High fixed carbon and low sulfur, low nitrogen and low ash content are crucial for carburizing agents, as these indicators directly affect carburizing efficiency, casting quality and production costs, and are the core parameters for evaluating the performance of carburizing agents. The specific analysis is as follows:
1. High fixed carbon: The cornerstone of carbon increase efficiency
Core function: Fixed carbon is the effective component that truly participates in carbon increment in the carbon additive, and its content directly determines the carbon increment effect. The higher the fixed carbon content, the more carbon elements can be provided by the carbon additive per unit mass, and the higher the carbon addition efficiency.
Economy: High fixed carbon can reduce the amount of carbon additive used and lower production costs. For instance, if the fixed carbon content is increased from 90% to 95%, the carbon increase efficiency may rise by 10% to 15%, while reducing the interference of impurities such as ash on the smelting process.
Process compatibility: In induction furnace smelting, high fixed carbon carburizing agents can dissolve more quickly and disperse evenly, avoiding fluctuations in casting performance caused by uneven carbon absorption.
2. Low ash content: Reduces impurity interference and enhances smelting efficiency
The hazard of ash: Ash is a non-carbon impurity in carbon additives (such as metal oxides, silicates, etc.). If its content is too high, it will form a slag layer, encapsulating carbon particles and hindering their dissolution, significantly reducing the carbon absorption rate. For instance, when the ash content rises from 2% to 5%, the carbon absorption rate may drop by 20% to 30%.
Process burden: High ash content will also increase the amount of slag, prolong the slag cleaning time, and raise power consumption and labor intensity. In a melting groove induction furnace, ash accumulation may clog the melting groove and reduce electrical efficiency.
Quality risk: Impurities in the ash may seep into the castings, causing defects such as porosity and shrinkage cavities, which can affect mechanical properties and surface quality.
3. Low sulfur: Avoids spheroidization interference and ensures the performance of cast iron
The hazards of sulfur: Sulfur is a “harmful element” for ductile iron. It can interfere with the function of spheroidizing agents (such as magnesium and rare earths), causing distortion and reduction in the number of graphite spheres, and even the appearance of flake graphite, seriously reducing the strength and toughness of the castings.
Process requirements: When producing ductile iron, the sulfur content in the original molten iron must be strictly controlled at ≤0.015%. Therefore, the sulfur content of the carbon additive must be extremely low (usually ≤0.05%) to avoid the risk of sulfur addition.
Exceptions to gray cast iron: Gray cast iron requires a certain sulfur content (0.06%-0.12%) to stabilize cementite and prevent the expansion of graphitization. However, the sulfur content of the carburizer still needs to be moderately controlled to avoid excessive sulfur content leading to a tendency of white cast iron.
4. Low nitrogen: Prevents porosity defects and optimizes metallographic structure
The duality of nitrogen: In gray cast iron, nitrogen can stabilize pearlite and bend and passivate graphite, thereby enhancing mechanical properties (such as tensile strength and hardness). However, when the nitrogen content exceeds the equilibrium concentration (about 140ppm), the castings are prone to develop fissure-like nitrogen pores, leading to an increase in the scrap rate.
Process control: The nitrogen content of the carburizer used for gray cast iron is usually controlled at 70-120 PPM, while for ductile iron, which is more sensitive to porosity, a carburizer with a lower nitrogen content (such as ≤200ppm) should be selected.
High-end application requirements: In precision castings such as engine crankshafts, excessive nitrogen content may lead to substandard mechanical properties. Therefore, low-nitrogen carbon additives are crucial.
Comprehensive impact: Efficient, high-quality and low-cost casting guarantee
High-efficiency carbon increase: High fixed carbon and low ash content ensure rapid carbon dissolution and absorption, reducing smelting time.
High-quality castings: Low sulfur and low nitrogen prevent spheroidization failure and porosity defects, ensuring mechanical properties and surface quality.
Cost control: By reducing the amount of carbon additive used, lowering power consumption and the rate of defective products, the overall production cost has significantly decreased.
Instance verification
Graphite carbon additive: Fixed carbon ≥99%, ash content ≤0.5%, sulfur ≤0.05%, nitrogen ≤200ppm. It is suitable for ductile iron and has a carbon addition efficiency of over 90%.
Calcined anthracite carbon additive: Fixed carbon 90%-95%, ash 4%-5%, sulfur 0.3%-0.5%, nitrogen 800-1200ppm. It is suitable for gray cast iron, but the dosage needs to be controlled to avoid excessive sulfur and nitrogen.
Post time: Aug-25-2025