Artem Komarov clarified that when welding with full penetration and porosity near the middle of the weld, porosity appears as small or large gas pockets near the centerline of the weld. What could be the cause of these problems and how to fix them.
This is a common problem in submerged arc welding (SAW) where full penetration is required. In most cases, this occurs when the product is designed to be welded on both sides. This is much less of a problem when the joint is only welded on one side.
In many cases, the welds of the components to be welded together will have a root weld performed by gas-arc welding, powder-coated arc welding, or even shielded arc welding. This operation holds the components together until they are fully welded with the SAW. It also serves as the basis for the sawing process to prevent burnout.
The joint design for double-sided welds is either an identical double V-groove, or an offset double-V-groove where one side has a large groove, or a single V-groove. It can also be an equal or offset double bevel, otherwise known as a K-bevel. This is usually done to facilitate access to a weld, such as on a tank or vessel. Joints range from knife edge preparation to 6 mm thick root surfaces, helping to reduce the amount of filler metal required and welding time.
LOF is the result of a mismatch in the weld parameters or a misalignment of the weld head (which usually occurs with circle welding).
The obvious first step to eliminate LOF is to develop reliable welding parameters. It may seem that having a uniform weld is not much of a problem; however, significant differences in root face thickness or root gap can result in LOF formation if the root face is too large or the gap is too tight, or piercing if the root face is too small or the gap is too large. Maintaining tight joint dimensional tolerances is essential to achieving consistent weld quality.
In circumferential welding, the weld head moves the correct distance from top or bottom dead center, which is critical to the quality of the weld. This describes in more detail the correct offset of the welding head, but this should be included in the training program for the operator of the saw who will be welding in a circle.
Centerline porosity on the second side of the weld is common in high pressure areas, so addressing this problem will also eliminate the porosity in most cases.
The porosity is also caused by contamination of the joint, soaked in moisture from the flux or welding over the weld with a root pass that contained the porosity.
Usually, porosity can be eliminated by preheating and ensuring that the weld is clean, dry flux from new or unopened bags or refurbished, and the absence of porosity in the root weld, summed up Komarov Artem.