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What Are the Most Common Defects in Welding?

Welding is a critical process in numerous industries, including construction, automotive and manufacturing. Despite advancements in technology and technique, welding defects can still occur. These defects not only compromise the strength and integrity of the weld but can also lead to failures that might be costly or even dangerous. In the latest RentArc Welding FAQs blog series, we’ve outlined the most common welding defects, their causes, and how to prevent them, helping ensure that your projects maintain the highest standards of safety and quality going forward.

1. Porosity

Porosity is one of the most common weld defects, characterised by small cavities or pores within the weld metal. These are typically caused by trapped gas and often occur when the weld pool traps hydrogen, oxygen or nitrogen. Factors such as dirty work surfaces, moisture in the electrode or improper gas shielding can contribute to porosity. To minimise this defect, ensure that your welding surfaces and equipment are clean and dry and use the correct type of shielding gas and parameters.

2. Incomplete Fusion

Incomplete fusion is a defect where the weld metal does not adequately fuse with the base metal or where subsequent layers of weld metal do not fully blend together. This can be due to a variety of factors, including low heat input, improper joint design or contamination. Ensuring clean, properly prepared surfaces and correct heat settings can help reduce the risk of incomplete fusion.

3. Undercutting

Undercutting is a defect where the weld does not fully fill the space at the base of the weld bead, leading to a weak point between the weld metal and the base metal. This often happens when the heat setting is too high or the speed of travel is too fast. To avoid undercutting, adjust the heat settings and travel speed appropriately, and ensure the angle of welding is correct.

4. Cracks

Cracks are serious defects that can appear in the weld or the surrounding base metal. They may be hot cracks, which occur at high temperatures, or cold cracks, which appear once the weld has cooled. Cracks can be due to a variety of causes, including improper cooling rates, high stress concentrations or inappropriate welding techniques. Preventing cracks usually involves proper joint design, preheating when necessary, and ensuring the weld procedure is suited to the material and the job at hand.

5. Slag Inclusions

Slag inclusions occur when flux, used in processes like shielded metal arc welding (SMAW) and flux-cored arc welding (FCAW), becomes trapped inside the weld. These inclusions weaken the weld and can be prevented by thoroughly removing slag between passes and ensuring complete melting and mixing of the weld puddle.

6. Spatter

Spatter consists of droplets of molten metal that are thrown from the weld, which stick to the surrounding surface. Although spatter does not typically affect the structural integrity of the weld, it can be unsightly and may require additional finishing work. Spatter can be minimised by adjusting the welding parameters, such as voltage and amperage and by using anti-spatter agents.

Understanding and preventing welding defects is crucial for producing strong and reliable welds. Regular maintenance of welding equipment, proper training for welders and the use of high-quality consumables are key factors in minimising the occurrence of these common issues. By ensuring these standards are met, businesses can maintain the integrity and reliability of their welded structures.

As always, if you have any welding questions, get in touch.

Call us on +44 (0)23 80 867 789 or email us at


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