Vacuum Refining Furnace: Fundamental Principles

Vacuum Refining Furnace: Fundamental Principles

The Vacuum Degassing Process

The core principle involves transferring molten steel into a vacuum chamber for refining. The process initiates by submerging two snorkel tubes (risers) into the ladle containing the molten steel. Inert gas, typically argon, is injected into the molten steel through porous plugs located in the side walls of these risers. Under the combined influence of the molten steel's high temperature and the low-pressure (vacuum) environment in the upper chamber, the argon bubbles rapidly expand. This causes the density of the argon-steel mixture within the riser to decrease significantly along its height.

Driven by this density differential, the lighter mixture is forced upward into the vacuum chamber. Upon entering the vacuum, the argon bubbles expand explosively, and the molten steel is dispersed into a fine spray or droplets. This dramatic increase in surface area, combined with the high vacuum, creates ideal conditions for dissolved gases (such as hydrogen and nitrogen) to escape from the steel into the vacuum space.

The degassed molten steel, now denser, returns to the ladle through the downcomer snorkel. This circulation cycle repeats multiple times, progressively reducing the gas content in the steel to very low levels.

Process Control:

During the cycle, sampling and temperature measurements are conducted at regular intervals—typically every 10 minutes initially, increasing to every 5 minutes as the treatment concludes. Based on the analytical results, alloying materials, deoxidizers, or other additives can be introduced into the vacuum chamber via an automated, controlled feed system without breaking the vacuum, allowing for precise composition adjustment.

Advantages of Vacuum Degassing:

1.  Superior Degassing Efficiency: The injection of argon generates abundant bubble nuclei. The subsequent atomization of the steel into droplets within the vacuum massively increases the surface area available for gas removal, leading to highly effective degassing.
2.  Minimal Temperature Loss: The inherent temperature drop during treatment is relatively small, generally between 30-50°C. Furthermore, the process can be equipped with heating capabilities (e.g., induction), meaning the steel only requires minimal superheat from the primary furnace.
3.  Broad Processing Flexibility: The same vacuum unit can efficiently process heats of varying capacities and is compatible with molten steel sourced from different primary furnaces, such as electric arc or induction furnaces.

Due to these significant benefits, vacuum cycle degassing technology has seen rapid development and widespread adoption. Globally, over 100 RH (Ruhrstahl-Heraeus) type units are in operation, with the largest capable of processing 350-ton heats. In China, this technology has been implemented in several major plants, including Daye Steel Plant (commissioned 1967), Shanghai Heavy Machinery Plant (1972), Wuhan Iron & Steel's No. 2 Steelmaking Plant (No. 1 RH in 1979, No. 2 in 1990), and Baoshan Iron & Steel's Steelmaking Plant (December 1985). Operational practice has proven this equipment to be highly effective and flexible.

Process Capabilities of Induction Melting Vacuum Degassing Furnaces:

Such furnaces can perform a range of conventional and advanced processes, including:

   Melting solid charge materials under atmospheric or vacuum conditions.

   Melting under a precisely controlled protective atmosphere.

   Homogenizing alloy distribution and fine-tuning chemical composition.

   Vacuum degassing to remove hydrogen and nitrogen.

   Removal of volatile residual elements (e.g., lead, zinc, bismuth) under low pressure.

   Deoxidation via the carbon-oxygen reaction enhanced by vacuum.

   Decarburization, where the carbon-oxygen reaction is significantly promoted at low pressure, allowing for effective production of ultra-low-carbon steels.

   Bottom stirring through argon injection via porous plugs for enhanced mixing and reaction kinetics.
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