Welding defects: types, causes and prevention

Even an experienced welder makes errors if conditions or settings are not correct. Being able to recognise common welding faults understand what causes them and know how to prevent them is a central part of delivering quality — and of passing a welding test.

In the standards, a welding defect is called an «imperfection». The imperfections have names and numbers in DS/EN ISO 6520. An AMU objective in the blacksmith training regulations uses exactly that: the apprentice must be able to assess the typical welding defects of the welding processes according to DS/EN ISO 6520 and assess welding work according to DS/EN ISO 5817 and DS/EN ISO 10042.

Pores are small cavities in the weld created by gas trapped in the weld pool. They are often caused by impurities such as grease, paint, rust or moisture on the joint surfaces, or by the shielding gas being blown away by draft. Prevention is about clean joint surfaces and protecting the weld zone from draft.

Missing penetration means the weld has not penetrated fully with the base material or all the way to the root. It gives a weak joint. The causes can be too low heat input incorrect joint preparation or too high welding speed.

Burn-in is a notch along the weld seam edge where the base material has melted away without being filled back up. It weakens the joint and is typically caused by too high current, incorrect electrode angle or too high speed.

With methods that form slag, slag can become trapped inside the weld if not removed between passes. This gives weak points. Good slag removal between passes and correct technique prevent the error.

Cracks are among the most serious defects because they significantly weaken the joint and can grow over time. They can occur during cooling and are related to the material's composition, stresses in the part and cooling rate. Preheating and correct sequence can in some cases prevent them.

A special kind of crack is the cold crack. It can appear hours after welding, and hydrogen in the weld metal is one of the causes. Hydrogen is measured in ml per 100 g of weld metal. ISO/TR 17844 divides it into five classes, from A (more than 15 ml) to E (at most 3 ml). Rutile and cellulosic electrodes belong to class A; basic electrodes can reach B to D. Example: ESAB's basic electrode OK 48.00 is marked H5, and the data sheet states less than 4.0 ml of hydrogen per 100 g of weld metal.

ISO 5817 (4th edition from February 2023) sets quality levels for fusion-welded joints in steel, nickel and titanium: B, C and D. B is the strictest. Aluminium has its own standard, ISO 10042. In the blacksmith programme, the apprentice must weld to level B in the subject Energiteknik 3 (energy technology 3).

Errors are detected by visual inspection and, where required, by non-destructive or destructive testing. The skilled welder uses the errors as learning: each error points back to a cause in preparation, setting or technique that can be corrected next time.