Welding is a key task in the construction of large-scale installation projects. Its quality and efficiency directly affect the safe operation and manufacturing period of the project. Due to the different levels of technical workers, the welding process is uneven and prone to many defects. The types and causes of defects are sorted out to reduce or prevent the occurrence of welding defects and improve the quality of project completion.
First, the weld size does not meet the requirements
Rough welding waves, uneven appearance, too low or too high weld reinforcement height, different welding wave widths, and excessive unilateral or sag of fillet welds are all weld sizes that do not meet the requirements. The reasons are:
1. Improper weld groove angle or uneven assembly gap.
2. The welding current is too large or too small, and the welding specification is improperly selected.
3. The rod feeding speed is uneven, and the angle of the welding rod (or welding handle) is improper.
Second, cracks
The shape of the crack end is sharp, the stress concentration is serious, and it has a greater impact on the bearing of alternating and impact loads and static tension. It is the most dangerous defect in the weld. According to the causes of occurrence, they can be divided into cold cracks, hot cracks, and reheating cracks. (Cold cracks) refer to cracks generated below 200°C. They are closely related to hydrogen. The main reasons for their occurrence are:
1. The preheating temperature and slow cooling measures after welding are not suitable for thick workpieces.
2. Improper selection of welding materials.
3. The rigidity of the welded joint is large and the process is unreasonable.
4. Brittle and hard structures are generated in the weld and its vicinity.
5. Improper selection of welding specifications.
(Hot cracks) refer to cracks generated above 300°C (mainly solidification cracks). The main reasons for their occurrence are:
1. The influence of composition. It is easy to occur when welding pure austenitic steel, some high-nickel alloy steels, and non-ferrous metals.
2. The weld contains more harmful impurity elements such as sulfur.
3. Improper selection of welding conditions and joint forms.
(Reheat cracks) are stress relief annealing cracks. Refers to the intergranular cracks in the heat-affected zone of high-strength welding areas due to post-weld heat treatment or high-temperature use. The main reasons for their occurrence are:
1. Improper heat treatment conditions for stress relief annealing.
2. The influence of alloy composition. Elements such as chromium, molybdenum, vanadium, and boron tend to increase reheat cracks.
3. Improper selection of welding materials and welding specifications.
4. Unreasonable structural design causes large stress concentrations.
Third, pores
During the welding process, the holes formed inside or on the surface of the weld metal because the gas does not have time to escape in time are caused by:
1. Insufficient drying of welding rods and flux.
2. The welding process is not stable enough, the arc voltage is too high, the arc is too long, the welding speed is too fast and the current is too small.
3. The oil and rust on the surface of the filler metal and the base material are not cleaned up.
4. The backward method is not used to melt the arc starting point.
5. The preheating temperature is too low.
6. The positions of arc starting and arc extinction are not staggered.
7. Poor protection of the welding area, too large a molten pool area.
8. AC power supply is prone to pores, and DC reverse connection has the least tendency to pores.
Fourth, weld nodules
During the welding process, the molten metal flows to the unmelted parent material outside the weld to form a metal nodule, which changes the cross-sectional area of the weld and is not conducive to dynamic loads. The reasons for this are:
1. The arc is too long and the bottom welding current is too large.
2. The current is too large during vertical welding, and the wire swing is improper.
3. The weld assembly gap is too large.
Fifth, arc pits
The weld has an obvious meat shortage and depression at the end. The reasons for this are:
1. Improper operation during welding arc closure, and the arc extinguishing time is too short.
2. During automatic welding, the wire feeding and power supply are cut off at the same time, and the wire is not stopped before the power is turned off.
Sixth, undercut
After the arc melts the parent material at the edge of the weld, it is not supplemented by the weld metal and leaves a gap. The undercut weakens the stress-bearing section of the joint, reduces the strength of the joint, causes stress concentration, and may cause damage to the undercut. The reasons for this are:
1. The current is too large, the arc is too long, the rod feeding speed is improper, and the arc heat is too high.
2. The voltage of submerged arc welding is too low and the welding speed is too high.
3. The inclination angle of the electrode and wire is incorrect.
Seventh, slag inclusion
There are non-metallic inclusions inside the weld metal or at the fusion line. Slag inclusion has an impact on the mechanical properties, and the degree of impact is related to the number and shape of the inclusions. The reasons for this are:
1. Each layer of welding slag is not cleaned up during multi-layer welding.
2. Thick rust is left on the weldment.
3. The physical properties of the electrode coating are improper.
4. The shape of the weld layer is poor and the groove angle is improperly designed.
5. The ratio of the weld width to the weld depth is too small, and the undercut is too deep.
6. The current is too small, the welding speed is too fast, and the slag does not have time to float out.
Eighth, incomplete penetration
There is a local lack of fusion between the parent materials or between the parent materials and the deposited metal. It generally exists at the root of the weld of single-sided welding, is very sensitive to stress concentration, and has a great impact on strength, fatigue, and other properties. The reasons for its occurrence are:
1. Poor groove design, small angle, large blunt edge, and small gap.
2. Incorrect angles of welding rods and welding wires.
3. The current is too small, the voltage is too low, the welding speed is too fast, the arc is too long, there is magnetic blow, etc.
4. There is thick rust on the weldment that has not been cleaned up.
5. Welding deviation during submerged arc welding.
Post time: Nov-04-2024