Question

Vacuum metallurgy is used to remove

• A. oxides
• B. slag
• C. carbon
• D. dissolved gases

Hint:

Hydrogen concentrations must be kept below \( 2\text{ ppm} \) in high-strength steels to prevent hydrogen-induced cracking (flaking or hydrogen embrittlement), which is easily achieved using vacuum degassing.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the primary impurity class or component removed from liquid steel using vacuum metallurgy (degassing) techniques.

Step 2: Key Formula or Approach:
The solubility of diatomic gases (like Hydrogen and Nitrogen) in liquid metal is governed by Sieverts' Law:
\[ [\%G] = K_G \sqrt{P_G} \]
where:
\( [\%G] \) is the weight percentage of the dissolved gas in the liquid metal,
\( K_G \) is the temperature-dependent Sieverts' constant, and
\( P_G \) is the partial pressure of the gas above the liquid metal bath.

Step 3: Detailed Explanation:

• Mechanism of Gas Removal: According to Sieverts' Law, reducing the partial pressure of a gas above the liquid metal dramatically lowers its solubility limit within the liquid.
By placing the molten metal in a vacuum vessel (where total pressure is reduced to very low values, e.g., \( \lt 1\text{ Torr} \)), dissolved hydrogen and nitrogen gases spontaneously desorb from the liquid metal and are evacuated as gas molecules (\( \text{H}_2 \) and \( \text{N}_2 \)).

• Vacuum Decarburization: Vacuum treatment also promotes the reaction between dissolved carbon and oxygen to form carbon monoxide gas:
\[ \underline{\text{C}} + \underline{\text{O}} \rightarrow \text{CO(g)} \]
A low pressure shifts this equilibrium to the right, allowing for the production of ultra-low carbon steels. However, the primary physical class of impurities targeted is collectively described as dissolved gases (Hydrogen, Nitrogen, and Oxygen).

• Comparison with Other Options:
-

Oxides (Option A) and

slag (Option B) are solid/liquid oxide phases that are removed mechanically by flotation or skimming, not primarily by vacuum treatment.


Step 4: Final Answer:
Hence, vacuum metallurgy is mainly used to remove dissolved gases, corresponding to Option (D).