Porosity is a common welding defect that can occur in both the root and cap passes of a weld. It can cause cavities in the weld. It is typically caused by contamination of the weld metal or base metal, improper welding techniques, or incorrect gas mixture. Porosity can weaken the strength of a weld and make it more susceptible to corrosion.
Identification of porosity in welds
Weld porosity is an imperfection within the weld that can significantly impact the weld’s overall strength. Porosity can occur for various reasons, including incorrect welding techniques, dirty or damaged welding equipment, and contaminated materials.
There are a few different ways to identify porosity in a weld. One common method is to use X-ray testing. This type of testing uses high-energy waves to penetrate the weld and create an image that it can analyze for imperfections.
Another method you can use to identify weld porosity is dye penetrant testing. This method involves applying a special dye to the surface of the weld. The dye will then penetrate any pores or voids that can be seen with the naked eye.
You can also identify porosity by using a microscope. This method is more time-consuming than the other two methods, but it can provide a more detailed analysis of the weld.
Naked eyes can also detect porosity. It comes by professionalism to detect porosity by the naked eye.
Forms of porosity in welds
Porosity is one of the most common welding defects. Gas is trapped in the weld metal or the heat-affected zone (HAZ). Porosity can be caused by several factors, including poor shielding, contaminated base metals, and high humidity.
There are three main types of porosity: surface, body, and internal. Surface porosity is the most common type and is usually caused by poor shielding. Toxic base metals typically cause body porosity. Internal porosity occurs when gas is trapped inside the weld metal or HAZ.
According to the physical distribution, porosity may occur in numerous ways.
Distributed porosity in welds
- The weld joint is porous. The pores are small and dispersed throughout the weld bead, known as distributed porosity. Surface breaking pores typically indicate a high degree of distribution porosity.
The nitrogen, oxygen, and hydrogen absorption in the molten weld pool, which is then discharged on solidification to become trapped in the weld metal, cause porosity. The gas shielding was typically poor.
The porosity barrier can benefit from further improvement in its performance. When distributed porosity occurs, nearly any percentage of air entrainment in the shielding gas causes small pores. More than 1.5 per cent produces gross surface-breaking pores. Leaks in the gas supply, excessive gas flow rate, draughts and excessive turbulence in the weld pool are typical reasons for porosity.
Hydrogen can be produced in several ways, including from moisture that has not been dried out of the electrodes, fluxes or the workpiece surface, and grease and oil on the workpiece’s surface. The presence of grease or oil on the workpiece’s surface or filler wire is also a typical cause of hydrogen.
- remove slag and oxide inclusions before depositing the next layer
- use stringer beads rather than weaving
- maintain a stable arc length
- avoid excessive heat input
- use an appropriate temperature
- seal air leak
- use low hydrogen electrode
- use low hydrogen gas
- use low hydrogen flux
- control the welding speed and heat input
- reduce or eliminate arc blowing
- post-weld heat treatment (PWHT)
Wormhole porosity in welding
In welding, wormhole porosity is a type of porosity that can occur in welded metal. Gas pockets are trapped in the weld metal as it solidifies. These gas pockets can cause the weld to be weaker and less corrosion resistant.
Reason for occurrence
Wormhole porosity can occur for a variety of reasons. Some of the most common causes are:
– Incomplete fusion: If the weld metal does not completely fuse with the base metal or other weld metals, it can trap gas pockets in the weld.
– Inadequate shielding: If there is not enough shielding gas around the welding arc, gas pockets can form in the weld metal.
– Improper cleaning: If the base metal or other weld metals are not properly cleaned before welding, contaminants can cause porosity.
- check that the workpieces are at the correct preheat temperature as recommended in the welding procedure specification (WPS)
- use an autogenous weld process where possible, or if not possible, select a filler metal with a low hydrogen content (<3ml/100g of deposited weld metal)
- check that the correct root gap has been used as specified in the WPS
- check that the correct electrode size and type have been used as specified in the WPS
- make sure that any inter-pass temperature is within the allowable range as specified in the WPS
- purge the inside of hollow sections before welding to remove air
- increase gas flow rate as specified in the WPS
- check that shielding gas is not leaking from hoses or connections
- use a wire brush, grinding wheel or power tool to remove surface contamination before welding
- use a solvent to remove oils and greases from the surface before welding
- clean the inside of hollow sections before welding to remove contaminants
- Avoid a joint geometry that results in a cavity by merely matching the two components.
Crater pipe’s porosity in welds
A crater pipe is a fissure in the weld pool that develops during its final solidification and is often associated with gas porosity.
Shrinkage on weld pool solidification is the reason for this flaw. As a result, circumstances that exacerbate the liquid to solid volume change will encourage its formation. The welding arc must be discontinued to speed up the weld pool’s solidification.
A crater pipe is identified by a concavity at the weld toe or weld root and increased bead width. To detect a crater pipe, section the weld in the transverse or longitudinal direction. If the fissure is through-weld, it will be visible on both sides of the sectioned face. If the fissure only extends partially through the thickness of the weld, it will be visible on one side of the cut surface.
- The best way to prevent this flaw is to use a smaller welding electrode and decrease the welding current. By doing so, you can slow down shrinkage and minimize liquid to solid volume change.
- Another method is to use a backing strip to help reduce shrinkage.
- The welded object is then placed back into the tank to be extracted after the end of production. A post-welding run-off tag is used to allow for extinguishing outside of the welded junction. To prevent porosity.
Porosity is a common problem in welding, but you can prevent it with proper care and attention to detail. Following the recommended prevention methods can help ensure that your welds are strong and free of porosity.