Welding Defects Explained: A Practical Guide from Detection to Repair

Welding is a method used to join metal parts. It is important in many industries such as construction, automotive manufacturing and machinery. But even with advanced technology, welding problems or ‘defects’ can still occur. These defects don’t just affect the appearance of the weld. They can also make parts weaker. In some cases, the entire product or structure may fail. To ensure safety and quality, it is vital to detect, prevent and repair these defects.

In this article, we will describe different welding defects and their causes. We will also describe how hidden problems can be detected through special tests. These tests are called non-destructive tests and do not damage the welded part. Finally, we will share tips on how to prevent defects and how to repair them. By following these steps, you can make stronger, safer welds.

What are welding defects?

Welding defects are problems or imperfections that occur during the welding process. These defects may affect the appearance or function of the weld. According to ISO 6520, a defect is called a ‘weld defect’ if it affects the intended function of the weld. If they do not affect the quality of the weld, they are called ‘weld discontinuities’.

The acceptance of a weld defect depends on three main factors:

• Defect type: Different kinds of defects, like cracks, porosity (tiny holes), or areas that didn’t fuse properly, may be more or less severe.
• Defect size: Small defects may be acceptable, but large defects may weaken the structure.
• Defect Location: If the defect is located in a critical load-bearing area, it is more serious than if it is located in a less critical location.

Some defects may be within acceptable limits, while others may reduce the overall quality of the product or even lead to its rejection.

Classification of welding defects

Welding defects can be divided into two main categories. These types depend on the location of the defect and its effect on the weld. The two types are external defects and internal defects. They occur in different ways and are detected in different ways. Therefore it is important to understand these two types to improve the quality of the weld.

External welding defects

External weld defects are problems that can be seen on the outside of the weld. These defects usually change the appearance of the weld. Some defects also make the weld weaker. Common external welding defects include: undercut / weld cracks / spatter / overlap / slag inclusions, etc.

Internal welding defects

Internal welding defects are hidden inside the weld and cannot be seen with the naked eye. To detect these defects, special tests called non-destructive testing are usually required. Internal defects may affect the structural integrity and mechanical properties of the weld. Common internal weld defects include: porosity/unfused/incomplete penetration/internal cracks/inclusions, etc.

Common Welding Defects

#1 Welding Cracks

Welding cracks are continuous or discontinuous cracks that form in the weld or heat affected zone. Cracks are usually characterised by length, depth, width and the direction of crack extension. Welding cracks may form along the weld seam, at the junction of the weld seam and the base metal, or even within the base metal. They can be classified into various types, such as hot cracks, cold cracks, and laminar cracks, according to where they appear and when they form.

• Hot cracks: occur during the welding process, usually caused by cooling contraction of the weld metal at high temperature.
• Cold cracks: occur during the cooling process after the completion of welding, commonly found in high-strength steel.
• Laminar Crack: Cracks perpendicular to the weld seam, usually occurring in materials with a laminated structure.

Causes

• Excessive cooling: Large temperature differences in the weld area create excessive stresses.
• Material brittleness: High carbon steel or brittle materials are more prone to cracking.
• Concentration of stress: Thermal stresses are not evenly distributed during the welding process.
• Improper process settings: excessive welding current, improper speed or inappropriate material selection

Preventive measures

• Control cooling rate: Reduce stress by preheating or slow cooling.
• Optimize welding parameters: Adjust current and welding speed to avoid overheating or overcooling.
• Selection of suitable materials: Use materials with better toughness to reduce the risk of cracking.
• Reasonable weld design: Reduce stress concentration and prevent crack extension.

Remedial measures

• Crack removal: Use grinding tools to remove cracked areas and re-weld.
• Heat treatment: Heat treat cold cracks to remove residual stress.

#2 Crater Defect

Craters are small holes or depressions that form at the end of a weld. This usually occurs in the molten pool, especially in thick plate or multi-layer welds. These craters create additional stress in the weld, increasing the chances of cracking or weakening over time. This is especially a problem for welds that are subjected to a lot of movement or repetitive stress, as it can cause the weld to crack more quickly.

Causes

• Incomplete molten pool fill: The weld is completed too quickly for the molten pool to be completely filled.
• Insufficient shielding gas: The gas protecting the weld is insufficient, so the edges of the pool do not fill properly.
• Current cut-off is too fast: The power supply is switched off too quickly, causing the molten pool to cool down too quickly and collapse.

Preventive measures

Gradual reduction of current: When finishing the weld, slowly reduce the current to allow the pool to fill smoothly.

Extend the cooling time: Depending on the thickness of the material, extend the time before finishing the weld. This will effectively ensure that the molten pool is completely filled.

Remedial measures

• Rewelding: If a crater forms, you can grind it down and then add more weld material to fill the area.
• Fill and inspect: After repairing with the correct filler material, you need to check the quality of the repair. You can do this by careful observation or by using non-destructive testing methods. This helps to ensure that there are no residual stress points.

#3 Undercut

Undercutting occurs when a groove or depression is formed where the weld meets the base metal. This is usually due to the fact that too much metal was melted during the welding process, resulting in erosion of the edge of the base metal. This defect weakens the weld, especially in parts that need to support weight or are under pressure. It can lead to cracking or even fracture, which reduces the strength and service life of the entire structure.

Causes

• Excessive welding current: Excessive current leads to overheating of the molten pool and erosion of the base metal.
• Excessive welding speed: Moving too quickly during welding can prevent the molten pool from properly filling the edges, which leaves dents.
• Wrong electrode angle: If the angle of the electrode isn’t right, the heat won’t spread evenly, causing problems.

Preventive measures

• Control welding current: Use appropriate welding current to avoid overheating of the base material.
• Optimise welding speed: Maintain a moderate welding speed so that the molten pool can completely fill the edge of the weld.
• Adjust electrode angle: Ensure that the electrode angle is appropriate so that the heat is evenly distributed between the weld and the base metal.
• Fix the electrode angle: Make sure the electrode is at the right angle so the heat spreads evenly between the weld and the base metal.

Remedial Measures

• Fill the weld: Grind the undercut area and refill the weld.
• Inspection and Repair: Ensure that the weld is flat and free of stress concentration by inspection after repair.

#4 Porosity

Porosity is the formation of bubbles or small holes inside or on the surface of the weld. These bubbles reduce the quality of the weld. Porosity weakens the weld, thus affecting the weight or stress it can withstand. Porosity can also lead to leaks, especially in pressure vessels or liquid pipelines.

Causes

• Contamination: Dirt, oil, rust or water on the weld area may trap gas, making it difficult for air to escape during the welding process.
• Not enough shielding gas: Using the wrong amount or type of shielding gas can allow air to enter the molten metal.
• Moisture in the materials: If the welding material (e.g. electrode or wire) contains moisture. This moisture can then create air bubbles during the welding process.

Preventive measures

• Clean the welding area: Remove dirt, grease, rust and other contaminants from the surface of the weldment before welding.
• Appropriate shielding gas: Ensure that the correct type and flow rate of shielding gas is used to avoid air entering the molten pool.
• Dry the welding material: Ensure that the welding material is dry before use to avoid moisture entering the molten pool.

Remedial measures

• Welding repair: Clean the affected area and re-weld the repair.
• Non-destructive testing: Test the weld after repair to ensure that no holes or porosity are remaining in the weld.

#5 Spatter

Spattering occurs when small droplets of molten metal splatter around the weld and adhere to the surface. This creates rough metal particles that make the weld seam look messy. It also means that sanding and cleaning takes extra time and can cause problems if paint or coatings are to be added later.

Causes

• Excessive welding current: When the welding current is too high, the molten metal becomes unstable and scatters..
• Inappropriate welding material: Use of inappropriate welding wire or electrode leads to unstable melting process and increased spattering.
• Improper gas flow: The flow rate of shielding gas is too high or too low, leading to instability in the welding zone.

Preventive measures

• Adjust the welding current: Use the appropriate current to ensure the stability of the molten pool and reduce spattering.
• Choose the right material: Select the right wire or electrode for the job to ensure smooth melting of the metal.
• Adjust gas flow: Set the shielding gas flow correctly to maintain a stable welding process.

Remedial Measures

• Sanding: Sand or use a chemical cleaner to remove splatter and smooth the surface.
• Prepare Surface: After removing spatter, prepare the surface so that the coating or protective layer will bond well.

#6 Overlap

Overlap is a raised metal build-up where the weld metal accumulates on the surface of the base metal, but is not fully fused to the base metal. This problem weakens the weld and increases the risk of stress concentrations, which may lead to structural failure. In addition, overlap can affect the appearance of the weld and subsequent finishing processes.

Causes

• Welding current too low: Insufficient current results in the melted weld metal not being able to fully fuse with the base metal.
• Welding speed is too slow: the welding speed is too slow, resulting in excessive metal accumulation on the surface of the base material, forming an overlap.
• Improper welding angle: The welding torch angle is incorrect, resulting in the molten pool failing to fully contact the base metal.

Preventive measures

• Optimize welding current: Ensure that sufficient current is used to promote full fusion of the metal with the base metal.
• Adjust weld speed: Adjust weld speed to the thickness and material of the part to avoid excessive metal build-up.
• Ensure Proper Welding Angle: Maintain proper welding angle to ensure even distribution of molten metal and fusion with base material.

Remedial measures

• Remove overlap: Remove overlapping areas by mechanical grinding or cutting and re-weld.
• Inspection of the weld: Ensure that the weld has no overlap problems and has the required strength by non-destructive testing after repair.

#7 Laminar Tear

Lamellar tearing is a type of tearing that occurs in the weld area along the layers of the base metal. It is usually perpendicular (or at right angles) to the direction of stress in the weld, and is more common in thicker sheet metal. This type of tearing weakens the weld and makes it unable to withstand stress, especially in structures that carry heavy loads. If this happens, the welded joint will fail.

Causes

• Parent material delamination defects: Uneven layer structure within the parent material, especially with non-metallic inclusions or layer defects.
• Uneven Stress: Heat and stress can cause material layers to pull apart and tear during welding.

Preventive measures

• Check the base material: Ensure the quality of the base material, especially high-strength metals, before use. Avoid using materials with non-metallic bit or layer defects.
• Choose the right welding method: Control the heat during welding to reduce the chance of tearing. For thick plates, use a layered welding process to distribute heat and stress.

Remedial measures

• Replacement of base material: If the laminar tear is severe, it may be necessary to replace the base material.
• Welding repair: For localised laminar tearing, welding repair can be used, adjusting the process to reduce heat input and distribute the stresses evenly.

#8 Slag Inclusion

Slag inclusions are residues of slag that have not been completely removed from the weld, usually within the weld or between weld layers. It weakens the weld and increases stress concentrations, which can lead to cracks and fractures. In pressurized welds, slag can weaken the seal or reduce the fatigue resistance of the weld.

Causes

• Incomplete slag removal: The slag from the previous weld layer wasn’t cleaned off in time during the welding process, which caused it to get trapped in the new layer.
• Poor electrode quality: Use of unqualified electrodes leads to excessive or incomplete removal of slag.
• Welding speed is too fast: welding speed is too fast, resulting in the slag can not fully float out of the molten pool and entrapped in the weld.

Preventive measures

• Remove slag in time: After each layer of welding, especially when welding multiple layers, be sure to clean the slag immediately to prevent it from entering the next layer.
• Use high quality electrodes: Choose high quality and suitable welding rod. In this way, it will help the slag to float up and be easy to remove.
• Control welding speed: Adjust the welding speed according to the material and thickness. Excessive speed will prevent proper removal of the slag.

Remedial Measures

• Removal of inclusions: Remove slag inclusions by grinding or mechanical cutting and re-weld the repair.
• Non-destructive testing: Perform testing after repair to ensure that there is no slag residue or new inclusions in the weld.

#9 Incomplete Fusion

Incomplete fusion occurs when the weld metal does not fully bond with the base metal or the previous weld pass. This weakens the connection between the weld and base metal, reducing the overall strength of the weld. In high-stress conditions, this can increase the risk of failure, leading to cracking or splitting of the weld.

Causes

• Welding speed too fast: The welding speed is too fast, resulting in the weld metal not being able to fully melt the base metal or the previous weld pass.
• Insufficient welding current: Low current provides inadequate heat, preventing proper fusion.
• Incorrect welding angle: Improper welding angle results in the molten metal pool not being able to fully cover the base metal.

Preventive measures

• Reduce the welding speed: Adjust the welding speed so that the molten pool has enough time to melt and fully combine with the base material.
• Increase the welding current: Increase the current appropriately to provide enough heat to ensure that the weld and base metal fuse.
• Adjust the welding angle: Ensure that the welding angle is correct so that the molten pool can evenly cover the joint between the weld and the base metal.

Remedial Measures

• Re-welding: Clean the area of incomplete fusion and re-weld to ensure proper bonding.
• Non-Destructive Testing (NDT): After repairs, use NDT to confirm that the weld is fully fused and meets strength requirements.

#10 Incomplete penetration

Incomplete penetration happens when a weld doesn’t fully go through the thickness of the base metal, leaving the weld bond incomplete.. This problem is more common in thick plates or deep grooves. It can weaken the overall strength of the weld, especially when the structure is subjected to heavy loads or stresses. This can lead to cracks, fractures or even failure of the welded section, reducing the safety and durability of the structure.

Causes

• Insufficient welding current: When the welding current is too low, there isn’t enough heat for the weld metal to fully penetrate the base metal.
• Improper welding position: Wrong welding angle or welding position, resulting in the molten pool not being able to fully penetrate to the bottom of the base material.
• Excessive welding speed: excessive speed does not allow enough time for the weld metal to fully penetrate the base metal.

Preventive measures

• Adjust the welding current: Set the current high enough based on the material thickness to ensure the weld fully penetrates.
• Optimize welding angle: Adjust the welding angle correctly so that the molten pool can be evenly distributed and fully penetrate deep into the base material.
• Control welding speed: Maintain proper welding speed to ensure that the weld has enough time to achieve full penetration.

Remedial Measures

• Grinding and rewelding: Clean up bad weld areas and reweld with the correct current and angle.
• Non-Destructive Testing: Use methods like ultrasonic or X-ray testing to check if the weld is properly fixed.

#11 Distortion

Distortion occurs when a welded part changes shape due to heat after welding. This can affect the size and appearance of the part, and in precision construction can also affect performance and durability. In some cases, rework may be required to resolve the problem.

Causes

• Uneven welding heat input: Uneven heat distribution leads to inconsistent expansion and contraction in different parts of the weldment.
• Uneven cooling: different cooling speeds after welding, resulting in concentration of thermal stress and deformation.
• Improper welding sequence: Incorrect sequence makes localized areas receive too much heat, increasing the risk of distortion.

Preventive measures

• Use fixtures to fix: Fix the weldment during the welding process to prevent distortion.
• Control heat input: Use lower heat input to ensure even heat distribution.
• Segmented welding: Use symmetrical or segmented welding to reduce localized heat concentration.

Remedial Measures

• Heat correction: Localized heating and use of mechanical means to correct distortion.
• Mechanical correction: Correction of deformations after cooling by means of jigs or hammering.
• Non-destructive testing: Test dimensional accuracy after correction to ensure compliance with requirements.

#12 Burn Through

Burn-through is the excessive melting of metal during welding, resulting in a hole being formed through the base metal. This problem can severely weaken the strength and integrity of the weldment.

Causes

• Excessive current: The current is too high and the temperature of the molten pool is too high, resulting in penetration of the base metal.
• Welding speed is too slow: slow moving speed, the molten pool stays too long, resulting in excessive melting.
• Material too thin: thin material cannot withstand excessive heat.

Preventive measures

• Reduce current: Adjust the current to avoid excessive heating.
• Accelerate speed: Appropriately accelerate the welding speed, reduce the molten pool residence time.
• Choose the right process: Choose the appropriate welding process and parameters for thin plates.

Remedial measures

• Repair the weld: Fill the burned-through area and re-weld the repair.
• Non-destructive testing: Perform testing after repair to ensure the integrity of the weld.

#13 Mechanical Damage

Mechanical damage is the destruction of the weld or base metal by mechanical forces during the welding process. This weakens the welded structure and affects stability and longevity.

Causes

• Improper operation: Damage caused by incorrect use of tools or equipment.
• External interference: Welding is subjected to external shock or vibration.

Preventive measures

• Standardized operation: Strictly follow the specifications for welding operations.
• Avoid interference: Ensure the working environment is stable and avoid external influence.

Remedial measures

• Repair damage: Clean up the damaged area and make patch welding.
• Strength check: check the structural strength after repair to ensure no hidden danger.

#14 Excessive Reinforcement

Excessive reinforcement is an excessive buildup of weld metal that causes the weld to protrude above the surface of the base metal. This can directly affect the appearance and may increase stress concentrations in the weld area.

Causes

• Too low a current: Insufficient welding current results in excessive metal buildup.
• Welding speed too slow: Welding speed is too slow, resulting in excessive metal deposition.

Preventive Measures

Control current and speed: Adjust the welding current and speed to ensure proper fusion and avoid excessive buildup.

Remedial Measures

• Repair the weld: Remove excessive metal buildup by grinding or polishing.
• Non-destructive testing: Perform testing after repair to ensure that the weld is free of stress concentration problems.

#15 Weld Whiskers

Weld whiskers is the appearance of thin filaments in the metal during the welding process, which affects the quality of the weld.

Causes

• Current fluctuation: The welding current is unstable, resulting in uneven melting of the metal.
• Material impurity: The welding material contains impurities, which affects the uniformity of the metal.

Preventive measures

• Use of high-quality materials: ensure that the welding material is pure and free of impurities.
• Control welding parameters: Keep the current stable and prevent fluctuations.

Remedial Measures

• Remove whiskers: Grind off the whiskers and re-inspect the weld.
• Inspection and Repair: After removing the whiskers, inspect the weld to ensure it meets quality standards.

#16 Misalignment

Misalignment is when the weld in a welded part is not properly aligned with the base metal, resulting in a welded joint that is out of position.

Cause

• Inaccurate Positioning: Improper alignment prior to welding, resulting in offset during welding.
• Fixture not securely fastened: The fixture is loose, causing the part to move during the welding process.

Preventive Measures

• Accurate Positioning: Ensure that the weldment is properly aligned and secured.
• Secure with fixtures: Use proper fixtures to prevent parts from moving during welding.

Remedy

• Reposition Weld: Adjust misaligned parts and re-weld to ensure alignment.
• Inspection and Repair: Check after repair to ensure that the weldment is correctly positioned and not offset.

How to detect invisible welding defects (non-destructive testing methods)

Magnetic Particle Flaw Detection

Magnetic particle inspection is a simple method of finding cracks or defects on the surface of a weld using a magnetic field.. You magnetize the weldment and the cracks allow the magnetic powder to attach to them, thus showing the location of the defect. This method is usually applied to ferromagnetic materials, such as steel products.

Magnetic particle flaw detection is a simple method used to find cracks or defects on the surface of a weld by using a magnetic field.

Ultrasonic Inspection

Ultrasonic inspection is like ultrasound for physical examination. It is done by emitting high-frequency sound waves, which are reflected back when they encounter problems (such as cracks or voids) in the weld. By analyzing these echoes, it is possible to determine if there are any internal problems. This method is applicable to many materials.

Radiographic Inspection

Radiography is similar to an X-ray in that the rays penetrate the weld and show internal defects as irregular areas on the image. It is often used to find internal holes, cracks, and other defects, making it ideal for welded structures that need very high quality.

Weld discontinuity vs welding defects (how to distinguish)

lthough weld discontinuities and weld defects look similar, there is actually a clear difference. Discontinuities refer to some irregularities in the weld, like minor porosity or dimpling. These do not necessarily affect the serviceability of the weldment, so they are usually acceptable. This is not the case with weld defects, which directly affect the strength and safety of the weld and must be repaired. Simply put, discontinuities are controllable, while weld defects require immediate attention. Here are their differences

Weld discontinuity:

• Irregularities such as minor porosity or dimpling.

• Usually does not affect performance within specified limits.

• Acceptable and not necessarily requiring repair.

Welding Defects:

• Includes serious problems such as cracks, unfusion, and burn-through.

• Can affect the strength or safety of the weld.

• Must be repaired or rewelded.

Conclude

Welding defects can weaken a product and shorten its life, but you can minimize these problems with a few simple steps. First, make sure that the weld settings (e.g., current and speed) are matched to the material and process. Also, use high-quality welding materials to avoid impurities. Keeping the welding process stable is also important to prevent interference. Finally, it’s a good idea to do non-destructive testing after welding to check for hidden defects. Yonglihao provides professional metal welding services, and our experience and technology can help you achieve high-quality welding results. Need help? Contact us anytime!