Welding defects can be divided into external defects and internal defects. And the main causes of welding defects can be separated as follows: poor process conditions, operator error, wrong technique, incorrect consumables, and bad weld grooves.

Internal defects include:

1. Cracks

2. Incomplete penetration

3. Incomplete fusion

4. Slag Inclusion

5. Flux Inclusion

6. Porosity.

External defects include:

1. Excess height does not meet the requirements

2. Welding tumor

3. Undercutting

4. Crater

5. Arc burns

6. Surface pores

7. Surface Cracks

8. Welding deformation

9. Warpage 

The presence of welding defects is related to the groove processing and assembly accuracy, the rigor degree of the welding process, the technology of the welder and other factors. The most harmful in welding defects is the crack, the followed is lack of penetration, incomplete fusion and slag inclusion, pores, and structural defect. The number and the nature of allowed defects are determined by the using conditions and the quality assessment criteria of the product. If the surplus height of welding line is too high, it is allowed for those products by the static load, but it is not allowed for those products by cyclic fatigue loading in high frequency, even the normal excess weld metal also has to be cut off.

Anyway, the welded steel pipe with such kinds of welding defects will cause the loss of property and the human safety problem. We should have a basic understanding of these defects and try to improve them.