"Welding knowledge" eight defects in welding quality and preventive measures

Date：2022-06-15 14:35:11 Views：689

The quality defects existing in the welding engineering mainly include the following aspects: all defects that can be seen by the naked eye or a low power magnifying glass and located on the weld surface, such as undercut (undercut), weld beading, arc crater, surface porosity, slag inclusion, surface crack, unreasonable weld position, etc., are called external defects, and the internal porosity, slag inclusion, internal crack, incomplete penetration Non fusion, etc. are called internal defects. However, it is common that the welding slag, spatter and scar are not cleaned after welding.

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, the transition part 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.

reason:It is caused by poor straightness of weld groove processing, improper groove angle or unequal assembly clearance. Excessive current during welding will make the welding rod melt too fast, it is difficult to control the weld formation, and too small current 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 method of strip transportation is improper, such as too fast or too slow, and the electrode angle is incorrect. Improper selection of welding process parameters in submerged arc automatic welding process.

Prevention and control measures:The weld groove shall be processed according to the design requirements and the provisions of the welding specification. Mechanical processing shall be used as much as possible to make the groove angle and the straightness of the groove edge and the straightness of the groove edge meet the requirements. Manual gas cutting and manual shovel cutting shall be avoided to process the groove. During assembly, the weld gap shall be uniform to lay a foundation for ensuring welding quality. Select appropriate welding process parameters through welding process qualification. Welders shall work with certificates, and the trained welders shall have certain theoretical basis and operation 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 shall be adopted, and the（ φ 2.0mm~3.0mm). The strip conveying speed shall be uniform, advancing longitudinally rhythmically, and swinging laterally with a certain width to make the weld surface neat and beautiful.

2. undercut (meat)

Phenomenon:During welding, the arc will melt the dents or grooves at the edge of the weld without being supplemented by the molten metal, leaving a gap. Too deep undercut will weaken the strength of the welded joint, cause local stress concentration, and produce cracks at the undercut after bearing.

reason:Undercut is mainly caused by excessive welding current, too long arc, improper electrode angle, improper speed of electrode transportation, and too short length of electrode at the end of welding. Generally, it is a common defect in vertical welding, horizontal welding and overhead welding.

Prevention measures the current should not be too large during welding. The arc should not be too long or too short. Short arc welding should be used as far as possible. Master the appropriate electrode angle and skilled electrode handling method. When the electrode swings to the edge, it should be slightly slow to make the molten electrode metal fill the edge, and it should be slightly faster in the middle. The depth of weld undercut shall be less than 0.5mm, the length shall be less than 10% of the total length of the weld, and the continuous length shall be less than 10mm. Once the depth or yield exceeds the above tolerance, the defect shall be cleaned up, and the welding rod with smaller diameter and the same brand shall be used. The welding current is slightly larger than the normal, and then repair welding shall be carried out.

3. cracks

Phenomenon:In the welding process 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 crack, transverse crack, crater crack, root crack, etc., and can also be divided into hot crack, cold crack and reheat crack.

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

Prevention and control measures:Mainly from the elimination of stress and the correct use of welding materials and improve the operation process to solve. 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 rods should be dried before use, and the oil, water and other impurities in the groove should be carefully cleaned. During welding, reasonable welding parameters shall be selected to control the input heat at the cooling temperature of 800~3000 ℃, so as to improve the microstructure of weld and heat affected zone. When the welding ambient temperature is low and the material is thin, in addition to increasing the operating ambient temperature, preheating shall be carried out before welding. At the end of welding, thermal insulation and slow cooling and post welding heat treatment shall be tried to eliminate the delayed cracks caused by the residual stress of the weld during cooling.

4. crater

Phenomenon:Arc crater is a 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 reasons are that the arc extinguishing time is too short at the end of welding, or the current used in welding thin plates is too large.

Prevention and control measures:At the end of the weld, make the welding rod stay for a short time or conduct circular strip transportation for several times. Do not stop the arc suddenly to make enough metal fill the weld pool. Proper current shall be ensured during welding, and arc striking plate can be added to main components to lead the arc pit out of the weldment.

5. 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. The slag inclusion in the metal weld will reduce the plasticity and toughness of the metal structure, increase the stress, and lead to cracks in the cold and hot brittleness, so that the components are damaged.

reason:The base metal of the weld is not cleaned completely, and the welding current is too small, which makes the molten metal solidify too fast and the slag can not float out. The chemical composition of welding base metal and welding rod 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 improper strip transportation methods make the molten slag and molten iron mixed together and cannot be separated, preventing the floating of molten slag. 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 is the cause of slag inclusion.

Prevention and control measures:Electrodes with only good welding process performance shall be used, and the welded steel must meet the requirements of design documents. Reasonable welding process parameters are selected through welding process qualification. 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 weld, short arc welding shall be adopted in addition to correct selection of welding current. Meanwhile, the electrode shall be correctly transported to make the electrode swing properly to make the slag float out of the surface. Preheating before welding, heating during welding, and heat preservation after welding to make it cool slowly to reduce slag inclusion.

6. air hole

Phenomenon:The gas absorbed in the molten weld metal in the welding process is not discharged from the weld pool before cooling, but remains in the weld to form holes. It can be divided into inner and outer pores according to the position of the pores; According to the partial conditions and shapes of the porosity defects, the existence of porosity in the weld will reduce the weld strength, also produce stress concentration, and increase the low-temperature brittleness, hot cracking tendency, etc.

reason:The welding rod itself is inferior, and the welding rod is not dried as required due to moisture; Deterioration or peeling of electrode coating; Corrosion of welding core, etc. 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 generated due to high temperature gasification. The welder is not skilled in operation, or his eyesight is poor. He can not 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 redness of welding rod and reduce the protection effect; 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 weld crater and on the surface of welding rods shall be cleaned. Select the appropriate current and control the welding speed. The workpiece shall be preheated before welding, and the arc shall be evacuated slowly at the end of welding or midway stop, 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.

7. spatter and welding slag shall not be cleaned after welding

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

reason:The welding material is damped 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 at 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 and control measures:Select appropriate welding equipment according to the welding base metal. The welding rod shall be equipped with drying and constant temperature equipment. The drying room shall be equipped with dehumidifier, air conditioner, and not less than 300mm from the ground and wall. The system of receiving, receiving, 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 sundries. Overlap the protective shed for construction in winter and rainy season to ensure the welding environment. Before welding non-ferrous metals and stainless steels, protective coatings can be applied to the base metal on both sides of the weld. Welding rod, thin coated welding rod and argon protection can also be selected to eliminate splashes and reduce slag. Welder operation requires timely cleaning of welding slag and protection.

8. arc scar

Phenomenon:Due to careless operation, the welding rod or welding handle is in contact with the weldment, or the ground wire is in poor contact with the workpiece, which will cause an arc for a short time, leaving an arc scar on the surface of the workpiece.

reason:The electric welding operator is careless and fails to take protective measures and maintain tools.

Prevention and control measures:The welder shall regularly 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 shall 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. In case of arc scratch, 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.