What is Welding Porosity And How to Prevent It?

Porosity is a costly and dangerous issue for welders. Porosity in welding refers to cavity formation in welds, which can vary in shape and size. Poor joint preparation, improper shielding gas usage and excessive welding speed are some of the causes of porosity in welding.

Welding porosity can lead to reduced strength, heat loss, increased costs and poor corrosion resistance. Pre-weld treatment, proper shielding gas usage, reduced arc time and staying updated on industry welding techniques can help prevent porosity in welding.

Continue reading to learn more. We’ll look at what causes welding porosity, how it affects the welding process and how to prevent it.

What is Welding Porosity?

Porosity is the presence of cavities in the weld metal caused by the freezing in of gas released from the weld pool as it solidifies. These cavities can vary in shape and size, ranging from discrete, tiny gas bubbles to larger craters on the material’s surface.

There are several different types of welding porosity, including:

Surface porosity: This looks like open gaps on the welded area’s surface.
Subsurface porosity: This can be difficult to detect with the naked human eye due to its location beneath the surface.
Cratering porosity: It appears as uneven and distinct crater shapes in the welded area and is most commonly caused by an insufficient amount of molten filler material that would otherwise fill the cavity.
Wormholing porosity: It produces defects that look like “wormholes,” named after their resemblance to a worm burrowing its way through the material.

Cause and prevention of defects in welding

#1. Distributed porosity and surface pores.

Distributed porosity is normally found as fine pores throughout the weld bead. Surface-breaking pores usually indicate a large amount of distributed porosity.

It is caused by the absorption of nitrogen, oxygen, and hydrogen in the molten weld pool which is then released on solidification to become trapped in the weld metal.

Nitrogen and oxygen absorption in the weld pool usually originates from poor gas shielding. As little as 1% air entrainment in the shielding gas will cause distributed porosity and greater than 1.5% results in gross surface-breaking pores.

Leaks in the gas line, too high a gas flow rate, draughts, and excessive turbulence in the weld pool are frequent causes of porosity.

Hydrogen can originate from a number of sources including moisture from inadequately dried electrodes, fluxes, or the workpiece surface. Grease and oil on the surface of the workpiece or filler wire are also common sources of hydrogen.

Surface coatings like primer paints and surface treatments such as zinc coatings may generate copious amounts of fume during welding. The risk of trapping the evolved gas will be greater in T joints than butt joints especially when fillet welding on both sides.

Special mention should be made of the so-called weldable (low zinc) primers. It should not be necessary to remove the primers but if the primer thickness exceeds the manufacturer’s recommendation, porosity is likely to result especially when using welding processes other than MMA.

How to Fix it?

The gas source should be identified and removed as follows:

Air entrainment

Seal any air leak
Avoid weld pool turbulence
Use filler with an adequate level of deoxidants
Reduce the excessively high gas flow
Avoid draughts

Hydrogen

Dry the electrode and flux
Clean and degrease the workpiece surface

Surface coatings

Clean the joint edges immediately before welding
Check that the weldable primer is below the recommended maximum thickness

#2. Wormholes.

Characteristically, wormholes are elongated pores that produce a herringbone appearance on the radiograph. Wormholes are indicative of a large amount of gas being formed which is then trapped in the solidifying weld metal.

Excessive gas will be formed from gross surface contamination or very thick paint or primer coatings. Entrapment is more likely in crevices such as the gap beneath the vertical member of a horizontal-vertical, T joint which is fillet welded on both sides.

When welding T joints in primed plates it is essential that the coating thickness on the edge of the vertical member is not above the manufacturer’s recommended maximum, typically 20µm, through over-spraying.

How to prevent Wormholes?

Eliminating the gas and cavities prevents wormholes.

Gas generation

Clean the workpiece surfaces at and adjacent to the location where the weld will be made
Remove any surface contamination, in particular, oil, grease, rust, and residue from NDT operations
Remove any surface coatings from the joint area to expose bright material
Check the primer thickness is below the manufacturer’s maximum

Joint geometry

Avoid a joint geometry that creates a cavity

#3. Crater pipe.

A crater pipe forms during the final solidification of the weld pool and is often associated with some gas porosity.

This imperfection results from shrinkage on weld pool solidification. Consequently, conditions that exaggerate the liquid-to-solid volume change will promote its formation. Extinguishing the welding arc will result in the rapid solidification of the weld pool.

In TIG welding, autogenous techniques, or stopping the welding wire entering the weld pool before extinguishing the welding arc, will affect crater formation and may promote pipe imperfection.

How to prevent Crater pipes?

Crater pipe imperfection can be prevented by controlling the rate at which the welding arc is extinguished or by the welder technique manipulating the welding arc and welding wire

Removal of stop

Use run-off tag to enable the welding arc to be extinguished outside the welded joint
Grind out the weld run stop crater before continuing with the next electrode or depositing the subsequent weld run

Welder technique

Progressively reduce the welding current to reduce the weld pool size (use slope-down or crater fill functions)
Add filler (tig) to compensate for the weld pool shrinkage

Causes For Porosity In Welding

From most common to least, let’s look at some of the causes of porosity in welds:

The cylinder is out of gas. This happens quite often.
Air or a draft of some kind disturbs the delivery of the shielding gas during the welding process. Overhead or floor fans even as far as 25 feet away can wreak havoc on the gas delivery. Welders also need to be aware of open doors and air being discharged from machinery. These drafts, if more than 4 to 5 miles per hour, can affect shielded metal arc welding (SMAW) and flux-cored arc welding (FCAW) operations.
The presence of moisture can lead to problems. It might be simple water or morning dew, but also could be condensation from welding on heavy plate and lap joints, which might occur particularly when temperatures reach below 50 degrees F. The easy fix is to preheat the metal to 200 to 220 degrees F to evaporate the moisture.
Plugged or restricted gas metal arc welding (GMAW) gun nozzles—typically from weld spatter—impede the delivery of shielding gas. To rectify this obstacle, the welder needs to look at the nozzle opening before starting a weld. This double-check might prevent weld spatter from falling into the weld.
The weld nozzle is held too far away from the weld puddle. The volume of shielding gas reaching the weld is diminished, and dilution of the shielding gas with the atmosphere severely affects the weld.
The GMAW gun is laid at an angle that will spread the gas flow out and actually suck in the atmosphere from the back side, opposite the nozzle direction. A 5- to 15-degree angle, perpendicular to the joint, is an acceptable angle for forehand or backhand methods with GMAW or FCAW guns and SMAW electrodes.
Paint, grease, oil, glue, and sweat release large volumes of gas when exposed to arc welding temperatures. This is especially true with solid-wire GMAW and gas tungsten arc welding (GTAW), but FCAW and SMAW processes are vulnerable as well. The flux makeup was not designed to handle such contamination.
When mill scale and rust are welded over, decomposition gases are formed, and oxidation begins, which can involve the presence of moisture. The strong possibility of cold lapping and lack of fusion at the weld toe also exists. When a metal oxidizes, it is no longer truly a metal and can’t be expected to respond to welding the same as a metal, especially when welding flux is not used.
Plating compounds with zinc, such as in the galvanization process, can create a problem. Zinc melts at approximately 420 degrees F. At welding temperatures far in excess of 2,000 degrees F, zinc changes from a solid to a gas in a fraction of a second. Also, zinc dust is a byproduct of the welding process. The release of both gases and dust make welding galvanized metal an unpleasant experience. (In an effort to prevent letters and calls of protest, let me say electrodes and welding procedures have been developed to weld galvanized material successfully. However, training and lots of practice are absolutely necessary to overcome the presence of all that trapped gas.)
SMAW electrodes, FCAW electrodes, and submerged arc welding (SAW) flux absorb moisture in an unprotected environment. To address moisture in the welding process, codes are pretty clear about the use of dryers and ovens to store these materials. SAW flux in particular is like a sponge. Once the container is opened, the welder should store the package according to the manufacturer’s directions.
The gas flow is too high. Gas flow of 50 to 60 cubic feet per hour (CFH) at the GMAW nozzle and 20 to 30 CFH at the GTAW torch should be plenty. If not, ask why. Wide-open gas flow at the nozzle actually creates turbulence and can pull outside air into the weld zone. Additionally, it’s a terrible waste of gas and adds unnecessary cost to the project. The only exception might be if the shielding gas contains more than 50 percent helium.
A pinched or smashed gas hose doesn’t deliver the shielding gas properly. If the gas hose is more than 20 ft. long, the possibility of it kinking is pretty good.
Improper use of antispatter compounds, sprays, or gels can be a major contributor to porosity. When used in excess, the antispatter material becomes a contaminant, boiling into a gas when exposed to the high temperatures of the welding arc. Also, jamming the GMAW gun into a container of antispatter gel can result in the gel dripping back into the weld puddle. An operator should use the anti- spatter material properly or not at all.
Weld filler metals contaminated with paint, grease, oil, tape, and glue can release gases when exposed to the very hot welding arc. Even dirty gloves used during GTAW can contaminate the consumables. Cleaning solid wire and flux-cored wire with wire wipes and GTAW fillers with steel wool is a good idea.
Contaminated GMAW gun liners can introduce unwanted elements to the weld pool. All the grease, oil, dust, and dirt found in the shop environment collects on the wire and ends up in the gun’s whip liner. Stainless steel and high-nickel-alloy wires are especially susceptible to attracting these contaminants.
GMAW right on the edge of an outside corner joint might create problems given the awkward position of the nozzle. The nozzle often does not cover the joint properly, causes turbulence, and draws in outside air into the weld joint.
If the weld joint is open at the root, it will suck in air from the back side. Unprotected liquid metal can absorb air easily.
The welding gas itself could be contaminated. If the welding gas is a suspect, the shop needs the gas supplier to certify that the gas has the correct dew point.
A contaminated gas hose could be a culprit, in particular, hoses that have been used for other activities prior to being used in a welding application. In one real-world example, a hose was grabbed from a storeroom to repair a cut hose that was attached to the wire feeder. Unfortunately, a bug had built a nest in the hose while it was sitting undisturbed in the storeroom. In another example, an air hose that was previously used as an air line for a tool on a line with an oil lube system on it was quickly connected to welding equipment only to find out later that the hose was full of air tool oil.
Damaged O-ring seals on the GMAW gun whip where it plugs into the wire feeder or the GTAW torch cap where it screws into the torch could introduce unwanted air into the welding process.
Cut or burnt hose anywhere from the regulator flowmeter to the connection at the feeder could create issues.
A defective gas solenoid at the wire feeder or the GTAW machine is a possible contributor to conditions that create porosity.

Detecting Porosity in Welded Material

Porosity, depending on the type involved can be detected in several different ways.

Visual Inspection

The most common and cost-effective method to detect porosity is a visual inspection. Surface porosity can be seen and measured by looking at the completed weld in adequate light, paying particularly close attention to the starts and stops in the weld pass.

Destructive Testing

Destructive examination methods are often used to qualify welders or procedures and can be performed in a variety of ways. Nick-break specimens are made by notching a strap and pulling the weld apart in a tensile machine and examining the cross section for porosity.

Fillet weld specimens can be cut in a saw and polished, followed by an acid solution applied that will enhance the cross-section of the weld making any porosity indication clearly visible.

Non-Destructive Testing

Radiography is very commonly used in pipe welding as it can give a very definitive look through the weld and detect porosity indications in all weld passes present. Porosity can also be detected with dye-penetrant or mag-particle examination on the surface or barely sub-surface level.

Ultrasonic examination, widely used in structural steel weld examination, can detect the presence of a myriad of weld discontinuities throughout the entire thickness of the completed weld. Porosity, depending on the size and amount, may be difficult to detect with ultrasonic examination.

Ultrasonic examination may show an indication that could be any one of several different discontinuities; incomplete fusion, porosity, or trapped slag. The true nature of the discontinuity may not be known until the weld metal has been removed and the discontinuity revealed.

How to Prevent Porosity in Welding

How to prevent porosity in welding? The most viable method of repairing welding porosity is to completely remove the porous section. Attempting to weld over porosity rarely solves the underlying problem.

The most effective way to deal with welding porosity is to prevent it entirely, which is very possible with proper welding and workspace preparation techniques.

#1. Keep it clean.

Preparation of material surfaces prior to welding can prove to be as critical to a clean weld as welding itself. The after effects of fabrication can lead to surface contamination and porosity if proper care is not taken to clean.

This in turn can lead to unsound welds with poor mechanical properties, which require rework or replacement at the expense of time and money.

#2. Check Your Gas Flow.

Monitor the flow from you gas shield. The more powerful the flow of gas is, the more air is disturbed. This can lead to contaminants mixing with the weld puddle, which in turn causes an impure weld.

Although flow rates can vary, it is important to select the correct flow rate for each application. Doing this can improve efficiency and ensure quality weld. If you are not sure what the correct flow rate is, you should seek advice from your gas supplier.

#3. Check Your Equipment.

Over time hoses may begin to leak or wiring may become exposed or frayed. Checking all connections before striking an arc will help make sure you get an accurate flow from your gas shield.

Check the tip of the weld gun to make sure you have a clean tip, sometimes the tip may become clogged which will eventually cause impurities in the weld. Check the tension to your drive rolls or wire spool hub. Poor tension can cause poor wire feeding performance.

#4. Workspace Conditions.

Welding shops can get very hot, however you should think twice before opening a door to get a breeze. You must monitor your workplace for strong air flows or currents that could affect the weld puddle or gas shield. Watch your voltage / arc length.

The further away the gun is from the weld site, the more likely air and gas will seep into the weld puddle causing bubbles to form which will in turn make a weak weld.

#5. Keeping up with industry welding techniques.

Staying updated on the latest welding industry standards requires initiative and curiosity and is an important part of a welder’s duties.

Doing so will allow welders to identify and complete high-quality welds. This way, they can complete high-stakes welding projects while avoiding costly mistakes.

This can be made much easier with formal welding training. Some employers may also prefer formally trained welders for their technical experience, as they may require less on-the-job training for entry-level welding roles than those without technical training.