Active gas oxygen content
The oxygen content in the active gas determines the wetting and flow behavior of the molten pool. 1-8% O₂ improves the weld surface and stabilizes the arc. Too much oxygen oxidizes alloying elements, creates oxide inclusions, and makes the weld brittle.
Physico-Chemical Mechanism of Action
Oxygen is surface-active: it lowers the surface tension of molten steel. This improves wetting and flattens the flow of the molten pool. The arc burns more steadily, and droplet transfer becomes finer. At the same time, oxygen reacts with iron, manganese, and silicon to form oxides. These oxides float as slag or remain as inclusions in the microstructure. Excess oxygen can react with carbon to form CO gas and cause porosity.
Metallurgical Relevance & Welding Risks
Too little oxygen (below 1%) leads to poor wetting, high spatter, and an unstable arc. Too much oxygen (above 8%) oxidizes alloying elements like manganese and silicon. The weld loses toughness, becomes brittle, and is prone to porosity and hot cracking. The weld bead shows heavy discoloration and mechanical properties drop.
Workshop Practice & Instructions
For mild and low-alloy steels, oxygen contents of 2-5% in argon-CO₂ mixtures are common. For thicker plates or spray transfer, the content can rise to 8%. Use filler materials with sufficient deoxidizers (Mn, Si). Check the gas composition with a flow meter and never mix yourself. Do not exceed 8% O₂ because impact energy drops significantly. For high-alloy steels like CrNi, the O₂ content is max 2% or use pure argon with CO₂.