What are the eight defects in welding engineering?
The quality defects existing in welding engineering mainly include the following aspects: the defects that can be seen by naked eye or 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 gas that can only be found by destructive test or special non-destructive testing method Hole, slag inclusion, internal crack, incomplete penetration and incomplete fusion are called internal defects. But most of them are welding slag and spatter and welding scar after welding.
- 1. Weld size does not meet the specification requirements
- 2. Biting
- 3. Crackle
- 4. Crater
- 5. Slag inclusion
- 6. Stoma
- 7. Do not clean spatter and welding slag after welding
- 8. Arc scarring
Weld size does not meet the specification requirements
Phenomenon: the height of the weld is too large or too small in 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 is not neat, and the undercut of the weld at the fillet weld is too large.
Cause: poor straightness of weld groove processing, improper angle of groove or unequal size of assembly clearance. Too much current in welding makes the electrode melt too fast, it is difficult to control the weld formation, and the current is too small. During arc striking, the electrode will produce “adhesion phenomenon”, resulting in incomplete welding or weld beading. The welder’s operation proficiency is not enough, the method of strip transportation is improper, such as too fast or too slow, and the electrode angle is not correct. In the process of automatic submerged arc welding, the selection of welding process parameters is improper.
Prevention and control 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, so as to avoid using artificial gas cutting and manual chipping to process groove.
During the assembly, ensure the uniform and consistent weld gap, and lay a foundation for ensuring the welding quality. Through the welding procedure qualification, the appropriate welding procedure parameters are selected. Welders shall be employed 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 that under the condition of ensuring the fusion with the bottom layer, the welding current shall be smaller than that between each layer, and the small diameter (φ 2.0mm ~ 3.0mm) welding rod shall be used for cladding welding. The strip conveying speed shall be uniform, and it shall be pushed forward longitudinally in a rhythmic manner, with a certain width of transverse swing, so as to make the weld surface neat and beautiful.
Phenomenon: when welding, the arc will melt the concave or groove out of the weld edge without the supplement of molten metal, leaving a gap. Too deep undercut will weaken the strength of the welded joint, cause local stress concentration, and produce cracks at undercut after bearing.
Cause: the main reason is that the welding current is too large, the arc is too long, the electrode angle is not properly controlled, the speed of strip transportation is not appropriate, and the length of the electrode left at the end of welding is too short. 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, and the arc should not be too long or too short. Try to use short arc welding. Master the proper electrode angle and skilled operation method. When the electrode swings to the edge, it should be slightly slow to make the molten electrode metal fill the edge, and slightly faster in the middle.
The undercut depth of the weld 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 10 mm. 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 one, and the repair welding shall be carried out.
Phenomenon: in the welding process or after welding, there is metal fracture in the welding area, which occurs inside or outside the weld, and may also occur in the heat affected area. According to the generated parts, it can be divided into longitudinal cracks, transverse cracks, crater cracks, root cracks, etc., and also into hot cracks, cold cracks and reheat cracks.
Cause: the heat affected zone of weld shrinks and produces large stress. The base metal contains more hardened structure, and cracks are easy to form after cooling. There is a high concentration of hydrogen in the weld. And other harmful elements and impurities, easy to produce cold and hot cracks.
Prevention and control measures: mainly from the elimination of stress and correct use of welding materials and perfect operation process to solve. Pay attention to the groove form of the welding joint, and eliminate the cracks caused by the uneven heating and cooling of the weld due to the thermal stress.
If different thickness of steel plate butt welding, the thick steel plate will be thinned. The selected materials must meet the requirements of the design drawing, strictly control the source of hydrogen, dry the electrode before use, and carefully clean the oil, water and other impurities in the groove.
In order to improve the microstructure of weld and heat affected zone, the reasonable welding parameters should be selected and the input heat should be controlled between 800 ~ 3000 ℃. When the welding environment temperature is low and the material is thin, in addition to improving the operating environment temperature, preheating should also be carried out before welding. At the end of welding, it is necessary to keep warm and cool slowly and heat treatment after welding, so as to eliminate the delayed crack caused by residual stress of weld during cooling.
Phenomenon: crater is the phenomenon of sliding at the end of weld, which not only weakens the weld strength, but also produces cracks in the cooling process.
Cause: the main reason is that the arc extinguishing time is too short at the end of welding, or the current used in welding sheet is too large.
Prevention and control measures: when the weld is closed, make the electrode stay for a short time or make several circular strips, and do not stop the arc suddenly so that there is enough metal to fill the weld pool. Ensure proper current during welding, and add arc strike plate to main components to lead out the crater outside the weldment.
Phenomenon: non-metallic inclusions such as oxides, nitrides, sulfides, phosphates, etc. are found in the weld through non-destructive testing, forming a variety of irregular shapes, common slag inclusions such as cone-shaped and needle shaped. Slag inclusion in metal weld will reduce the plasticity and toughness of metal structure, and increase the stress, resulting in the cold and hot embrittlement easy to produce cracks, which will damage the components.
Cause: the base metal of the weld is not cleaned, 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 electrode is not pure, such as oxygen, nitrogen, sulfur, phosphorus, silicon and other components in the molten pool during welding, it is easy to form non-metallic slag inclusions.
The welder is not skilled in operation, and the strip conveying method is improper, so that the slag and molten iron can not be separated together, preventing the slag from floating up. The reason for slag inclusion is that the angle of the weld groove is small, the electrode coating falls off 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 operation.
Prevention and control measures: the welding rod with only good welding process performance shall be used, and the welded steel must meet the requirements of the design documents. The reasonable welding parameters are selected by welding procedure qualification.
Pay attention to the cleaning of welding groove and edge range, and the welding electrode groove should not be too small; for multi-layer welds, the welding slag of each layer of welds should be carefully removed. When the acid electrode is used, the slag must be at the back of the molten pool; when the alkaline electrode is used to weld the vertical corner joint, in addition to the correct selection of welding current, the short arc welding should also be used, and the strip transportation should be correct to make the electrode swing properly to make the slag float out of the surface.
Preheat before welding, heat during welding, and heat preservation after welding to make it cool slowly to reduce slag inclusion.
Phenomenon: the gas absorbed in the weld metal melted in the welding process is not discharged from the molten pool before cooling, and the residual gas forms cavities in the weld. According to the location of porosity, it can be divided into internal and external porosity; according to the porosity defects of different parts and shapes, the presence of porosity in the weld will reduce the strength of the weld, also produce stress concentration, increase the low-temperature brittleness, hot crack tendency, etc.
Causes: the electrode itself is poor, the electrode is not dried as required due to moisture; the coating of the electrode is deteriorated or peeled off; the core is rusted, etc. There is residual gas in the base metal smelting; there are rust, oil and other impurities on the welding rod and weldment. During the welding process, gas is generated due to high temperature gasification.
The welder is not skilled in operation, or the vision is poor to distinguish molten iron and coating, so that the gas in coating is mixed with metal solution. Too much welding current can make the electrode red and reduce the protection effect; too long arc length; too large fluctuation of power supply voltage, resulting in unstable combustion of arc, etc.
Prevention and control measures: qualified welding rod shall be selected. Welding rod with cracked coating, peeling off, deterioration, eccentricity or serious corrosion of welding core shall not be used. Oil and rust spots near the welding junction and on the surface of welding rod shall be cleaned. If the current is suitable, control the welding speed.
Preheat the workpiece before welding. When the welding is finished or stopped in the middle, 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 8 m / s, rainfall, dew, snow, etc., effective measures such as windbreak and canopy shall be taken before welding operation.
Do not clean spatter and welding slag after welding
Phenomenon: This is the most common common fault, which is not beautiful and harmful. The melting spatter will increase the hardening structure of the surface of the material, and it is easy to produce hardening and local corrosion defects.
Cause: the welding material is deteriorated by damp in the preserved skin of traditional Chinese medicine, 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 and welding materials do not meet the requirements, the polarity of the welding secondary line is not correct, the welding current is large, there are sundries and oil dirt on the edge of the weld groove, and the welding environment does not meet the welding requirements. The operator is not skilled in operation and protection according to 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. In the drying room, there shall be dehumidifier, air conditioner, and the distance from the ground and the wall shall not be less than 300 mm. A system (especially for pressure vessel) shall be established for receiving, using and keeping the welding rod. The edge of the welding joint shall be cleaned to remove the water, oil and sundries. Construction of overlapping protective shed in winter and rainy season ensures the welding environment. Before welding non-ferrous metal and stainless steel, the base metal of both sides of the weld line can be coated with protective coating as protection. It can also choose electrode, thin coating electrode and argon protection to eliminate spatter and reduce slag. Welders are required to clean up welding slag and protect in time.
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 arc for a short time, leaving arc scar on the workpiece surface.
Cause: the welding operator was careless, did not take protective measures and maintenance of tools.
Prevention and control measures: the welder shall check the insulation of the welding handle wire and the grounding wire frequently, and wrap them in time in case of any damage. The grounding wire shall be installed firmly and reliably. Do not strike arc outside the weld bead during welding. The welding tongs shall be isolated from the base metal or properly hung.
Cut off the power supply in time when not welding. In case of arc scratch, it is necessary to polish it with electric grinding wheel in time. Because in stainless steel and other parts with corrosion resistance requirements, arc scar will become the starting point of corrosion, reducing the performance of materials.
Source: China Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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How Mang Types of Cracks in Welding Engineering?
Cracks can occur just about everywhere in a weld: in the weld metal, the plate next to the weld metal, or in any other piece affected by the intense heat of welding.
Here are the three major types of cracks, what causes them, and how you can prevent them.
✓ Hot cracks:
This type of crack occurs during welding or shortly after you’ve deposited a weld, and its cause is simple: The metal gets hot too quickly or cools down too quickly. If you’re having problems with hot cracking, try preheating your material. You can also postheat your material, which means that you apply a little heat here and there after you’ve finished welding in an effort to let the metal cool down more gradually.
✓ Cold cracks:
This type of crack happens well after a weld is completed and the metal has cooled off. (It can even happen days or weeks after a weld.) It generally happens only in steel, and it’s caused by deformities in the structure of the steel. You can guard against cold cracking by increasing the thickness of your first welding pass when starting a new weld. Making sure you’re manipulating your electrode properly, as well as pre- and postheating your metal, can also help thwart cold cracking.
✓ Crater cracks:
These little devils usually occur at the ending point of a weld, when you’ve stopped welding before using up the rest of an electrode. The really annoying part about crater cracks is that they can cause other cracks, and the cracking can just kind of snowball from there. You can control the problem by making sure you’re using the appropriate amount of amperage and heat for each project, slowing your speed of travel, and pre- and postheating.
What is stronger MIG or TIG welding?
TIG welding produces cleaner and more precise welds than MIG welding or other Arc welding methods, making it the strongest. That said, different welding jobs may require different methods, while TIG is generally stronger and higher in quality, you should use MIG or another method if the job calls for it.
What’s the strongest type of weld?
As we said, MIG is the most versatile and the easiest one to learn; TIG is the most aesthetically pleasing; stick and arc produce the strongest welds and can operate under less than desirable conditions. We also discussed the best beginner’s welder and the type that produces the strongest weld.
What’s the hardest type of welding?
TIG is the hardest form of welding for various reasons such as being a tedious process, and it is harder to master than other forms of welding. Now, I will show you why TIG is the hardest form of welding.
The process of TIG is slow: If speed is a factor you’re considering, then TIG is reasonably slow. It takes some time to get used to it as a beginner. Again, it takes time to complete a weld job and allow the welded pieces to cool down.
The welder must use hand and food at the same time: A foot operated variable amperage control device is used, and it gives you the ability to slowly start the heat and you can slow it down at the end, as well. Because of the multi-tasking aspect, it can seem a little trickier than the other types of welding.
The welder has more control in TIG welding: Unlike traditional welding, the welder has a control over the heating, the gas cooling, current, etc. This makes it seem daunting for a beginner to master.
Despite the con of TIG welding, it is nevertheless, the best welding method based on the visual appearance. It is little wonder industry experts prefer people with TIG welding skills to those without.