There are many types of weld defects, and the common defects inside and outside the weld can be summarized as follows:
First: the weld size is not satisfactory
Thick welding wave, uneven shape, low or high weld reinforcement height, different welding wave width
Fillet welds with a single side or excessive sags are all substandard weld sizes. The reasons are:
1. Improper weld bevel angle or uneven mounting gap.
2. The welding current is too large or too small, and the welding specifications are not selected properly.
3. The speed of the moving rod is not uniform, and the angle of the welding rod (or welding rod) is improper.
The shape of the crack tip is sharp, and the stress concentration is serious. It has a great impact on the bearing of alternating and impact loads and static tension. According to the cause, it can be divided into cold crack, hot crack and reheat crack.
(Cold crack) refers to cracks generated below 200 ° C. It is closely related to hydrogen. The main reasons for this are:
1. Preheating temperature and slow cooling after welding are not suitable for large and thick workpieces.
2. Welding material selection is not suitable.
3. Welded joints have high rigidity and unreasonable technology.
4. Brittle and hard structure is generated in and near the weld.
5. Improper selection of welding specifications.
(Hot cracks) refers to cracks (mainly solidification cracks) that occur above 300 ° C. The main causes are:
1. Effect of composition. It is easy to occur when welding pure austenitic steel, some high nickel alloy steels and non-ferrous metals.
2. Welds contain more harmful impurities such as sulfur.
3. Improper selection of welding conditions and joint types.
(Reheat cracking) means stress-annealing cracking. Refers to the intergranular cracks generated in the heat-affected zone due to post-weld heat treatment or use at high temperature in the high-strength weld zone. The main reasons for this are:
1. Improper heat treatment conditions for stress relief annealing.
2. Influence of alloy composition. Elements such as chromium molybdenum vanadium boron have a tendency to increase reheat cracking.
3. Incorrect selection of welding consumables 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 late escape of gas can cause:
1. The electrodes and fluxes 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 the filler metal and base material are not removed.
4. The back-off method is not used to melt the arc starting point.
5. The preheating temperature is too low.
6. The positions of the pilot arc and the quench arc are not staggered.
7. Welding area is poorly protected and the area of the molten pool is too large.
8. The AC power source is prone to air holes, and the DC air holes have the lowest tendency to air holes.
During the welding process, the molten metal flows to the metal lump formed on the unmelted base metal outside the weld, which changes the cross-sectional area of the weld and is not good for dynamic loading. The reason for this is:
1. The arc is too long and the welding current on the bottom layer is too large.
2. The current is too large during vertical welding, and the moving bar is not swinging properly.
3. Weld gap is too large.
Fifth. Arc Crater
There is obvious lack of meat and dents at the end of the weld. The reason for this is:
1. Improper operation during arc welding, arc extinguishing time is too short.
2. During automatic welding, the wire feeding and power supply are cut off at the same time, without stopping the wire first and then power off.
After the arc melted the base metal at the edge of the weld, it was not supplemented by the weld metal and left a gap. Undercuts weaken the joint section of the joint, reduce the strength of the joint, cause 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 moving the bar 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 electrode and wire is incorrect.
Seven, slag inclusion
There are non-metallic inclusions inside the weld metal or at the fusion line. The inclusion of slag has an influence on the mechanical properties, and the degree of influence is related to the number and shape of inclusions. The reason for this is:
1. Each layer of welding slag is not removed during multilayer welding.
2. Thick rust left on the weldment.
3. Improper physical properties of electrode cover.
4. Poor weld layer shape and improper bevel angle design.
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 is too late to float.
Eight, not welded
There is a local non-fusion phenomenon between the base materials or between the base materials and the deposited metal. It generally exists in the root of single-sided welding, is sensitive to stress concentration, and has a large impact on properties such as strength fatigue. The reason for this is:
1. Poor groove design, small angle, large blunt side, small gap.
2. The angle of the welding rod and 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.
4. Thick rust on the weldment has not been removed.
5. Welding deviation during submerged arc welding.