Weldability test: general welding defects and their causes

Date：2021-10-14 11:54:33 Views：2142

Solderability means that under the action of welding heat, the solder, weldment and copper foil will not melt, and the solder will melt and wet the welding surface, resulting in the diffusion of solder metal, forming a metal attachment layer between the copper foil and weldment, and connecting the copper foil and weldment together to obtain a firm and reliable welding point, The above process is the effect of mutual physical interaction process. If it can completely infiltrate with tin, it is good weldability. Otherwise, it is necessary to judge the weldability according to the effect.

The quality defects existing in welding engineering mainly include the following aspects: all defects that can be seen by the naked eye or low power magnifying glass and located on the weld surface, such as undercut (undercut), weld bead, arc pit, surface pores, slag inclusion, surface cracks Unreasonable weld position is called external defect, and internal air hole, slag inclusion, internal crack, incomplete penetration, incomplete fusion and other internal defects that can only be found by destructive test or special nondestructive testing method are called internal defects. However, it is common that the welding slag, spatter and welding scar are not cleaned after welding.

The weld size does not meet the specification requirements

Phenomenon: the height of the weld is too large or too small during the inspection of the weld; 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.

Cause: poor straightness of weld groove processing, improper groove angle or unequal assembly gap. Excessive current in welding will make the electrode melt too fast, it is difficult to control the weld formation, and too small current will cause "adhesion" of the electrode during arc striking, resulting in incomplete welding or overlap. The welder is not skilled enough, the strip transportation method is improper, such as too fast or too slow, and the electrode angle is incorrect. Improper selection of welding process parameters in automatic submerged arc welding process.

Prevention measures: weld groove shall be processed according to the design requirements and welding specifications, and mechanical processing shall be selected as far as possible to make the groove angle, straightness of groove edge and straightness of 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. Appropriate welding process parameters are selected through welding process qualification. Welders shall work with certificates, and the trained welders 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 fusion with the bottom layer, the welding current smaller than that between layers shall be adopted, and the weld with small diameter shall be used（ φ 2.0mm ~ 3.0mm) covered electrode welding. The strip transportation speed shall be uniform, advance longitudinally rhythmically, and swing laterally with a certain width to make the weld surface neat and beautiful.

Undercut (meat)

Phenomenon: during welding, the arc will melt the depression or groove 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 crack will occur at the undercut after bearing.

Reasons: 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 shall not be too large during welding. The arc shall not be too long or too short. Short arc welding shall be used as far as possible. Master the appropriate electrode angle and skilled electrode transportation method. When the electrode swings to the edge, it shall be slightly slow to make the molten electrode metal fill the edge, and 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 it shall be repaired and filled.

crackle

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 position, it can be divided into longitudinal crack, transverse crack, crater crack, root crack, etc., as well as hot crack, cold crack and reheat crack.

Cause: large stress is generated after shrinkage of weld HAZ. 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 technology. Pay attention to the groove form of welded joints and eliminate the cracks caused by uneven heating and cooling of welds due to thermal stress. 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, strictly control the source of hydrogen, dry the welding rod before use, and carefully clean the oil stain, moisture and other impurities in the groove. 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 ambient temperature is low and the material is thin, in addition to increasing the operating ambient temperature, it shall also be preheated before welding. At the end of welding, thermal insulation, slow cooling and post weld heat treatment shall be tried to eliminate the delayed cracks caused by weld residual stress during cooling.

Crater

Phenomenon: crater is a sliding phenomenon at the end of the weld, which not only weakens the weld strength, but also produces cracks during cooling.

Cause: the arc extinguishing time is too short at the end of welding, or the current used when welding thin plates is too large.

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

Slag inclusion

Phenomenon: non metallic inclusions such as oxides, nitrides, sulfides, phosphates, etc. are found in the weld through NDT, forming a variety of irregular shapes, and common slag inclusions such as cone and needle are found. Slag inclusion in metal weld will not only reduce the plasticity and toughness of metal structure, but also increase stress, resulting in cracks in cold and hot brittleness and damage to components.

Cause: the base metal of the weld is not cleaned up, the welding current is too small, the molten metal solidifies too fast, and the slag has no time to float out. The chemical composition of welding base metal and electrode is impure. 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 slag and molten iron mixed together and cannot be separated, which hinders the floating of slag. The weld groove angle is small, the electrode coating falls off in blocks and is not melted by the arc; During multi-layer welding, the slag is not cleaned up, and the slag is not pulled out in time during operation.

Prevention measures: only electrodes with 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, in addition to correctly selecting the welding current, short arc welding shall also be adopted. At the same time, the electrode shall be transported correctly 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, so as to reduce slag inclusion.

stoma

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 stomata, it can be divided into internal and external stomata; According to the partial conditions and shapes of pore defects, the existence of pores in the weld will reduce the weld strength, produce stress concentration, and increase the low-temperature brittleness, hot crack tendency, etc.

Cause: the welding rod itself is inferior, the welding rod is damp and does not dry according to the specified requirements; Electrode coating deterioration or peeling; 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 produced due to high-temperature gasification. The welder is not skilled in operation, or has poor eyesight, and 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 redness of welding rod and reduce the protection effect; The arc length is too long; The fluctuation of power supply voltage is too large, resulting in unstable arc combustion, etc.

Prevention 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. Select the appropriate current and control the welding speed. Preheat the workpiece before welding. At the end of welding or midway stop, the arc shall 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 welding operation site and improve the temperature of operation environment. During outdoor welding, if the wind speed reaches 8m / s, rain, dew, snow, etc., effective measures such as wind protection and canopy shall be taken before welding.

Spatter and welding slag shall not be cleaned 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 electrode 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 welding secondary line is incorrect, the welding current is large, there are sundries and oil dirt pollution at the edge of 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 rod shall be equipped with drying and constant temperature equipment. The drying room shall be equipped with dehumidifier, air conditioner, not less than 300mm from the ground and wall. The system of receiving, receiving, using and keeping of welding rod shall be established (especially for pressure vessels). The edge of the welded junction shall be cleaned to discharge water, oil, sundries and rust. The overlapping protective shed shall be constructed in winter and rainy season 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. Welding rod, thin coated welding rod and argon protection can also be selected to eliminate spatter and reduce slag. Welders are required to clean up welding slag and protect in time.

Arc scar

Phenomenon: due to careless operation, the electrode or welding handle contacts with 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 workpiece surface.

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

Prevention and control measures: the welder shall 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 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.

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