Oxide inclusion
Oxide inclusions are solid, non-metallic oxygen compounds (e.g., Al₂O₃, SiO₂) that form in the weld pool and remain trapped in the solidified weld. They act like internal notches: they disrupt stress flow, reduce toughness, and can lead to cracking.
Physical-Chemical Mechanism of Action
Oxide inclusions form when oxygen in the weld pool reacts with elements like aluminum, silicon, manganese, or titanium. This reaction creates solid oxide particles. Normally, these particles rise to the surface as slag. If the weld metal solidifies too quickly or the slag isn't fully removed, the particles get trapped in the weld. They are brittle and have a different thermal expansion than the steel. This creates stress concentrations at the particle boundaries.
Metallurgical Relevance & Welding Risks
Oxide inclusions act as internal notches. They reduce the impact toughness and fatigue strength of the weld. Under dynamic loading, cracks can start from these inclusions. Line-shaped or planar inclusions are especially dangerous because they weaken the cross-section. In high-strength steels, oxide inclusions can lead to brittle fracture. Corrosion resistance also drops because the inclusions are starting points for local corrosion.
Workshop Practice & Action Items
Avoid oxide inclusions by cleaning the weld edges: thoroughly remove rust, scale, grease, and paint by grinding or brushing. Use a suitable shielding gas with adequate flow (e.g., argon or mixed gas, 10–15 l/min) to prevent oxygen from getting in. Guide the electrode or wire evenly so the slag can float up. For multi-pass welding, completely remove the slag after each pass with a chipping hammer and wire brush. Keep preheat and interpass temperatures in the specified range to avoid too-fast solidification. After welding, visually inspect the weld and use non-destructive testing to check for inclusions.