The quality of carbon additives can be measured by multiple indicators, and the following are some main criteria for judgment:
1, Fixed carbon content
Definition: Fixed carbon is the truly useful part of a carbon enhancer, and the higher its value, the better the carbon enhancement effect. The fixed carbon content is obtained by calculating the moisture, volatile matter, ash content, and sulfur content in the sample.
Standard: Generally speaking, the fixed carbon content of high-quality carbon enhancers should be greater than 96%.
2, Ash content
Definition: Ash is the residue left after the combustion of a carbon enhancer at high temperatures, and the lower its content, the better.
Impact: Carbonizers with high ash content are prone to generating large amounts of smoke and slag during the smelting process, increasing energy consumption, reducing carbon absorption rate, and prolonging operation time, increasing power consumption and labor.
Standard: The ash content of graphite carbonization agent is relatively low, about 0.5% to 1.0%; The ash content of calcined anthracite is relatively high, and even high-quality ones will not be lower than 4% to 5%.
3, Sulfur content
Impact: Sulfur is a harmful element that interferes with spheroidization, and the sulfur content should be strictly controlled for the raw iron liquid used in the production of ductile iron. For gray cast iron, moderate sulfur can stabilize pearlite, but excessive sulfur is harmful.
Standard: The sulfur content of carburizer used for ductile iron should be as low as possible, generally requiring S ≤ 0.015%; The sulfur content of carburizer used for gray cast iron can be slightly higher, but it also needs to be determined according to specific needs.
4, Volatile matter
Definition: Volatile matter is the gas and water released by a carbon enhancer during the heating process, and the lower its content, the better.
Impact: Volatile matter entering the molten steel will increase the gas content in the molten iron, increasing the risk of producing porosity defects in castings.
Standard: The volatile matter of properly processed carbon additives should be below 0.5%.
5, Granularity
Influence: The particle size of the carbon additive should be selected according to different melting methods, furnace types, and furnace sizes. A uniformly and moderately sized carbon enhancer can increase the absorption rate of carbon.
Standard: For example, the particle size for induction furnaces is between 0.2 and 6mm; The particle size of steel and other black metals is between 1.4-9.5mm; High carbon steel requires low nitrogen and particle size between 0.5 and 5mm.
6, Water content
Impact: The introduction of water into the steel will increase the hydrogen content in the molten steel, which has a negative impact on the quality of the castings.
Standard: The total moisture index of the carbon additive should be as low as possible. The moisture content of the carbon additive in small bags should be ≤ 1%, and the moisture content of the raw material coke before crushing and packaging should be ≤ 3%.
7, Nitrogen content
Influence: The role of nitrogen content in gray cast iron has duality. Moderate nitrogen can stabilize pearlite, bend flake graphite, passivate the tip, and improve the mechanical properties of gray cast iron; However, excessive nitrogen can lead to crack like nitrogen porosity defects in castings.
Standard: Grey cast iron should be treated with coal or petroleum coke carbonization agents, and its nitrogen content should be around 1000ppm to meet the requirements of most casting materials. However, in the production of ductile iron, it is necessary to choose graphite carburizer or graphitization carburizer with lower nitrogen content.
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