BACKGROUND ON SURFACE PREP
Pressure Weld Inspection Processes
ASME (American Society of Mechanical Engineers), ANSI (American National Standards Institute), and ASTM (American Society for Testing and Materials) standards govern pressure vessel welds. Two common types of inspection for pressure welds are X-ray inspection and ultrasonic inspection. With X-ray inspection, subsurface cracks and inclusions can be detected. This is an expensive process, but to ensure safety in critical weld joints (such as those found in submarines and nuclear power plants), 100 percent X-ray examination is typically conducted.
When an inspection uncovers a weld defect, it is usually one of several varieties. Here is a look at some common defects that can cause welds in pressure vessels to fail inspection and some best practices for preventing them.
Porosity is caused when a gas is trapped in the molten weld pool (see below). As the weld cools and solidifies, the gas forms bubbles that appear as voids upon inspection. Numerous issues can cause porosity in a weld. It’s important to check that proper welding techniques are followed and appropriate consumables are being used.
Proper preparation and cleaning of the weld surface before welding and between welding passes are also important to help prevent porosity. This is particularly true in applications involving aluminum. During aluminum welding, any liquid hydrocarbons in the weld area are a potential source of hydrogen, which will readily dissolve into an aluminum weld pool. When the weld solidifies, the hydrogen atoms will collect and form small pockets of trapped gas, or porosity. Ensuring that the weld area is clean and dry is the best way to prevent this problem.
After-rust is light rust-colored surface contamination that appears in a stainless steel weld zone several hours or days after welding. It is the nemesis of shops and contractors that work with stainless steel. The most common source of after-rust is cross contamination. T
Nitrides are a highly adherent contaminant created when plasma cutting with compressed air or nitrogen. They make the edges brittle and create porosity in some welding processes, especially gas metal arc welding. Because nitrides can exist 0.005 to 0.010 inch. below the surface of the material, you cannot remove them with brushes. Also, nitrides often cause bonded abrasive grinding wheels to load and subsequently smear. Flap discs or flap wheels work well for this issue because they are aggressive enough to remove a small amount of base material, but the grain and cloth of the accessories wear down at a rate that resists loading.
Inclusions often result from surface contaminants that become mixed into the weld pool and are trapped during solidification. In multipass welding applications, slag that is not completely removed can be a source of inclusions. Thorough cleaning with a suitable wire brush before welding and between passes is a very effective means of eliminating this type of defect.