Welding is a key work in the construction of large-scal […]
Welding is a key work 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 skilled workers, the welding process is uneven, and there are 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.
01. The weld size is not up to the requirements
Rough welding wave, uneven shape, too low or too high welding seam reinforcement height, different welding wave width, one side of fillet welding seam or excessive sag are all unsatisfactory welding seam size. The reasons are:
1. The bevel angle of the weldment is improper or the assembly gap is uneven.
2. The welding current is too large or too small, and the welding specification is improperly selected.
3. The speed of the strip is uneven, and the angle of the welding rod (or welding handle) is improper.
The shape of the crack end is sharp and the stress concentration is serious, which has a great influence on the alternating and impact load and static tension, and is the most dangerous defect in the weld. According to the causes, it can be divided into cold cracks, hot cracks and reheat cracks.
Cold cracks: refers to cracks generated below 200 °C, which are closely related to hydrogen, and the main reasons for their occurrence are:
1. It is not appropriate to use preheating temperature and slow cooling measures after welding for large and thick workpieces.
2. Inappropriate selection of welding materials.
3. The rigidity of the welded joint is large and the process is unreasonable.
4. The weld and its vicinity produce brittle and hard tissue.
5. Improper selection of welding specifications.
Hot cracks: refers to cracks (mainly solidification cracks) generated above 300 °C. The main reasons for their occurrence are:
1. The influence of ingredients. It is easy to appear when welding pure austenitic steel, some high nickel alloy steel and non-ferrous metals.
2. The weld contains a lot of harmful impurities such as sulfur.
3. Improper selection of welding conditions and joint form.
Reheat cracks: that is, to eliminate stress annealing cracks. Refers to the intergranular cracks in the heat-affected zone due to post-weld heat treatment or use at high temperature in the high-strength welding zone. The main causes are:
1. Improper heat treatment conditions for stress relief annealing.
2. The influence of alloy composition. Elements such as chromium molybdenum vanadium boron have a tendency to increase reheat cracking.
3. Improper selection of welding materials and welding specifications.
4. Unreasonable structural design causes large stress concentration.
During the welding process, the cavities formed in or on the surface of the weld metal due to the lack of time for the gas to escape in time are caused by:
1. The welding rod and flux are not dried enough.
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 filler metal and base metal have not been cleaned.
4. The arc ignition point is not melted by the retreat method.
5. The preheat temperature is too low.
6. The positions of arc ignition and arc extinction are not staggered.
7. The welding area is poorly protected and the molten pool area is too large.
8. The AC power supply is prone to air holes, and the air holes of the DC reverse connection tend to be the least.
04. Welding flash
During the welding process, the metal nodules formed by the molten metal flowing to the unmelted base metal outside the welding seam change the cross-sectional area of the welding seam, which is unfavorable to the dynamic load. The reason for this is:
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 transport bar swings improperly.
3. The welding seam assembly gap is too large.
05. Arc crater
The weld has obvious lack of flesh and depression at the finish. The reason for this is:
1. Improper operation when receiving arc during welding, and the arc extinguishing time is too short.
2. During automatic welding, the wire feeding and the power supply are cut off at the same time, and there is no need to stop the wire first and then cut off the power.
After the arc melts the base metal at the edge of the weld, it is not replenished by the weld metal and leaves a gap. The undercut weakens the stress section of the joint, reduces the strength of the joint, causes stress concentration, and may cause damage at the undercut. The reason for this is:
1. The current is too large, the arc is too long, the speed of the strip is improper, and the heat of the arc 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 welding rod and welding wire is incorrect.
07. Slag inclusion
There are non-metallic inclusions inside the weld metal or at the fusion line. Slag inclusions have an impact on mechanical properties, and the degree of influence is related to the number and shape of inclusions. The reason for this is:
1. During multi-layer welding, each layer of welding slag is not cleaned.
2. Thick rust is left on the weldment.
3. The physical properties of the electrode coating are improper.
4. The shape of the welding layer is poor, and the groove angle is improperly designed.
5. The ratio of the fusion width to the penetration depth of the weld is too small, and the undercut is too deep.
6. The current is too small, the welding speed is too fast, and the slag is too late to surface.
08. Incomplete penetration
There is local lack of fusion between the base metal or between the base metal 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 influence on properties such as strength fatigue. The reason for this is:
1. The groove design is poor, the angle is small, the blunt edge is large, and the gap is small.
2. The angle of welding rod and welding wire is incorrect.
3. The current is too small, the voltage is too low, the welding speed is too fast, the arc is too long, and there is magnetic bias blowing, etc.
4. Thick rust on the weldment has not been removed.
5. Welding deviation during submerged arc welding.
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