The quality defects existing in welding engineering mainly include the following aspects: all defects that can be seen with the naked eye or a low-power magnifying glass and are located on the surface of the weld, such as undercut (bite), weld bead, arc crater, surface porosity, pinch Slag, surface cracks, unreasonable weld positions, etc. are called external defects, and internal pores, slag inclusions, internal cracks, incomplete penetration, incomplete fusion, etc., which must be discovered by destructive testing or special non-destructive testing methods, are called internal defect. But the common ones are not cleaning the slag and spatter after welding and the welding scars that are not cleaned.
The weld size does not meet the specification requirements
Phenomenon: The height of the weld during inspection is too large or too small; or the width of the weld is too wide or too narrow, and the transition between the weld and the base metal is not smooth, the surface is rough, and the weld is longitudinal or transverse Irregularity, and the amount of depression in the fillet weld is too large.
Reason: The flatness of the welding seam groove processing is poor, the groove angle is improper or the assembly gap size is not equal. Excessive current during welding will cause the welding rod to melt too fast, and it is difficult to control the welding seam formation. If the current is too small, it will cause the welding rod to "adhesion" during arc welding, resulting in impermeable welding or weld bead. The welder is not proficient in operation, the method of transporting the rod is improper, such as too fast or too slow, and the angle of the welding rod is incorrect. In the submerged arc automatic welding process, the welding process parameters are not selected properly.
Preventive measures: Process the weld groove according to the design requirements and welding specifications, and try to use mechanical processing to make the groove angle and the straightness of the groove edge and the straightness of the groove edge meet the requirements, and avoid manual gas cutting and manual shovel Cutting grooves.
During the pairing, the uniformity of the weld gap is ensured, which lays the foundation for ensuring the welding quality. Through the welding process qualification, select the appropriate welding process parameters. Welders must be certified, and trained welders have a certain theoretical foundation and operational skills.
The last layer of multi-layer welding seam on the welding surface is to ensure that it is fused with the bottom layer, and the welding current should be smaller than the welding current between each layer, and the welding rod with a small diameter (φ2.0mm~3.0mm) should be used for overlay welding. The conveying speed is required to be uniform, and it is advanced in a rhythmic longitudinal direction with a certain width of horizontal swing, which can make the surface of the welding seam neat and beautiful.
Undercut
Phenomenon: The dents or grooves on the edge of the weld that are melted by the arc during welding are not supplemented by the molten metal and leave gaps. An undercut that is too deep will weaken the strength of the welded joint, causing local stress concentration, and cracks will occur at the undercut after loading.
Reasons: The main reasons are that the welding current is too large, the arc is too long, the angle of the electrode is not properly controlled, the speed of the electrode is improper, and the length of the electrode is too short at the end of the welding to form an undercut. Generally, it is a common defect in vertical welding, horizontal welding and overhead welding.
Preventive measures When welding, the current should not be too large, the arc should not be drawn too long or too short, and short arc welding should be used as much as possible. Master the appropriate electrode angle and skilled handling techniques. When the electrode swings to the edge, it should be a little slower, so that the molten electrode metal fills up the edge, and in the middle, it should be slightly faster.
The depth of the weld undercut should be less than 0.5mm, the length should be less than 10% of the full length of the weld, and the continuous length should be less than 10mm. Once the depth or yield exceeds the above tolerance, the defect should be cleaned up, and the welding rod with a smaller diameter and the same brand should be used, and the welding current is slightly larger than normal, and the repair welding should be carried out to fill it up.
crack
Phenomenon: Metal fracture occurs in the welding area during the welding process or after welding. It occurs inside or outside the weld, and may also occur in the heat-affected zone. According to the location it produces, it can be divided into longitudinal cracks, transverse cracks, and arc craters. Cracks, root cracks, etc., can be divided into hot cracks, cold cracks and reheat cracks.
Reason: The heat-affected zone of the weld has a large stress after shrinking. The base metal contains more hardened structure, which is prone to cracks after cooling. There is a fairly high hydrogen concentration in the weld. And other harmful elements, impurities, etc., are prone to cold and hot cracks.
Preventive measures: Mainly start with the elimination of stress, the correct use of welding materials and the perfect operation process. Pay attention to the groove form of the welded joint to eliminate the cracks caused by the uneven heating and cooling of the weld.
For example, when the steel plates of different thicknesses are butt welded, the thick steel plates must be thinned. The selected materials must meet the requirements of the design drawings, and the source of hydrogen must be strictly controlled. The welding rod should be dried before use, and the grooves should be carefully cleaned of oil, moisture and other impurities.
In welding, select reasonable welding parameters to control the input heat between the cooling temperature of 800~3000℃ to improve the structure of the weld and the heat-affected zone. In the welding environment where the temperature is low and the material is thin, in addition to increasing the operating environment temperature, it should be preheated before welding. At the end of the welding, try to keep warm and slow cooling and post-weld heat treatment to eliminate the delayed cracks caused by the residual stress of the weld during the cooling process.
Arc crater
Phenomenon: Arc crater is the phenomenon of sliding at the end of the weld, which not only weakens the strength of the weld, but also produces cracks during the cooling process.
Reason: The main reason is that the arc extinguishing time is too short at the end of welding, or the current used when welding thin plates is too large.
Preventive measures: When the welding seam is finished, make the welding rod stay for a short time or do several circular transportation, and do not stop the arc suddenly so that there is enough metal to fill the molten pool. When welding, ensure proper current, and the main components can be added with arc ignition plate to lead the arc crater out of the weldment.
Slag
Phenomenon: Non-metallic inclusions, such as oxides, nitrides, sulfides, phosphides, etc., are found in the weld seam through non-destructive testing, forming a variety of irregular shapes, such as cones, needles and other slag inclusions. Slag inclusion in the metal weld will reduce the plasticity and toughness of the metal structure, and will increase the stress, which will cause cracks in cold and hot brittleness and damage the components.
Reason: The base metal of the weld is not cleaned up, and the welding current is too small, which causes the molten metal to solidify too fast, and the slag does not have time to emerge. The chemical composition of the welding base metal and electrode is impure. For example, if there are oxygen, nitrogen, sulfur, phosphorus, silicon and other components in the molten pool during welding, non-metallic slag inclusions are easily formed.
The unskilled welder and the improper method of transporting the strip make the slag and molten iron mixed together and cannot be separated and prevent the slag from floating. The angle of the weld bevel is small, and the electrode coating falls off in pieces and is not melted by the arc; during multi-layer welding, the slag is not cleaned up, and the slag is not removed in time during the operation, which all cause slag inclusion.
Preventive measures: Use welding rods with only good welding process performance, and the welded steel must meet the requirements of the design documents. Choose reasonable welding process parameters through welding process qualification.
Pay attention to the cleaning of the welding groove and the edge range, and the electrode groove should not be too small; for multi-layer welds, the welding slag of each layer of weld must be carefully removed. When using an acid electrode, the slag must be behind the molten pool; when using an alkaline electrode to weld vertical fillet seams, in addition to the correct selection of the welding current, short arc welding is also required. At the same time, the electrode must be transported correctly to make the electrode swing properly. In order to make the slag rise to the surface.
Use preheating before welding, heating during welding, and heat preservation after welding, so as to cool down slowly to reduce slag inclusion.
Stoma
Phenomenon: The gas absorbed in the weld metal melted during the welding process is too late to be discharged from the molten pool before cooling, and remains in the weld to form cavities. According to the position where the pores are generated, the internal and external pores can be divided; according to the division and shape of the pore defects, the presence of pores in the weld will reduce the strength of the weld, also cause stress concentration, increase low-temperature brittleness, hot cracking tendency, etc.
Reasons: the welding rod itself is inferior, and the welding rod is not dried according to the specified requirements when it is damp; the coating of the welding rod is deteriorated or peeling; the welding core is corroded, etc. There is residual gas in the base metal smelting; the welding rod and weldment are stained with rust, oil and other impurities. During the welding process, gas is generated due to high temperature gasification.
The welder’s operating skills are unskilled, or the eyesight is poor, and the molten iron cannot be distinguished from the coating, so that the gas in the coating is mixed with the metal solution. If the welding current is too high, the welding rod will become red, and the protection effect will be reduced; the arc length is too long; the power supply voltage fluctuates too much, resulting in unstable arc burning.
Preventive measures: Select qualified welding rods, do not use welding rods with cracked coating, peeling, deterioration, eccentricity or severe corrosion of the welding core, and clean up the oil stains and rust spots near the welding joint and on the surface of the welding rod. If the size of the current is appropriate, control the welding speed.
Preheat the workpiece before welding. When the welding is finished or stopped in the middle, the arc should be slowly withdrawn, which is beneficial to slow down the cooling rate of the molten pool and the discharge of gas in the molten pool, and avoid the occurrence of pore defects. Reduce the humidity of the welding operation site and increase the temperature of the operating environment.
When welding outdoors, such as wind speed of 8m/s, rainfall, dew, snow, etc., effective measures such as windshield and canopy should be taken before welding operation.
Spatter and welding slag are not cleaned up after welding
Phenomenon: This is the most common common problem, which is neither beautiful nor harmful. Fusion splash will increase the hardened structure of the surface of the material, and it is easy to produce defects such as hardening and local corrosion.
Reason: The welding material has been damp and deteriorated during the preservation of the traditional Chinese medicine, or the selected welding rod does not match the base material. The selection of welding equipment does not meet the requirements, the AC and DC welding equipment does not meet the welding consumables, the polarity of the secondary welding line is incorrect, the welding current is large, there is debris and grease pollution on the edge of the weld groove, and the welding environment does not meet Welding requirements, etc. The operator is technically unskilled, and fails to operate and protect according to the regulations.
Preventive measures: Choose suitable welding equipment according to the welding base material. Welding rods must be equipped with drying and constant temperature equipment, with dehumidifiers, air conditioners, and not less than 300mm from the ground and walls in the drying room, and establish systems for receiving and dispatching, using, and storing welding rods (especially for pressure vessels). The edges of the weld joints are cleaned up to remove moisture, oil stains and debris and rust. During the winter and rainy seasons, the protective shed is constructed to ensure the welding environment. Before welding non-ferrous metals and stainless steel, protective coatings can be applied to the base metal on both sides of the weld as protection. You can also choose welding rods, thin-coated welding rods and argon protection to eliminate spatter and reduce slag. The welder operation requires timely cleaning of welding slag and protection.
Arc scar
Phenomenon: Due to careless operation, the welding rod or the welding handle is in contact with the weldment, or the ground wire is in poor contact with the workpiece, causing an arc for a short time, leaving arc scars on the surface of the workpiece.
Reason: Welding operators are careless and fail to take protective measures and maintenance of tools.
Prevention measures: Welders should always check the insulation of the welding handle wire and grounding wire used, and if any damage is found, they should be wrapped up in time. Install the grounding wire firmly and reliably. Do not start the arc outside the weld bead during welding. The welding tongs should be isolated from the base metal or hung appropriately.
Cut off the power in time when not welding. If arc scratches are found, it must be polished in time with an electric grinding wheel. Because on stainless steel and other workpieces that require corrosion resistance, arc scars will become the starting point of corrosion, reducing the performance of the material.
