How to Prevent Porosity Defects In Welding?

Porosity in welding is a dangerous issue due to poor joint preparation, improper shielding gas usage, and excessive welding speed. It can lead to reduced strength, heat loss, increased costs, and poor corrosion resistance. Pre-weld treatment, proper shielding gas usage, reduced arc time, and stay updated on industry welding techniques can help prevent porosity in welding.

Well, in this reading, we’ll explore what porosity means in welding, its causes, diagram, and how to prevent it.

What is Welding Porosity?

Welding porosity refers to the presence of cavities in weld metal, formed by gas entrapment during the solidification process. Surface porosity, subsurface porosity, craring porosity, and wormholing porosity are different types of porosity.

Surface porosity, which appears as open gaps on the welded area’s surface, is difficult to detect due to its location beneath the surface. Subsurface porosity, which is difficult to detect, is often caused by an insufficient amount of molten filler material. Wormholing porosity, on the other hand, produces defects resembling wormholes.

Porosity reduces the strength of a weld and is caused by dissolved gases in a molten weld pool. These gases include hydrogen, oxygen, nitrogen, carbon monoxide, carbon dioxide, water vapor, hydrogen sulfide, and occasionally, argon, and helium. Hydrogen is the major cause of porosity in weld metals, with filler metal being the main source.

Other sources of porosity include contaminants of the base metal surface, such as oil and mill scale. Thorough cleaning is essential for material preparation, and zinc vapor can also be a source of porosity when the base metal is coated with zinc-bearing primer or galvanized.

Related: Stainless Steel Welding: Methods, Challenges and Best Practices

Cause and Prevention of Defects in Welding

Porosity in welding is a common issue caused by the absorption of nitrogen, oxygen, and hydrogen in the molten weld pool. This can result from poor gas shielding, leaks in the gas line, high gas flow rates, draughts, and excessive turbulence in the weld pool. Nitrogen can originate from various sources, including moisture from inadequately dried electrodes, fluxes, or the workpiece surface, grease and oil on the workpiece or filler wire, and surface coatings like primer paints and zinc coatings.

To fix porosity, identify and remove the gas source, such as air entrainment, avoiding weld pool turbulence, using filler with an adequate level of deoxidants, reducing excessively high gas flow, avoiding draughts, drying the electrode and flux, cleaning and degreasing the workpiece surface, and checking the weldable primer thickness.

Wormholes are elongated pores that produce a herringbone appearance on the radiograph and indicate a large amount of gas being formed which is then trapped in the solidifying weld metal. To prevent wormholes, eliminate the gas and cavities, clean the workpiece surfaces, remove surface contamination, and check the primer thickness.

Crater pipes form during the final solidification of the weld pool and are often associated with gas porosity. Preventing crater pipes can be achieved by controlling the rate at which the welding arc is extinguished or manipulating the welding arc and welding wire. Techniques include using run-off tags, grinding out the weld run stop crater before continuing with the next electrode or depositing the subsequent weld run.

What Causes Porosity Defects in Welding?

Porosity in welds can be caused by various factors, including gas outage, draft disturbances, moisture, nozzle restriction, and nozzle angle. Welders should be aware of drafts from overhead fans and machinery, and check the nozzle opening before starting a weld.

Proper gas flow is crucial, with 50-60 CFM at the GMAW nozzle and 20-30 CFM at the GTAW torch. Proper use of antispatter compounds, sprays, or gels can also contribute to porosity, boiling into a gas when exposed to high temperatures.

Contaminated GMAW gun liners can introduce unwanted elements to the weld pool, especially for stainless steel and high-nickel-alloy wires. The nozzle’s awkward position on the edge of an outside corner joint can cause problems, as does the open weld joint.

Damaged O-ring seals on the GMAW gun whip and cut or burnt hoses can also cause issues. A defective gas solenoid at the wire feeder or GTAW machine can also contribute to conditions that create porosity. To address moisture in the welding process, welders should store materials like SAW flux in dryers and ovens.

Related: 13 Types of Welding Defects, Their Causes and Remedies

How to Prevent Porosity in Welding

To prevent welding porosity, it is crucial to remove the porous section and prevent it entirely. Proper welding and workspace preparation techniques can help. Clean material surfaces before welding is essential for a clean weld, as after-effects can lead to surface contamination and porosity.

This can result in poor mechanical properties and require rework or replacement. Checking the gas flow from the gas shield is also essential. The more powerful the flow, the more air is disturbed, leading to contaminants mixing with the weld puddle, resulting in impure welds. Selecting the correct flow rate for each application can improve efficiency and ensure quality welds.

To ensure accurate flow from your gas shield, check your equipment, including the weld gun tip and tension to ensure a clean and efficient weld puddle. Workspace conditions, such as hot environments, should be monitored for strong air flows or currents that could affect the weld puddle or gas shield.

Keep the gun away from the weld site to prevent air and gas from seeping into the weld puddle, resulting in weak welds. Staying updated with industry welding techniques is crucial for identifying and completing high-quality welds, avoiding costly mistakes. Formal welding training can make this easier, and employers may prefer formally trained welders for their technical experience.

How to Detect Porosity is Welds 

Porosity imperfections can be detected using penetrant or magnetic particle inspection techniques, while radiography or ultrasonic inspection is used for sub surface imperfections. Radiography is more effective for detecting porosity imperfections, but small pores can be difficult to detect in thick sections.

Remedial action involves localized gouging or grinding, or if widespread, the entire weld is removed and re-prepared and re-welded according to the agreed procedure. The three ways to detect porosity in welding include visual Inspection , destructive testing , and non-destructive testing.

Visual inspection: Close attention is paid to surface porosity during a visual inspection of the weld pass, ensuring that the weld is properly inspected for any signs of surface porosity.

Destructive Testing: Destructive testing involves examining the porosity of a weld in various ways. Nick-break specimens are made by notching a strap and pulling the weld apart, while fillet weld specimens can be cut, polished, and treated with an acid solution to enhance the cross-section.

Non-destructive Testing: Porosity is a common issue in pipe welding, often detected through radiography or other non-destructive testing methods. Ultrasonic examination, widely used in structural steel weld examination, can detect numerous discontinuities throughout the weld’s thickness.

However, detecting porosity can be challenging depending on the size and amount. Ultrasonic examination may reveal discontinuities such as incomplete fusion, porosity, or trapped slag, but the true nature may not be known until the weld metal is removed.

Related: What is Welding Inspection? it Techniques, and Who is it

Material Susceptible to Porosity 

Porosity in materials can be caused by gases like hydrogen, nitrogen, and oxygen. To remove scale coatings, use a wire brush and degrease with a wire brush. For stainless steel, use a wire brush and degrease with a wire brush. For aluminum and alloys, use a chemical clean with a wire brush and degrease with a scrape. For nickel and alloys, use a wire brush and degrease with a wire brush.

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