Submerged Arc Welding (SAW) is widely recognized as one of the most efficient and stable welding processes in heavy industrial manufacturing. Due to its high deposition rate, deep penetration, and excellent weld bead appearance, it is extensively used in industries such as bridge construction, shipbuilding, pressure vessel manufacturing, and heavy machinery fabrication.

During the process, the welding arc burns beneath a thick layer of flux, which effectively protects the molten weld pool from atmospheric contamination while significantly reducing spatter and arc radiation.

However, despite its advantages, submerged arc welding defects can still occur if welding parameters are improperly set, welding consumables are poorly managed, or base materials are insufficiently prepared.

This article provides a comprehensive engineering analysis of the most common SAW welding defects, including their root causes and practical prevention strategies. The goal is to help welding engineers, operators, and manufacturers maintain high weld quality and structural reliability in critical applications.

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1. Porosity in Submerged Arc Welding

Porosity is one of the most common defects found in submerged arc welds. It appears as round or elongated gas cavities trapped within the weld metal. These voids reduce weld density and mechanical strength, and in severe cases can significantly compromise structural integrity.

Main Causes of Porosity

Improper Heat Input and Welding Parameters

If the welding current is too low or the welding speed is too high, the overall heat input becomes insufficient. As a result, the weld pool solidifies too quickly, preventing gases from escaping before solidification.

On the other hand, excessively high arc voltage may increase arc length and weaken the protective effect of the flux layer, allowing atmospheric gases such as nitrogen and oxygen to enter the molten pool.

Moisture in Welding Flux

Flux that has absorbed moisture during storage can release hydrogen and water vapor under arc temperatures. These gases may become trapped inside the molten weld metal.

Contaminated Base Materials

Rust, oil, paint, moisture, or mill scale on the base metal can decompose under welding heat and generate gases such as carbon monoxide and hydrogen, leading to porosity.

Prevention Measures

Effective prevention of porosity requires strict control of welding materials and parameters:

• Dry welding flux according to manufacturer recommendations, typically 250–350°C for 2–4 hours depending on flux type.

• Store flux in sealed containers or heated holding ovens after drying.

• Thoroughly clean the groove and surrounding area (at least 20 mm on each side) before welding.

• Maintain a balanced combination of welding current, arc voltage, and travel speed to ensure a stable weld pool and adequate gas escape time.

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2. Slag Inclusion in SAW Welding

Slag inclusion refers to non-metallic particles of flux or slag trapped inside the weld metal. These inclusions usually appear as irregular or elongated shapes within the weld and can weaken the structural continuity of the joint.

Causes of Slag Inclusion

Improper Groove Design

Although SAW provides deep penetration, unsuitable groove geometry can trap slag in the weld root. Excessively narrow grooves or overly thick root faces restrict slag movement and prevent it from floating to the surface.

Typical groove angles for submerged arc welding are generally 40°–60°, depending on plate thickness and welding procedure.

Poor Slag Fluidity

Flux composition directly affects slag viscosity and melting characteristics. If the slag becomes too viscous during welding, it may not separate efficiently from the molten metal.

Inadequate Interpass Cleaning

In multi-pass welding of thick plates, slag from previous weld passes may remain trapped if interpass cleaning is insufficient. These residues can easily become embedded in subsequent weld layers.

Prevention Measures

To minimize slag inclusion defects:

• Use high-quality flux with good slag fluidity and appropriate chemical composition.

• Optimize groove design according to plate thickness and welding procedure specifications.

• Ensure proper welding parameters to maintain adequate weld pool temperature and fluidity.

• Perform thorough interpass slag removal using chipping hammers, wire brushes, or grinding tools before depositing the next pass.

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3. Lack of Penetration and Lack of Fusion

Among all welding defects, lack of penetration and lack of fusion are considered particularly dangerous because they significantly reduce the load-bearing capacity of welded joints.

Lack of penetration occurs when the weld metal fails to fully penetrate the joint root. Lack of fusion occurs when the weld metal does not properly fuse with the base metal or with previous weld passes.

Causes

Insufficient Welding Current

Low welding current reduces arc energy and limits penetration depth, preventing the weld from reaching the joint root.

Improper Joint Preparation

If the root face is too thick or the root gap is too small, the arc may not adequately access the root area.

Incorrect Electrode Position

Misalignment of the welding wire relative to the joint centerline may result in uneven heat distribution, leaving one side insufficiently melted.

Excessive Weld Reinforcement

In multi-pass welding, excessive reinforcement from previous passes may create cavities or “shadow zones” where subsequent weld metal cannot properly fuse.

Prevention Methods

• Select adequate welding current to ensure sufficient penetration.

• Maintain proper groove geometry and root gap during assembly.

• Ensure correct wire alignment relative to the weld joint.

• Control weld bead thickness and reinforcement in multi-pass welding.

For thick plate welding, backing plates or ceramic backing strips may be used to improve root penetration and ensure complete fusion.

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4. Welding Cracks in Submerged Arc Welding

Cracks are the most critical welding defects because they can propagate under service loads and lead to catastrophic structural failure.

Cracks in SAW welding are generally classified into two main types: hot cracks and cold cracks.

Hot Cracks

Hot cracks occur during the solidification stage of the weld metal.

Causes

These cracks are often associated with the presence of low-melting-point impurities such as sulfur and phosphorus. During solidification, these elements segregate at grain boundaries and reduce ductility, making the weld susceptible to cracking under shrinkage stress.

Excessive heat input and high welding speed may also produce deep, narrow weld pools that increase solidification stress.

Prevention

• Use welding consumables with low sulfur and phosphorus content.

• Optimize welding parameters to produce a stable and moderately wide weld pool.

• Reduce excessive welding speed and heat input fluctuations.

Cold Cracks (Hydrogen Cracks)

Cold cracks typically occur hours or even days after welding and are strongly associated with hydrogen diffusion.

Causes

Cold cracking usually results from the combination of three factors:

1. High hydrogen content in the weld metal

2. Hard or brittle microstructures in the heat-affected zone

3. High residual stress due to structural restraint

Prevention

Effective prevention strategies include:

• Using low-hydrogen welding consumables

• Proper drying of welding flux

• Preheating high-strength steels before welding

• Applying controlled cooling or post-weld heat treatment when necessary

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Conclusion

The formation of submerged arc welding defects is rarely caused by a single factor. Instead, it is typically the result of combined influences involving welding materials, process parameters, joint design, and operational practices.

Achieving high-quality welds requires systematic control throughout the entire welding process—from proper storage of welding consumables and precise groove preparation to optimized welding parameters and strict operational discipline.

With the support of modern non-destructive testing (NDT) methods, manufacturers can further ensure weld integrity and detect potential defects at an early stage. By combining sound engineering practices with continuous process optimization, submerged arc welding can consistently deliver reliable, high-quality results for demanding industrial applications.