Welding defects can be divided into two main categories, internal defects and external defects, according to their location in the weld.

External defects are located on the outer surface of the weld and are directly visible. External defects mainly include weld size non-conformity, edge galling, weld tumour, collapse, surface porosity, surface cracks, burn through, etc.

Internal defects mainly include non-penetration, internal porosity, internal cracks, slagging, etc. Internal defects are located inside the weld and can only be detected by non-destructive flaw detection or by ring-breaking tests.

Causes of welding defects

1. Weld seam

The size of the weld seam does not meet the requirements. Mainly refers to the uneven height, width and narrow width of the weld seam, the residual height is too high and insufficient.

The size of the weld seam is too small to reduce the load-bearing capacity of the welded joint; the size of the weld seam will increase the welding workload, make the residual stress and welding deformation increase and cause stress concentration.

Improper welding bevel angle or uneven assembly gap, too much or too little welding current, improper way or speed and welding angle will cause the weld seam size not to meet the requirements.

2. Biting edge

When welding, the groove formed on both sides of the weld at the junction with the base metal is called the biting edge (or biting flesh). Bite edge will make the effective cross-section of the base metal reduced, weakening the strength of the welded joint, while at the same time at the bite edge is easy to stress concentration, after bearing may produce cracks at the bite edge, or even cause the structure of the broken ring.

The reason for biting edge is improper operation process, incorrect choice of welding specification, such as excessive welding current, too long arc, improper angle of welding rod, etc.

3. Welding tumours

During the welding process, the molten metal flows outside the weld to the unmelted base material on the formation of the metal tumor is the weld tumor. Not only does it affect the aesthetic appearance of the weld, but there are often imperfections below the weld, which can easily cause stress concentration. The weld gap is too large, the welding rod position and transport method is not correct, the welding current is too large or the welding speed is too slow, etc. can cause the weld tumor.

4. Burn through

During the welding process, the molten metal flows out from the back of the bevel, forming a perforation defect called burn-through.

The main reason for burn-through is that the welding current is too high, the welding speed is too slow, and burn-through can also occur when the assembly gap is too large or when the blunt edge is too thin.

5. Unfused

Unfused refers to welding, welding channel and the base material between or between the welding channel and the welding channel is not completely fused with the part; or refers to spot welding between the base material and the base material is not completely fused with the part.

The reasons for not fusing are that the welding line energy is too low; the arc is blown off; there is rust and dirt on the sidewall of the bevel; the welding layer is not completely cleared of slag, etc.

6. Pits, collapses and unfilled

Cratering refers to the formation of a localised low-lying portion of the weld surface or the back of the weld that is lower than the surface of the base material.

Collapse refers to the phenomenon of a single-sided fusion weld where the welding process is improper, resulting in an excess of weld metal through the back side, causing the front side of the weld to collapse and the back side to bulge. Due to insufficient filler metal, a continuous or intermittent groove formed in the surface of the weld, this phenomenon that is unfilled.

7. Slagging

After welding residue in the weld slag called slag.

Many reasons for slag, such as the edge of the weld and weld layer, between the weld channel clean-up is not clean; welding current is too small, resulting in melting more metal solidification speed up, slag in time to float out; improper transport, slag and iron separation is not clear, impede the slag float; welding parts and electrodes of improper chemical composition; molten pool contains too much oxygen, nitrogen composition, etc.

8. Stomata

Welding, the molten pool of air bubbles in the solidification failed to escape and residual cavities formed by called pores. Pores can be divided into dense pores, worm-like pores and needle-like pores. The main gases that form pores in the weld are hydrogen, nitrogen and carbon monoxide.

It not only reduces the effective area of the weld, which reduces the mechanical properties of the weld, but also destroys the denseness of the weld and can easily cause leakage.

Causes of porosity are, welding process, good protection of the welding area is damaged; base material welding area and wire surface with oil, rust and adsorption of water pollutants; welding rod moisture, baking is not sufficient; welding current is too large or too small, welding speed is too fast; welding arc is too long, arc voltage is high.

9. Cracking

The temperature at which welding cracks are formed can be divided into hot and cold cracks, and according to the location where the cracks occur can be divided into cracks in the weld metal and cracks in the heat-affected zone. In the welding process, the weld and heat-affected zone metal cools to a high temperature zone near the solid phase line resulting in a weld crack called a hot crack; the weld crack produced when the welded joint cools to a lower temperature is called a cold crack.

Weld cracking is the most dangerous welding defect, which seriously affects the performance of the welded structure and the safety and reliability of the welded structure. In addition to reducing the strength of the welded joint, cracks also cause severe stress concentrations due to a sharp notch at the end of the crack, prompting the development of cracks and damage.