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Summary and analysis of eight defects in welding

The defects existing in welding engineering mainly includes the following aspects: all or low power magnifying glass to the naked eye can see and on the surface of the weld defects, such as biting edge (bit), flash, arc pit, surface porosity, slag inclusion, surface crack, called external defects such as weld position is unreasonable and must have special NDT methods with destructive test or to find internal cracks, porosity, slag inclusion, incomplete fusion, not together referred to as the internal defects, etc.But the common is not clean up after welding slag and splash and not clean up the welding scar. 

The weld dimension does not meet the specification requirements

Phenomenon: the height of weld in the 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 material is not smooth, rough surface, the weld is not neat longitudinally and transversely, and the concave of the weld in the fillet weld is too large. 

Reason: weld groove processing flatness is poor, groove Angle is not appropriate or the size of the assembly gap is not equal and caused by.Welding current flow large, make the welding rod melting too fast, difficult to control the weld formation, the current is too small, in the welding arc will make the welding rod produce "bonding phenomenon", resulting in penetration or welding tumor.The welder operation skilled process is not enough, the transport method is not appropriate, such as too fast or too slow, as well as the electrode Angle is not correct.Improper selection of welding parameters in automatic submerged arc welding process. 

Preventive measures: according to the design requirements and the provisions of the welding code processing weld groove, as far as possible to choose mechanical processing to make the groove Angle and groove edge straightness and groove edge straightness to meet the requirements, avoid manual gas cutting, manual shoveling cutting groove.

In group matching, the gap of welding seam is guaranteed to be uniform, which lays a foundation for guaranteeing welding quality.Through the welding process evaluation, select the appropriate welding process parameters.Welder to hold a certificate on the post, after training welders have a certain theoretical basis and operational skills.

Multi-layer welding in the welding surface of the last layer of welding, in the condition of ensuring and the bottom layer of fusion, should be used than the welding current between the layers smaller, with a small diameter (2.0mm~3.0mm) electrode covering welding.The speed of the bar shall be uniform, and the bar shall advance longitudinally with rhythm and make a certain width of transverse swing, which can make the weld surface neat and beautiful.

Edge bite (meat bite)

Phenomenon: the arc during welding will weld edge fusion out of the sag or groove without the addition of molten metal and leave a gap.Too deep undercut will weaken the strength of the welded joint, causing local stress concentration, and cracks will appear in the undercut after bearing. 

Reason: the welding current is too large, arc is too long, the Angle of the electrode is not suitable and the speed of the electrode is not appropriate, and the length of the electrode indweling is too short at the end of the welding and the formation of edge bite.It is a common defect in vertical welding, horizontal welding and overhead welding. 

Preventive measures welding current should not be too large, arc not too long or too short, try to use short arc welding.Grasp the appropriate electrode Angle and skilled operation, electrode swing to the edge should be slightly slow, so that the molten electrode metal filled edge, and in the middle is a little faster.

The undercut depth of the weld shall be less than 0.5mm, the length shall be less than 10% of the full length of the weld, and the continuous length shall be less than 10mm.Once the depth or yield exceeds the above tolerance, the defects should be cleaned up, using a smaller diameter, brand of the same electrode, welding current than normal slightly larger, to fill up welding. 

The crack

Phenomenon: in the welding process or after welding, metal fracture occurs in the welding area, which is generated inside or outside of the weld, and may also occur in the thermal affected area. According to the position generated, it can be divided into longitudinal crack, transverse crack, arc pit crack, root crack, etc., and can be divided into thermal crack, cold crack and reheat crack.

Cause: the weld heat affected area after the contraction of a large stress.The parent material contains many hardened structures, which are easy to crack after cooling.There is a fairly high concentration of hydrogen in the weld.And other harmful elements and impurities, easy to produce cold, hot crack. 

Preventive 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 to eliminate the cracks caused by thermal stress due to uneven heating and cooling of the weld.

Such as different thickness of steel butt welding, thick steel to do thinning treatment.Selection of materials must meet the requirements of the design design, strictly control the source of hydrogen, welding rod should be used before drying, and carefully clean groove oil, water and other impurities.

During welding, reasonable welding parameters should be selected to control the input heat between 800 ℃ and 3000℃ to improve the microstructure of the weld and the heat-affected zone.In the welding environment temperature is lower, the material is thinner, in addition to increase the operating environment temperature, also should be preheated before welding.At the end of welding, heat preservation and slow cooling and post-welding heat treatment should be taken to eliminate the delayed cracks caused by residual stress in the welding seam during the cooling process.

Arc pit

Phenomenon: arc pit is a sliding phenomenon at the end of the weld, which not only weakens the weld strength, but also produces cracks in the cooling process. 

Reason: the arc quenching time is too short at the end of welding, or the current used in welding sheet metal is too large. 

Preventive measures: weld end, make the electrode to do a short time to stay or do a few times circular bar, do not suddenly stop the arc so that there is enough metal to fill the pool.Ensure proper current when welding, the main components can be added to lead the arc pit out of the welding parts. 

slag

Phenomenon: non-metallic inclusions such as oxide, nitride, sulfide, phosphide, etc. are found in the welding seam by nondestructive testing, forming a variety of irregular shapes, common slag inclusion materials such as cone, needle, etc.Metal weld slag inclusion will reduce the metal structure of plastic, toughness, but also increase the stress, resulting in cold and hot brittleness easy to produce cracks, so that the members are destroyed.

Reason: the weld base material is not cleaned cleanly, the welding current is too small, the molten metal solidifies too quickly, the slag is too late to emerge.The chemical composition of welding base material and electrode is not pure, such as oxygen, nitrogen, sulfur, phosphorus, silicon and other components in the welding pool, it is easy to form non-metallic slag inclusion.

The welder operation is not skilled, the transport method is not appropriate, so that slag and molten iron mixed together, separated from the block slag floating.Welding groove Angle is small, welding electrode coating into a block off not melted by arc;In multilayer welding, slag cleaning is not clean, the operation of slag is not removed in time is the cause of slag clamping. 

Preventive measures: use only good welding performance electrode, welded steel must meet the requirements of the design document.Reasonable welding process parameters were selected through welding process evaluation.

Pay attention to the cleaning of welding groove and edge area, welding electrode groove should not be too small;For multi - layer welding seam should be carefully removed each layer of welding slag.When acid electrode is used, slag must be behind the molten pool.In the use of alkaline welding electrode welding Angle seam, in addition to the correct selection of welding current also need to use short arc welding, at the same time, the transport bar should be correct, make the electrode swing appropriately, in order to make the slag surface.

Preheating before welding, heating during welding, and heat preservation after welding, slow cooling to reduce slag inclusion.

stomatal

Phenomenon: the gas absorbed in the molten weld metal in the welding process is not discharged from the weld pool before cooling, and the residual gas forms holes in the weld.According to the stomatal parts can be divided into internal and external stomatal;According to the partial situation and the shape of the stomatal defects, the presence of stomatal in the weld will reduce the strength of the weld, also produce stress concentration, increase the low temperature brittleness, hot cracking tendency.

Reason: welding rod itself inferior, welding rod damp did not according to the requirements of drying;Electrode skin deterioration or peeling;Core corrosion, etc.Residual gas in smelting of base metal;The welding rod and welding parts are stained with rust, oil and other impurities, in the process of welding, due to high temperature gasification gas.

The welder is not skilled in operation, or poor vision of molten iron and coating can not distinguish clearly, so that the coating gas and metal solution mixed together.Welding current over ambassador electrode red to reduce the protection effect;Arc length is long;The voltage fluctuation of power supply is too large, resulting in unstable arc combustion.

Prevention and control measures: choose qualified electrode, shall not use the coating cracking, peeling, deterioration, eccentric or seriously corroded core of the electrode, should be near the welding joint and the surface of the electrode oil, rust and other clean.Select the size of the current, if appropriate, control the welding speed.

Preheat the workpiece before welding, and when the welding is finished or stopped halfway, the arc should be evacuated slowly, which is conducive to slowing down the cooling speed of the molten pool and the exhaust of gas in the molten pool, so as to avoid the appearance of blowhole defects.Reduce the humidity of welding operation site and improve the operating environment temperature.

When welding outdoors, such as wind speed up to 8m/s, rain, dew, snow, etc., effective measures should be taken to prevent wind and build canopy before welding operation. 

Do not clean spatter and slag after welding

Phenomenon: this is the most common one of the common, not beautiful, harm is very big.Soluble spatter will increase the surface of the material hardened structure, easy to produce hardening and local corrosion and other defects. 

Reason: welding material in the preservation of Chinese medicine skin affected by damp deterioration, or the selected electrode and the base material does not match.The selection of welding equipment does not meet the requirements, ac and dc welding equipment and welding materials do not meet the requirements, welding second line polarity is not correct, welding current is large, welding groove edge debris and oil pollution, welding environment does not meet the requirements of welding.The operator is not skilled, not according to the procedures of operation and protection. 

Preventive measures: select suitable welding equipment according to the welding base material.Electrode shall be equipped with a drying thermostat, desiccating machine, air conditioner, no less than 300mm away from the ground and wall in the drying room, and a system of receiving, using and keeping electrode shall be established (especially for pressure vessels).Clean up water, oil and sundries.In winter and rainy season, the construction of overlap protection shed ensures the welding environment.Before welding nonferrous metal and stainless steel, the wire base material on both sides of the weld can be coated with protective coating.Also can choose electrode and thin flux electrode and argon protection and other methods to eliminate spatter and reduce slag.Welder operation requires timely cleaning of welding slag and protection.

Arc scar

Phenomenon: due to careless operation and welding rod or welding and welding parts contact, or the earth and the workpiece contact bad short time caused arc, and the workpiece surface left arc scar. 

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

Preventive measures: welders should often use the welding wire and ground wire insulation inspection, found that the damage should be timely wrapped up.Install the grounding wire firmly and reliably.Do not introduce arc on weld bead.Welding tongs should be placed separately from the base metal or hung up appropriately.

Cut off the power when not welding.If arc abrasives are found, they must be polished by 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 the material.


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