What is carbon additive? Carbon additive is a carbon or petroleum coke follow-up product of black or gray particles (or blocks) with high carbon content. The material is mainly coal and petroleum coke. The petroleum coke is divided into general calcined petroleum coke. (calcined coke or calcined coke) and graphitized petroleum coke. The carbon additive is used to reduce the oxygen content in the molten iron mainly during metal smelting and improve the mechanical properties of the casting. In the new synthetic cast iron smelting process, the amount of scrap steel used is greatly increased. By adding a carbon additive to adjust the carbon content of the molten iron, the production cost is greatly reduced while ensuring the quality of the molten iron.
Types of carbon additive
According to the existence form of carbon atoms, carbon additive can be divided into graphitized carbon additive and non-graphitized carbon additive. Graphitized carbon additive mainly includes graphitized petroleum coke and graphite electrode. Graphitized petroleum coke carbon additive is the most widely used in the foundry industry. The production process is to convert the raw material petroleum coke in a graphitization furnace through a high temperature of 2200-2600 °C, so that the carbon crystallization of the petroleum coke amorphous layer structure is transformed into The high-temperature heat treatment process of the three-dimensional ordered graphite crystal, that is, through the graphitization process, reaches the graphitization state. The non-graphitized carbon additive mainly includes: calcined petroleum coke, coal carbon additive, etc., calcined petroleum coke refers to petroleum coke heated at 1200-1500 ° C high temperature, the treatment temperature is low, and the crystal structure has not changed. Due to the high ash content and volatile matter, coal-based carbon additive is rarely used in electric arc furnace smelting. The indicators for measuring the quality of carbon additives are fixed carbon content, sulfur, volatile matter, ash, nitrogen content and water. After high temperature graphitization, the graphitized carbon additive has a much lower sulfur and nitrogen content than the non-graphitized carbon additive.
Carbon additive index: fixed carbon, ash, volatile matter
The fixed carbon of carbon additive is different from the carbon content. The fixed carbon value is calculated based on the moisture, volatile matter, ash and sulfur content of the sample, and the carbon content can be directly detected by the instrument. The fixed carbon content is high and the ash content is low, so the carbonization effect is better. The high ash content in the carbon additive generates a large amount of soot and slag during the smelting process, increases energy consumption, and increases the labor intensity of the slag.
Carbon additive index: sulfur
The sulfur content of ordinary calcined coke has a relatively high grade, generally less than 3% of sulfur to 0.1%, and the sulfur content of graphitized carbon additive is generally less than 0.05%. In ordinary cast iron, sulfur element has stable cementite and prevents the expansion of graphitization. In the production of ductile iron and vermicular graphite iron, a graphitized carbon additive with low sulfur content should be used. If the sulfur content is too high, the ball will be affected. The effect is to consume more spheroidizing agent and increase the production cost; the calcined petroleum coke carbon additive is used in the production of some gray iron castings because of the lower calcination temperature and higher sulfur content, generally less than 1%.
Carbon additive index: nitrogen content
Nitrogen content is an important indicator of the quality of carbon additive. In general, when the foundry purchases the carbon additive, the first is to pay attention to the price of the carbon additive; the second is the fixed carbon, sulfur, ash, volatile matter, and moisture content in the carbon additive, and less attention is paid to the nitrogen content. Ordinary calcined petroleum coke carbon additive, the calcination temperature is low, and the nitrogen and sulfur contents are high. When producing gray iron castings, the scrap steel itself will bring in a part of nitrogen. If the scrap is added in a large proportion, the amount of carbon additive will increase accordingly. If ordinary carbon calcined carbon additive with high nitrogen content is used, the molten iron is used in molten iron. The nitrogen content will increase significantly. Excessive dissolved nitrogen in the molten iron hinders graphitization, increases the stability of the carbide, promotes the formation of D-type graphite, improves the hardness, and deteriorates the processing properties of the casting.
For gray cast iron, the proper amount of nitrogen can shorten the length of the graphite sheet, increase the degree of bending, passivate the end, reduce the aspect ratio, stabilize the pearlite of the matrix, refine the grain structure and improve the tensile strength of the casting. And the role of hardness. According to the data, when the iron is N>0.012%, there is a possibility of nitrogen hole defects in the casting. Nitrogen pores are commonly found in the interior, surface or near surface of castings. They are round, rectangular and irregular in shape and are one of the common defects in casting production. The graphitized carbon additive is subjected to high-temperature desulfurization and denitrification treatment, and the content of nitrogen and sulfur is very low. The content of molten iron in the molten iron using graphitized carbon additive is generally low.
Factors affecting the absorption rate of carbon additive: The factors affecting the absorption rate of carbon additive are mainly the way of adding carbon additive, the chemical composition of molten iron, the temperature of molten iron, and the particle size of carbon additive.