There are many causes of weld defects in pressure vessels. In this blog, we focus on the three most common causes.
Moisture on the Weld Surface
Porosity is one of the most common defects caused by the presence of moisture on the weld surface. It occurs when gas gets trapped in the molten pool, resulting in the formation of air bubbles as the weld cools and solidifies. Inclusions are another type of defect that can arise from the presence of dirt and/or contaminants on the weld surface being mixed in with the molten weld pool. Proper cleaning of the surface, both prior to welding and in between passes is the most effective way to prevent these conditions.
Improper Use of Cleaning Tools
After-rust is a common defect seen in stainless steel welds. It typically is a result of cross contamination when a stainless steel brush used for cleaning was previously used on a steel surface. In such cases, steel particles collect on the brush and are then deposited on the stainless surface. This condition is typically observed days after the weld has solidified and can easily be prevented by ensuring that brushes used for cleaning are either new or have not been used on non-stainless steel surfaces.
Nitrides are produced during plasma cutting with compressed air or nitrogen. The formation of nitrides can result in frailness and/or porosity around the edges of the weld. They often extend below the weld surface, which prevents their removal by traditional brushing and cleaning processes. Doing so typically requires the use of more aggressive cleaning tools that are capable of reaching base material and resisting loading.