The quality defects existing in welding engineering mainly include the following aspects: all defects that can be seen by the naked eye or low magnification and located on the weld surface, such as undercut (undercut), weld beading, crater, surface pores, slag inclusions, surface cracks, unreasonable weld position, etc., are called external defects. Internal pores, slag inclusions, internal cracks, lack of penetration, lack of fusion and other defects that can only be found by Destructive tests or special nondestructive testing methods are called internal defects. However, it is common that the welding slag and spatter are not cleaned after welding and the welding scar is not cleaned.

1、 Weld size does not meet the specification requirements

Phenomenon: the height of the weld is too large or too small during the inspection; 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, the longitudinal and transverse of the weld are not neat, and the concave amount of the weld at the fillet weld is too large.

Cause: poor straightness of weld groove processing, improper angle of groove or uneven size of assembly gap. Excessive current during welding will make the welding rod melt too fast, it is difficult to control the weld formation, and the current is too small, which will make the welding rod produce "bonding phenomenon" during welding arc striking, resulting in incomplete welding or weld beading. The welder is not proficient enough, the strip transportation method is improper, such as too fast or too slow, and the electrode angle is incorrect. In the process of submerged arc automatic welding, the welding process parameters are improperly selected.

Prevention and control measures: the weld groove shall be processed according to the design requirements and welding specifications, and mechanical processing shall be used as far as possible to make the groove angle and the straightness of the groove edge and the straightness of the groove edge meet the requirements, and manual gas cutting and manual shovel cutting shall be avoided. During assembly, ensure the uniformity of weld gap, so as to lay a foundation for ensuring welding quality. Select appropriate welding process parameters through welding process evaluation. Welders should work with certificates, and the trained welders have a certain theoretical basis and operating skills. The last layer of Multi-layer Weld on the welding surface is the weld. Under the condition of ensuring the fusion with the bottom layer, the welding current smaller than that between layers should be adopted, and a small diameter should be used（ φ 2.0mm~3.0mm) electrode overlay welding. The strip conveying speed shall be uniform, advancing longitudinally and rhythmically, and swinging laterally with a certain width to make the weld surface neat and beautiful.

2、 Crack

Phenomenon: during or after welding, metal fracture occurs in the welding area, which occurs inside or outside the weld, and may also occur in the heat affected zone. According to the location, it can be divided into longitudinal cracks, transverse cracks, crater cracks, root cracks, etc., and can also be divided into hot cracks, cold cracks and reheat cracks.

Cause: large stress is generated after shrinkage of weld heat affected zone. The base metal contains more hardened structure, which is easy to crack after cooling. There is a fairly high hydrogen concentration in the weld. And other harmful elements and impurities, which are easy to produce cold and hot cracks.

Prevention measures: mainly from the elimination of stress, the correct use of welding materials and perfect operation process. Pay attention to the groove form of the welded joint and eliminate the cracks caused by thermal stress due to uneven heating and cooling of the weld. If steel plates of different thicknesses are butt welded, the thick steel plates shall 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 oil stain, water and other impurities in the groove should be carefully cleaned. During welding, select reasonable welding parameters to control the input heat between 800~3000 ℃ cooling temperature, so as to improve the microstructure of weld and heat affected zone. When the welding environment temperature is low and the material is thin, in addition to increasing the operating environment temperature, it should also be preheated before welding. After 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.

3、 Slag inclusion

Phenomenon: non metallic inclusions such as oxides, nitrides, sulfides, phosphates, etc. are found in the weld through nondestructive testing, forming a variety of irregular shapes, and common slag inclusions such as cone and needle are found. Slag inclusion in metal welds will reduce the plasticity and toughness of metal structures, increase stress, and lead to cracks in cold and hot brittleness, so that components are damaged.

Cause: the base metal of the weld is not cleaned up, and the welding current is too small, which makes the molten metal solidify too fast, and the slag has no time to float out. The chemical composition of welding base metal and electrode is not pure. If there are oxygen, nitrogen, sulfur, phosphorus, silicon and other components in the molten pool during welding, it is easy to form non-metallic slag inclusions. The unskilled operation of welders and the improper method of strip transportation make the slag and molten iron mixed together and cannot be separated, preventing the slag from floating up. The weld groove angle is small, and the electrode coating falls off in blocks without being melted by the arc; During multi-layer welding, the slag is not cleaned up, and the slag is not removed in time during operation, which are all the reasons for slag inclusion.

Prevention measures: use welding rods with only good welding process performance, and the welded steel must meet the requirements of the design documents. Select reasonable welding process parameters through welding process evaluation. Pay attention to the cleaning of welding groove and edge range, and the welding rod groove should not be too small; For multi-layer welds, the welding slag of each layer of welds shall be carefully removed. When using acid electrode, the slag must be behind the molten pool; When using alkaline electrode to weld vertical fillet joints, in addition to correctly selecting the welding current, short arc welding should also be used. At the same time, the electrode should be transported correctly, so that the electrode can swing properly to make the slag float out of the surface. Preheat before welding, heat during welding, and keep warm after welding to make it cool slowly to reduce slag inclusion.

4、 Stomata

Phenomenon: the gas absorbed in the weld metal melted in the welding process is not discharged from the molten pool before cooling, but remains in the weld to form holes. According to the position of pores, they can be divided into internal and external pores; According to the partial situation and shape of pore defects, the existence of pores in the weld will reduce the weld strength, also produce stress concentration, and increase the low-temperature brittleness, hot cracking tendency, etc.

Cause: the welding rod itself is inferior, and the welding rod is not dried according to the specified requirements due to moisture; Electrode coating deterioration or peeling; Corrosion of welding core, etc. There is residual gas in base metal smelting; The welding rod and weldment are stained with rust, oil and other impurities. During the welding process, gas is produced due to high temperature gasification. The welder is not skilled in operation, or his eyesight is poor, and he cannot distinguish the molten iron from the coating, so that the gas in the coating is mixed with the metal solution. Excessive welding current will cause the welding rod to redden and reduce the protection effect; The arc length is too long; The power supply voltage fluctuates too much, resulting in unstable arc combustion, etc.

Prevention and control measures: qualified welding rods shall be selected. Welding rods with cracked coating, peeling, deterioration, eccentricity or serious corrosion of welding core shall not be used. Oil stains and rust spots near the welding junction and on the surface of welding rods shall be cleaned. Choose the appropriate current and control the welding speed. Preheat the workpiece before welding. When welding ends or stops halfway, the arc should be evacuated slowly, which is conducive to slowing down the cooling speed of the molten pool and the discharge of gas in the molten pool, so as to avoid pore defects. Reduce the humidity of the welding operation site and improve the temperature of the operation environment. During outdoor welding, if the wind speed reaches 8m/s, rain, dew, snow, etc., effective measures such as wind proof and canopy shall be taken before welding operation.

5、 Do not clean up spatter and welding slag after welding

Phenomenon: This is the most common problem, which is not beautiful and harmful. Miscibility splash will increase the hardened structure on the surface of the material, which is easy to produce defects such as hardening and local corrosion.

Cause: the welding material is damp and deteriorated in the preservation of traditional Chinese medicine skin, or the selected welding rod does not match the base metal. The selection of welding equipment does not meet the requirements, the AC and DC welding equipment does not meet the welding materials, the polarity connection method of the welding secondary line is incorrect, the welding current is large, there are sundries and oil stains on the edge of the weld groove, and the welding environment does not meet the welding requirements. The operator is not skilled and does not operate and protect according to the regulations.

Prevention measures: select appropriate welding equipment according to the welding base metal. The welding rods shall be equipped with drying and constant temperature equipment. The drying room shall be equipped with dehumidifiers, air conditioners, and the distance from the ground and the wall shall not be less than 300mm. The system of receiving, sending, using and keeping welding rods shall be established (especially for pressure vessels). The edge of the welded junction shall be cleaned to remove water, oil and debris corrosion. In winter and rainy season, the construction overlap protective shed ensures the welding environment. Before welding non-ferrous metals and stainless steel, protective coatings can be painted on the base metal on both sides of the weld. Welding rods, thin coated electrodes and argon protection can also be selected to eliminate splashes and reduce slag. Welder operation requires timely cleaning of welding slag and protection.

6、 Arc scar

Phenomenon: due to careless operation, the welding rod or welding handle contacts the weldment, or the poor contact between the ground wire and the workpiece causes an arc for a short time, leaving an arc scar on the surface of the workpiece.

Cause: the electric welding operator was careless and did not take protective measures and maintain the tools.

Prevention and control measures: welders should often check the insulation of the used welding handle wire and grounding wire, and wrap it up in time in case of damage. The installation of grounding wire should be firm and reliable. Do not strike an arc outside the weld bead during welding. The welding tongs shall be isolated from the base metal or hung properly. Cut off the power supply in time when not welding. If electric arc scratch is found, it must be polished with electric grinding wheel in time. Because on stainless steel and other workpieces with corrosion resistance requirements, the arc scar will become the starting point of corrosion and reduce the performance of the material.

Source: Ship knowledge

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