Q: I have installed a gas diffuser in our medium-frequency 200-kg induction furnace. I would like to know how it works to reduce gas porosity in the stainless steel casting process: Is it a chemical reaction or mechanical action that happens in degassing with gas diffuser? Is there any experience you can reference to indicate how much time — how many minutes?— may be needed to flow the argon gas with a gas diffuser into the molten metal?
A: I assume that the “gas diffuser” you’re referring to is a porous plug installed in the bottom of the furnace. Inert gas, usually argon, is bubbled into the liquid metal through the porous plug and through the melt.
The porous plug produces very small bubbles that have a high surface area. Any gas, and hydrogen in particular, dissolved in the liquid metal can diffuse into the argon bubbles, going from a high concentration in the metal to a lower concentration in the argon. The diffusion rate is based on the surface area of the bubbles, so the smaller the bubbles, the better will be their diffusion into the metal.
The gas bubbles, now including hydrogen, float to the surface of the bath and into atmosphere, gradually reducing the hydrogen content of the metal. When the metal is poured into a mold and solidifies, any remaining gas above the solubility limit will be expelled, creating a gas hole defect in the casting.
Degassing time will depend on how much gas (hydrogen) has dissolved in the metal prior to degassing. This will depend on gas content of the charge material and gas pickup during melting, usually from high humidity in the air. Typically, degassing times would be from 5 to 20 minutes, but testing needs to be conducted on-site to meet the foundry specific conditions.
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