What is a Low and Micro-Carbon Ferrochrome Furnace?

What is a Low and Micro-Carbon Ferrochrome Furnace?

In stainless steel production, low-carbon and micro-carbon ferrochrome are critical base materials. They are characterized by high chromium content and extremely low carbon, and they are used in steel grades that are sensitive to carbon, such as 304, 316 stainless steel, and various specialty steels. Producing such alloys cannot rely on ordinary electric or high-carbon ferrochrome furnaces; it requires a metallurgical unit capable of precise control over temperature, atmosphere, and chemical reaction rates—this is what is referred to as a low and micro-carbon ferrochrome furnace.

The core principle of this furnace is simple: make the chemical relationship between chromium and carbon controllable. At high temperatures, carbon and chromium have a strong affinity, and if not carefully managed, carbon will remain in the molten metal, making it difficult to reduce to low or micro-carbon levels. Therefore, production of low and micro-carbon ferrochrome relies not only on equipment but also on precise process control and optimized operational parameters.

The production process typically starts with melting. Raw materials include high-carbon ferrochrome, chromium oxide-bearing materials, and recycled ferrochrome scrap. Melting is usually carried out in an electric arc furnace or a submerged arc furnace. This step is not intended for decarburization but to obtain a uniform, fluid molten metal, which provides a stable base for subsequent refining. The typical temperature range is 1550–1650°C, sufficient to ensure fluidity while preventing excessive reactions between carbon and chromium.

After melting, the molten metal is transferred to the refining stage, where the carbon content is controlled. For low-carbon ferrochrome, a medium-frequency induction furnace or an argon-oxygen decarburization (AOD) furnace is typically used. The principle of AOD refining is to oxidize carbon in the molten metal using oxygen while diluting the furnace atmosphere with argon, which reduces the activity of oxygen and ensures that oxygen reacts primarily with carbon rather than chromium. The process requires precise control: oxygen flow, argon proportion, blowing rhythm, and furnace temperature must be coordinated to gradually reduce carbon to the target range while minimizing chromium loss. In practice, the process often starts with a higher oxygen proportion to accelerate decarburization and shifts to a higher argon proportion as carbon decreases to protect chromium. Furnace temperature is maintained in a stable range of 1650–1750°C.

For micro-carbon ferrochrome (carbon ≤0.03%), oxygen-argon decarburization alone is insufficient. In this case, the aluminothermic reduction process is employed. Aluminum, a strong reducing agent, reduces Cr₂O₃ to metallic chromium while forming Al₂O₃ slag, without introducing additional carbon. The aluminothermic reaction is highly exothermic, so the furnace must have strong insulation, sufficient hearth depth, and a stable slag-metal interface. Feeding sequence, aluminum particle size, and quantity must be strictly controlled; otherwise, localized overheating, slag entrapment, or aluminum residue may occur, affecting product quality.

Throughout the refining process, slag plays a role far beyond impurity collection. It controls oxygen potential and reaction environment. The composition and viscosity of the slag determine the stirring efficiency of the molten metal and the oxidation tendency of chromium. The ratios of CaO, MgO, and Al₂O₃ must be optimized; if the slag is too acidic, chromium enters the slag and is lost, while overly viscous slag reduces mass transfer efficiency and slows decarburization.

Once the carbon content reaches the target, final adjustments are made, including adding metallic chromium, silicon, and trace elements to ensure stable composition. The molten metal then enters ingot casting or continuous casting, with controlled cooling rates to prevent segregation and maintain a uniform microstructure.

Overall, a low and micro-carbon ferrochrome furnace is not about achieving extreme temperature or rapid melting. Its strength lies in process control, atmosphere management, and temperature stability. Temperature, gas composition, slag layer, and molten metal stirring must remain within a controlled range to gradually reduce carbon while preserving chromium. In essence, the furnace functions as a precision-controlled engineering system rather than a conventional high-temperature furnace. Its value lies in transforming complex chemical and thermodynamic processes into controllable, reproducible industrial operations, providing stable and reliable raw materials for stainless steel and specialty steel production.