Primary prevention and improvement measures for welding defects

1. Stoma

Cause Analysis:
1. The welding rod or welding wire is affected by moisture, especially the low-hydrogen welding rod, which is prone to produce pores and the front arc starter of the low-hydrogen welding rod falls off;
2. When the arc is broken, the welding wire leaves the molten pool too quickly, the molten pool lacks gas protection, and arc crater pores appear;
3. When re-igniting the arc, the weld before the arc was broken was not effectively polished, and the arc was not ignited;
4. The wind force on site is strong and wind protection measures are not in place;
5. There is dirt, condensation or moisture on the weld;
6. The electrode is skewed or the arc is too long;
7. Welding technology is not proficient enough.
Prevention and control measures:
1. Welding rods or welding wires should be kept dry. Low-hydrogen welding rods should be dried according to regulations, and limited quantities should be collected and stored in heat preservation barrels. Welding rods that are not used up on the day should be dried again. The arc starter of the low-hydrogen welding rod must be kept intact. Handle with care during loading and unloading to prevent the arc starter from falling off and damaging the coating;
2. When the arc is broken, the electrode should stay at the arc breaking point for a short time or perform remelting welding to control the entry of harmful gases.
3. When re-starting the arc, use a grinding wheel to grind the arc pit defect to the original arc breaking point, and re-weld at the weld bead before the arc breaking, and completely cover the weld bead part when the arc was broken;
4. Windproof measures should be taken. Low-hydrogen electrodes are extremely sensitive to wind and should be strictly windproof. When using ultra-short arc welding, welding operations cannot be carried out without windproof measures. Experience shows that wind leakage may occur even at wind speeds below level 2.
5. The pipe mouth must be kept clean and dry, free of rust, oil stains, impurities, etc.;
6. When the welding rod deviates from the arc, the arc should be broken, the welding rod should be replaced and polished;
7. If the welder is not proficient in welding technology, he should strengthen targeted practice and master the ability to control defects as soon as possible.
2. Slag inclusion:
Cause Analysis:
1. The groove angle is small, the welding current is small, the slag is accumulated more or the welding speed is fast, the molten pool cools quickly, and the slag does not have time to float to the surface of the molten pool, causing slag inclusion;
2. The root weld is too thin or polished too much, the hot weld is easy to burn through, and high current is not allowed. The root cleaning method is improper, and there are corners or too deep grooves on both sides of the weld. During rapid hot welding, the slag flowing to the corners and deep grooves has no time to float up and forms slag inclusions;
3. The welder's operating technique is improper, the welding rod moves too fast in the center and corners of the weld, the molten pool is not melted deep enough, and there is residue covering or mixed in it.
4. The welding rod is tilted, the other side is not well fused, and slag inclusion occurs;
5. If the welding rod and weld are stained with dirt and not cleaned up, slag inclusion will be formed during welding;
6. When welding vertically downhill, the welding speed is fast, the welding rod angle and the method of moving the welding rod are improper. Especially for low-hydrogen electrodes, when filling welding, "railway slag inclusions" are easy to appear above and below the 3 points, that is, parallel strip slag inclusions appear on both sides of the weld at the same time.
Prevention and control measures:
1. The gap between the two sets of welds should not be too small. A larger current should be selected within the process parameter range to ensure the welding depth. At the same time, the welding speed should not be too fast.
2. The root weld should not be too thin or over-grinded. Remove the surface slag (or black spots) and flatten the joint. When cleaning the root, the weld should be cleaned into a "U"-shaped groove to avoid forming an angle, so that the slag is not easy to float out during welding. The root weld grinding should be conducive to hot welding and control burn-through and slag inclusion;
3. The welder should correct incorrect operating techniques, control the angle and swing amplitude of the welding rod, and ensure the welding depth;
4. When the welding rod is deflected, the arc should be cut off, the welding rod should be replaced and polished;
5. Carefully clean the impurities on the pipe and welding rod or the slag on the previous weld. Before welding, clean the weld stubble, slag and dirt inside the joint to prevent impurities from entering the weld. During the welding process, always keep the molten pool unobstructed to ensure that the slag liquid surface is well separated from the molten pool metal;
6. When welding vertically downhill, the welding speed should not be too fast to ensure the penetration depth. Use the correct electrode angle and electrode movement method. Especially for low-hydrogen electrodes, when filling welding, pay more attention to the three-point position, the technique should be steady, and the welding angle should be controlled (the parallel angle of the three-point position is recommended to be 10-15 degrees). The welding speed should not be too fast to prevent parallel strip slag from appearing on both sides of the weld.
3. Incomplete penetration:
Cause Analysis:
1. The welding current is too small, the penetration is too shallow, the welding speed is too fast, the electrode residence time on both sides of the weld is too short, or the electrode angle is biased to one side, so that the parent material (root weld) or interlayer metal is not fully melted and covered by the filler metal;
2. When welding the root, the groove angle is too small, the joint gap is small, the blunt edge is thick or the misalignment is large, the pipe wall thickness is uneven, the penetration depth is insufficient, and the rod transportation method is improper, which can easily cause incomplete root welding;
3. The welding rod angle is improper during welding, causing the arc heat source to dissipate or deviate, resulting in incomplete welding on one side.
Prevention and control measures:
1. According to the requirements of welding regulations, control the groove size and blunt edge thickness, adjust the assembly gap, strictly control the misalignment of pipe joints, and pipes with different wall thicknesses should be processed into a gentle slope shape as required;
2. Select a larger current within the process parameter range to ensure the depth of penetration. Adjust and reasonably control the wire feeding angle and welding speed to keep the arc in the correct direction;
3. The edge of the groove should move slowly and stay a little longer to keep the electrode in place so that the heat is enough to melt the blunt edge of the parent material and the previous layer of weld metal. Pay attention to the shape of the molten pool when welding at the root. The shape of the penetration depth is a small circle in front of the molten pool that moves forward with the welding, which can effectively ensure the root penetration depth.
4. Not fused (not completely welded):
Cause Analysis:
1. The welding current is too small, the welding speed is too fast, the electrode residence time on both sides of the weld is too short, or the electrode angle is biased to one side, so that the base material or interlayer metal is not fully melted and is covered by the filler metal;
2. Small joint gap, thick blunt edge or large misalignment, improper electrode angle and operation method, insufficient penetration, and too fast welding speed may cause the root weld to fail to fuse;
3. Incomplete welding is mainly caused by the welding rod or wire not being swung into place, and secondly by arc deviation.
Prevention and control measures:
1. Select a larger current within the process parameter range, reasonably adjust the wire feeding angle and welding speed, feed the wire slower at the edge of the groove, and keep it for a longer time so that the heat is sufficient to melt the parent material and the previous layer of weld metal;
2. Adjust the gap between the two sets of welds, reduce the thickness of the blunt edge and the amount of misalignment, make the weld groove smooth and the slag clean;
3. Adjust the electrode angle, keep the electrode in place, and keep the arc direction correct;
4. To prevent incomplete welding, the welding rod or welding wire should be returned to its original position. When arc deflection occurs, the welding rod should be replaced first, and the defective part should be polished before welding.
5. Welding:
Cause Analysis:
1. The root weld thickness is not enough or the grinding is too much, and the current is too large;
2. The molten pool temperature is too high and the welding speed is slightly slow, resulting in burn-through;
3. Improper welding rod (wire) angle;
4. The wire feeding speed is not well controlled, or the welding wire runs out of the groove, causing the welding wire to penetrate the molten pool and form a burnt nodule at the root of the weld.
Prevention and control measures:
1. Ensure the thickness of the root weld, avoid excessive grinding, and adjust the current reasonably;
2. When the molten pool temperature is too high during hot welding, the welding speed should be appropriately increased to reduce the welding pool temperature;
3. Adjust the wire feeder, control the wire feeding speed, and prevent wire channeling;
4. The molten pool temperature should be strictly controlled during flat welding and overhead welding. It should not be too high. Short arc welding should be used as much as possible. Use the correct electrode angle. It is recommended to change the angle in time at the 5 o'clock position.
5. The groove gap should not be too large, and the welding current should be appropriately smaller. For semi-automatic welding, the welding gun can be adjusted to a low gear.
6. Biting edge:

Cause Analysis:
1. The welding current is too large, the arc is too long, and the welding angle is improper;
2. When welding, the welding rod should stay on both sides of the groove edge for a little longer;
3. When filling, the weld filling metal is too low, the cover weld is too thick, the arc dwell time is too long, and the temperature in the weld area is too high, causing undercut.
Prevention and control measures:
1. Select a slightly smaller current within the welding process parameters, control the arc length or short arc welding, keep the wire moving evenly, shorten the wire moving time at the edge of the groove, and pay attention to the wire moving angle;
2. The last layer of filler metal should be slightly lower than the surface of the weld base material (0.5mm-1mm) to make the width of the cap weld clear, which is conducive to controlling the undercut and obtaining a good appearance;
3. For weld undercut depth and length that exceed the standard, it is recommended to grind and trim with a grinding wheel before re-welding.
7. Welding arc pit:
Cause Analysis:
1. Improper arc breaking and arc closing causes a depression at the end of the weld, and the depth of the depression exceeds the specification requirements, that is, it is lower than the parent material;
2. The welding pool is not full of metal, the arc closes too quickly or the current is too high;
3. When closing the arc at the 6 o'clock position, the welding temperature is too high and the pause interval is too short.
Prevention and control measures:
1. During the arc closure process, the electrode should stay at the arc closure position for a short time or perform remelting welding to make the deposited metal fill the molten pool;
2. Use a grinding wheel to grind the repair welding. The repair welding length should not be less than 50mm. Use low-hydrogen welding rods to return the arc (move the rod) and fill the molten pool;
3. When closing the arc at the six o'clock position, two people should not close the arc at the same time. They should wait until the temperature at the joint drops slightly before closing the arc. Otherwise, arc pits or poor weld formation will easily occur.
8. Cracks

Cause Analysis:
1. Improper construction methods, the pipeline is subjected to stress, or it is suspended for a long distance, which can easily cause stress cracks at the welding arc closing point (or stress concentration point);
2. Improper welding methods, repeated local welding and grinding lead to changes in the crystal structure of the parent material, increased hardness (increased brittleness) and decreased plasticity;
3. During the root welding process, if the butt joint is removed too early, the molten iron in the molten pool will not have enough time to fully solidify, and cracks are likely to occur at the end of the welding arc;
4. The misalignment is large, causing the center line of the weld to shift and form a center crack;
5. There are impurities in the weld. Due to the vibration of the internal butt welding machine, the welding slag falls on the weld and enters the molten pool during welding. The slag inclusion reduces the strength of the weld and is prone to root cracks.
6. The pipeline is condensed or the welding material is damp and not dry, and the diffused hydrogen content in the weld is high;
7. Clean the raised part of the end of the inner butt joint frequently to remove welding slag and dirt to prevent it from falling into the weld and entering the molten pool to cause defects (slurry overflow, poor molten iron fusion) and reduce the strength of the weld. The end of the welding rod can be flattened and bent (where there is no coating), inserted into the inside of the weld at the flange, and the welding slag and impurities can be scraped off to avoid and reduce defects in the root weld at the flange.
9. Repair cracks
Cause Analysis:
1. Cracks appear after winter repair, improper preheating method, low local preheating temperature or uneven overall heating, no insulation after welding, and temperature drops too fast;
2. The current is too high during root welding repair, improper operation, and repeated welding;
3. The pipeline may have stress (internal stress or external stress);
4. There is condensation on the pipeline and moisture on the welding material (the E8018-G welding rod used for filling the cover has not been dried and insulated). The content of diffusible hydrogen in the welding pool is high, which is prone to defects and reduces strength.
Recommended actions when repairing cracks:
1. Heat correctly, or heat the entire part, measure the temperature after grinding and repairing the part and add additional heating to prevent the weld temperature from dropping too quickly;
2. Use low current within process parameters. Fill and cover with E8018-G electrode and dry and keep warm.
3. Try to remove impurities in the pipe mouth, especially the residues, welding slag and dirt on the inner surface;
4. Try to eliminate the stress at the weld. If necessary, use a pipe crane to keep the pipe in a horizontal position;
5. When the root repair is long, it is recommended to weld in sections, first welding the two ends and then the middle, to eliminate and disperse the weld stress.
10. The weld bead height exceeds the standard
Cause Analysis:
1. The welding speed is a bit slow, and the grinding control after the last filling is not in place, resulting in the excess height of the cover weld exceeding the standard;
2. Improper transportation of flat welding and overhead welding electrodes, slow welding speed, inadequate control of filling welds, lack of experience or lack of responsibility.
Prevention and control measures:
1. Increase the welding speed at the parts where the excess height may exceed the standard, and weld and grind each layer strictly according to the standard thickness to prevent the neglect of process control and post-weld quality inspection due to rushing the progress;
2. The last filling should be controlled to be 0.5mm-1mm lower than the thickness of the parent material to create favorable conditions for controlling the excess height of the cover welding;
3. Grind the high welds to make them smooth.
11. The number of repairs, grinding length and depth exceed
Cause Analysis:
1. The welder repeatedly grinds and repairs obvious quality defects and local defects between layers or on the cover surface during the welding process;
2. The welder repaired the weld without notifying the quality inspector. Frequent repairs caused the weld material to become brittle, plasticity decreased, and structural strength decreased.
3. When using low-hydrogen electrodes to repair and fill defects in the covering layer, the weld is prone to overheating and produce pores.
Prevention and control measures:
1. When full-time quality inspectors and inspection supervisors discover such problems (i.e. welders repeatedly grinding and welding in one place) during on-site inspections, they should stop it in time and replace welders with higher technical levels to deal with the defects;
2. When repairing with low-hydrogen electrodes, the weld temperature should not be too high (repair experience shows that overheating of the weld is very likely to produce pores). Do not grind or weld immediately. Wait for a while until the weld temperature drops to about 150 degrees Celsius before grinding or welding.
Welding details determine success or failure, and the welding process is particularly important. Strengthening process control and reducing defect formation can effectively improve the welding pass rate and facilitate rework. To control welding quality, we must first fully understand and analyze the causes of welding defects on site, and then take targeted preventive measures and control measures based on the actual situation on site to avoid serious defects, reduce and eliminate welding defects, and effectively solve welding quality problems such as "porosity, slag inclusion, lack of fusion, lack of penetration, penetration, undercut, arc pit, crack, and excess height exceeding the standard". Below we analyze the common defects and quality problems that are prone to occur in winter construction, and refer to control measures.