Identify:
High-Quality: Moisture content ≤ 0.5%, typically dried.
Low-Quality: Moisture content may exceed 1%, or even higher, especially if stored in humid environments.
II. Physical Appearance and Appearance (Intuitive, Quick Initial Judgment)
1. Color and Gloss
High-Quality: The cross-section has a shiny metallic luster, similar to crushed coke, but denser and brighter.
The color is dark gray or silvery gray.
Low-Quality: The cross-section is dull and matte, appearing dark gray or black, with a "blackish" feel.
2. Texture and Hardness
High-Quality: Dense and heavy to the touch (high density).
When broken or pinched, it feels firm and resistant to breaking. When rubbed, it does not stain or only slightly stains.
Low-Quality: Loose and light to the touch.
The texture is brittle and easily crumbles or breaks. A large amount of black powder is left on the hands after rubbing.
3. Particle Form
High-Quality: Particles are uniform in size, with a concentrated distribution (e.g., primarily 3-5mm or 5-8mm).
Irregular shapes with sharp corners and edges allow these particles to dissolve and diffuse quickly in molten iron.
Poor quality: Uneven particle size and excessive powder.
Powdered recarburizers are easily burned, resulting in extremely low yields and environmental pollution.
III. Production Process and Raw Material Sources (Tracing the Origin)
1. High-quality recarburizer:
Raw materials: Typically derived from high-quality petroleum coke (such as needle coke), which undergoes a high-temperature graphitization treatment (above 2500°C).
This process converts the amorphous carbon into a graphite-like microcrystalline structure.
Advantages: Extremely high fixed carbon content, extremely low sulfur, nitrogen, and ash content, and a very high absorption rate (over 90%-95%). Because its carbon atoms are arranged more similarly to graphite, it can serve as a nucleus for graphite formation, helping to improve the structure and properties of cast iron.
2. Ordinary/low-quality recarburizer:
Raw materials: Derived from ungraphitized petroleum coke, pitch coke, metallurgical coke, or even coal.
Disadvantages: Low fixed carbon, high impurities, low absorption rate (usually only 60%-80%), and unstable recarburization.
IV. Verification of Usage Results (Practice is the Only True Test)
1. Absorption Rate and Yield: When added to molten iron, high-quality recarburizers melt quickly, emit minimal black smoke, and dissolve quickly after stirring.
With the correct process, the yield is stable and high.
2. Slag Condition: Using high-quality recarburizers results in low slag, which is loose and easy to remove.
3. Impact on Molten Iron: It does not cause excessive cooling of the molten iron, steadily increases the carbon equivalent, and does not introduce excessive impurities.
4. Final Casting Quality: The castings have excellent graphite morphology, stable mechanical properties, and are less susceptible to defects such as shrinkage and porosity.
When purchasing, don't just focus on price. Be sure to request an authoritative third-party test report from the supplier, focusing on the three core indicators: fixed carbon, sulfur, and ash content.
Samples can also be taken for a simple visual and tactile inspection. For key products, we first conduct small-batch trial verification to observe their absorption rate in actual production and the quality of the final castings before deciding whether to purchase them in large quantities.
