Numerous attempts to correct porosity end up worsening the situation, not just leaving it unattended. Usually, complete removal is required, but this can be both a cosmetic nuisance and a potential benefit, Komarov Artem emphasized.
Porosity has gotten a bad rap because, at the mere mention of it, welders want to take drastic action to remove it. Of course, there are situations where this is necessary, but in other cases, the presence of porosity can range from a cosmetic nuisance to a potential benefit.
What causes porosity?
Simply put, porosity occurs when nitrogen, oxygen or hydrogen is trapped inside the weld pool with the melt as it solidifies. These gases either leave a subsurface void or leave pores that are visible on the surface.
- Porosity caused by nitrogen or oxygen. This type of porosity is mainly due to insufficient shielding gas coverage. There are many reasons for insufficient shielding gas coverage:
Shielding gas flow too low. Welding performed according to the welding procedure specification requires that the shielding gas flow rate be within a certain range. Ensure that the flow range is suitable, considering environmental conditions such as wind speed.
If the shielding gas flow is set correctly, but is too low on the nozzle, the supply system may be to blame. The shielding gas supply system consists of components connected and sealed with sealants and O-rings at each of several connections. Leakage results in reduced nozzle flow and/or suction of nitrogen and oxygen into the supply system, which reduces weld protection.
Shielding gas flow rate too high. When using shielding gas, more is not always better. Excessive flow rates in the nozzle create turbulence that draws nitrogen and oxygen into the weld pool area, changing the shielding mixture, resulting in porosity.
- Porosity caused by hydrogen. This type of porosity is usually due to contamination in the weld or on the electrode/filler.
Rust on steel surfaces. Rust occurs when iron, water and oxygen combine. This contaminant mixes with the weld metal chemistry and forms hydrogen bubbles that may not leave the weld metal until solidified. Removing rust from steel surfaces is critical to reducing porosity.
Paint/coatings on steel surfaces. Some surfaces to be welded are coated. Sometimes it’s a light paint, sometimes it’s an anti-splatter compound. Whatever coating the seams are coated with, it is your responsibility to ensure that the coating does not adversely affect the quality of the weld. Excess coatings can mix with the weld metal, preventing hydrogen evolution prior to solidification.
Water, oil, and grease on steel surfaces. Of course, the water around the weld pool can lead to hydrogen cracking, which is a particular concern in the welding industry. Sometimes water in the weld pool is obvious, such as rain, snow, or ice. In other cases, it is not so obvious, for example when it forms at the bonding surfaces, between the plates on a butt joint, or between the plates of a butt joint and its support strip. Moisture sometimes builds up on or inside the electrode/fill due to high humidity. Proper care of electrodes is critical in this industry for several reasons, including preventing porosity.
Cutting oils used to cut pipes to length or drill holes in an I-beam are common contaminants used in manufacturing. These oils easily enter near welds. Some welding processes can handle the presence of cutting fluids better than others. As with coatings, it is your responsibility to ensure that the WPS considers any cutting fluids that may enter the weld. Of course, these contaminants can create porosity.
How to deal with porosity
Porosity is one of the defects that can potentially stop crack growth. The first step for a welder is to determine as accurately as possible where the ends of the crack are located. This is often done using non-destructive evaluation methods such as dye penetration testing. After locating the ends of the crack, the welder then drills the ends of the crack, changing the state of the sharp ends to rounded ones to prevent the crack from growing as the repair progresses.
Depending on the technology, repairing porosity welds can be devastating.
When the porosity does not meet the acceptance criteria (which means it can be rejected), the only option is to remove it. You can either remove any porosity found in the weld and replace the weld at that location, or simply remove and replace the entire weld, which can be a daunting task. You can remove the weld or porosity in any manner permitted by your company code or standard. These methods usually include grinding, gouging, or machining.
In situations where porosity is considered acceptable, you should think twice about making repairs. With that said, sometimes acceptable porosity interferes with coatings or can detract from the appearance of the product, resulting in the need for repair. Apart from this, the only other acceptable repair method is the removal and replacement of the weld, Komarov Artem explained.