What are the types of weld porosity?
Weld porosity is a common defect that occurs during the welding process, resulting from gas entrapment in the weld metal. Understanding the different types of weld porosity and their causes is crucial for preventing this issue and ensuring the integrity of welded structures. Below, we discuss the main categories of weld porosity and their characteristics.
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1.Precipitation-Type Porosity
Precipitation-type porosity occurs when gases become trapped as a result of differences in gas solubility between liquid and solid metal. As the liquid metal solidifies, gases in a supersaturated state are released, forming porosity. This type is often caused by external gases like hydrogen and nitrogen entering the weld pool.
Hydrogen Porosity: Hydrogen porosity results from the drastic difference in hydrogen solubility between liquid and solid metal. As liquid metal transitions to a solid state, the solubility of hydrogen drops rapidly, causing excess hydrogen to precipitate and form gas pockets. In low-carbon and low-alloy steel welding, hydrogen porosity often appears on the weld surface. These pores typically have a smooth, shiny internal surface and may look like threaded holes or funnel shapes. In some cases, hydrogen porosity can also be found inside the weld, particularly when high hydrogen content is present, such as from the moisture in electrode coatings or during aluminum and magnesium alloy welding.
Nitrogen Porosity: Similar to hydrogen porosity, nitrogen porosity usually forms on the weld surface and often appears in clusters, resembling a honeycomb pattern. Nitrogen contamination generally comes from insufficient shielding, allowing air to enter the welding zone.
Weld Porosity
2.Reaction-Type Porosity
Reaction-type porosity occurs when gases like carbon monoxide (CO) or water vapor (H₂O) are generated through metallurgical reactions in the weld pool and are unable to escape before the metal solidifies.
Carbon Monoxide Porosity: This type of porosity forms when CO gas, generated by metallurgical reactions, becomes trapped in the weld metal. In carbon steel welding, if the liquid metal has a high carbon content but insufficient deoxidation, reactions produce CO. At high temperatures, CO can rapidly escape, causing spatter. However, during the solidification process, the segregated alloying elements and oxides in the tail region of the weld pool increase the CO generation rate. As the metal cools, its viscosity rises, preventing CO from escaping, especially from dendritic depressions, leading to worm-like or stripe-shaped porosity.
Water Vapor Porosity: This occurs when oxides in metals such as copper or nickel react with dissolved hydrogen to form water vapor, which cannot escape. For example, during copper welding, Cu₂O reacts with dissolved hydrogen to produce water vapor, a primary cause of porosity. The process is similar when welding nickel.