The Difference Between VD VOD Vacuum Refining Furnace

VD and VOD vacuum refining furnace are two very common types of external vacuum refining equipment. Their core difference lies in whether they have oxygen blowing decarburization capabilities. Simply put:

• VD Furnace: 

Primarily used for degassing (especially hydrogen), desulfurization, inclusion removal, composition fine-tuning, and temperature homogenization. It cannot blow oxygen, therefore it cannot perform deep decarburization.

• VOD Furnace: 

Building upon the functions of VD, it adds top-blown oxygen functionality, specifically designed for smelting high-alloy steels such as ultra-low carbon and ultra-low nitrogen stainless steel. It can reduce carbon to extremely low levels (e.g., C ≤ 0.03% or even lower).

Differences between VD / VOD Vacuum Refining Furnace

VD Ladle Refining Furnaces - Vacuum Degassing Furnaces

1. Functions:

• Dehydrogenation: 

This is the most important function. Through high vacuum (typically operating vacuum below 67 Pa), the dissolution balance of gases in the molten steel is broken, causing hydrogen to escape and be extracted, preventing "white spot" defects in the steel.

• Denitrification: 

Has some effect, but not as significant as dehydrogenation.

• Deoxidation & inclusion removal: 

The carbon-oxygen reaction (C + O → CO↑) proceeds vigorously under vacuum, generating CO bubbles that rise to the surface. This reduces the oxygen content and also adsorbs and carries non-metallic inclusions, purifying the molten steel.

• Desulfurization: 

Lime and other slag materials are added under vacuum, and strong stirring with inert gas (Ar) creates favorable desulfurization kinetics.

Composition and temperature homogenization: Achieved through bottom-blowing Ar strong stirring.

2. Characteristics: 

• The VD furnace doesn't have an oxygen lance and cannot blow oxygen.

• It is typically used in conjunction with an electric arc furnace or converter. The primary refining furnace first completes the main dephosphorization, decarburization, and temperature rise tasks, controlling the carbon content within a certain range (e.g., above 0.10%), and then the molten steel enters the VD furnace for refining.

• Mainly suitable for bearing steel, gear steel, spring steel, high-strength engineering structural steel, and other steel grades requiring high gas purity.

VOD Ladle Refining Furnaces - Vacuum Oxygen Blowing Decarburization Furnaces

1.Function: 

• Deep decarburization through oxygen blowing under vacuum: 

This is its core and most unique function. Under vacuum conditions, the equilibrium constant of the carbon-oxygen reaction (C + O → CO↑) is greatly increased, allowing oxygen to preferentially react with carbon without over-oxidizing expensive alloying elements such as chromium (Cr). This is crucial for smelting stainless steel.

• It possesses all the functions of a VD furnace: 

Degassing, desulfurization, inclusion removal, etc.

2. Features: 

The VOD furnace is equipped with a liftable oxygen lance for blowing oxygen onto the surface of molten steel within a vacuum ladle.

• Special Process Flow: 

The primary refining furnace (usually an electric arc furnace) provides pre-melted steel with a high carbon content (e.g., C=0.3-0.6%, Cr=18%), which then enters the VOD. Under vacuum, oxygen blowing rapidly oxidizes carbon, while the oxidation of chromium is inhibited. Once the carbon content reaches the target level, oxygen blowing is stopped, and reducing agents such as ferrosilicon are added to reduce the oxidized chromium back into the molten steel (i.e., "chromium return"), increasing the chromium yield.

This process is specifically designed for producing ultra-low carbon and ultra-low nitrogen stainless steels (such as 304L and 316L), high-alloy steels, and electrical pure iron—products with extremely stringent carbon content requirements.

Simple way to remember it: 

• You can think of a VOD furnace as a "VD furnace + oxygen lance".

• If you need to produce ordinary alloy steel or high-strength steel, pursuing high purity and low gas content, use a VD furnace.

• If you need to produce stainless steel (especially ultra-low carbon type), the core task is to reduce carbon to extremely low levels without losing chromium, then you must use a VOD furnace or its derivative technologies (such as SS-VOD, strong stirring VOD).

The above explains the difference between VD and VOD vacuum refining furnace. We welcome your additions and discussions. 

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