Cause analysis of common welding defects and preventive measures

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Defects that can be seen with the naked eye or under low-magnification magnification and are located on the surface of the weld, such as undercutting, weld beads, arc pits, surface pores, inclusions, surface cracks, and improper weld position, are referred to as external defects； Internal pores, inclusions, internal cracks, lack of penetration, and lack of fusion, which can only be detected through destructive testing or specialized non-destructive testing methods, are known as internal defects. But what is common is the failure to remove slag and spatter after welding, as well as untreated weld scars. 1. Weld dimensions do not meet the specification requirements. 1.1 Phenomenon: During inspection, the height of the weld is either too large or too small ； Either the width of the weld is too wide or too narrow; the transition area between the weld and the base material is not smooth, the surface is rough, the weld is uneven in both the longitudinal and transverse directions; furthermore, there is an excessive indentation at the fillet weld area. 1.2 Reasons: 1.2.1 It is caused by poor straightness in the preparation of the weld groove, improper groove angles, or uneven assembly gaps. 1.2.2 Excessive current during welding causes the electrode to melt too quickly, making it difficult to control the shape of the weld. Too low a current can lead to the electrode sticking together during arc initiation, resulting in incomplete welding or weld defects. 1.2.3 The welder lacks sufficient skill; the wire feeding method is improper, such as being too fast or too slow, and the angle of the welding rod is incorrect. 1.2.4 During the submerged arc automatic welding process, the welding process parameters are not selected appropriately. 1.3 Control measures 1.3.1 Machine the weld grooves in accordance with the design requirements and welding specifications; prefer mechanical processing to ensure that the groove angles as well as the straightness of the groove edges meet the specified standards, and avoid using manual gas cutting or hand chiseling for groove preparation. During alignment, ensure that the weld gap is uniform, which lays the foundation for maintaining welding quality. 1.3.2 Select appropriate welding process parameters through welding procedure qualification. 1.3.3 Welders must work with valid certificates; trained welders possess a certain theoretical foundation and practical skills. 1.3.4 For multi-layer welds, the final layer on the welded surface should, while ensuring fusion with the underlying layers, use a welding current that is lower than that used for the layers in between, and should be covered with electrodes of small diameter (φ2.0mm~3.0mm) for surfacing welding. The welding speed should be uniform, advancing longitudinally in a rhythmic manner while making lateral movements of a certain width, which helps to achieve a neat and aesthetically pleasing weld surface. 2. Undercutting 2.1 Phenomenon: During welding, the arc melts the edges of the weld, leaving depressions or grooves that are not filled with molten metal, thus resulting in gaps. Excessive undercutting reduces the strength of the welded joint, leading to local stress concentration; cracks may form at the undercut area under load. 2.2 Reasons: Undercut is mainly caused by excessive welding current, an overly long arc, improper control of the electrode angle and welding speed, as well as too short a length of electrode left at the end of welding. It is generally a common defect in vertical welding, horizontal welding, and overhead welding. 2.3 Preventive and control measures 2.3.1 During welding, the current should not be too high; the arc length should neither be too long nor too short. Short arc welding should be used whenever possible. 2.3.2 It is necessary to master the appropriate electrode angle and skilled electrode movement techniques; the electrode should be moved more slowly at the edges so that the melted electrode metal can fill those areas, while a faster speed is required in the middle. 2.3.3 The depth of weld undercutting shall be less than 0.5 mm, the length shall be less than 10% of the total weld length, and the continuous length shall be less than 10 mm. Once the depth or width of the defect exceeds the aforementioned tolerances, the defective area should be cleaned thoroughly, and welding should be carried out using electrodes of smaller diameter but the same grade, with a welding current slightly higher than normal, in order to fill the defect. 3. Cracks 3.1 Phenomenon During or after welding, metal cracking occurs in the welded area; it can appear inside or outside the weld seam, and may also occur in the heat-affected zone. Based on their location, they can be classified as longitudinal cracks, transverse cracks, crater cracks, root cracks, etc. They can further be divided into thermal cracks, cold cracks, and reheat cracks. 3.2 Reasons 3.2.1 Large stresses are generated due to the contraction of the weld heat-affected zone. 3.2.2 The base material contains a large amount of hardened structure, making it prone to cracking after cooling. 3.2.3 There is a fairly high hydrogen concentration in the weld. As well as other harmful elemental impurities, etc., it is prone to cause cold and hot cracks. 3.3 Control measures: The issues are mainly addressed by eliminating stress, using welding materials properly, and improving the manufacturing processes. 3.3.1 Pay attention to the groove shape of the weld joint to eliminate cracks caused by thermal stress resulting from uneven heating and cooling of the weld. When welding steel plates of different thicknesses, the thicker plates need to be thinned. 3.3.2 The materials selected must meet the requirements of the design drawings; the source of hydrogen must be strictly controlled. Welding rods should be dried before use, and impurities such as oil and moisture from the weld joints must be carefully removed. 3.3.3 During welding, select appropriate welding parameters to keep the input heat within a cooling temperature range of 800–3000°C, thereby improving the microstructural condition of the weld and heat-affected zone. 3.3.4 When the welding ambient temperature is low and the material is thin, in addition to raising the operating environment temperature, preheating before welding is also necessary. After welding, measures should be taken to maintain heat and allow for slow cooling, as well as to carry out post-weld heat treatment, in order to eliminate delayed cracks that arise due to residual stresses in the weld during the cooling process. 4. Undercut is a downward movement that occurs at the end of a weld; it not only reduces the strength of the weld but also causes cracks during the cooling process. 4.1 Cause: Mainly, it is due to an excessively short arc extinction time at the end of welding, or the use of too high a current when welding thin sheets. 4.2 Preventive measures: When terminating a weld, allow the electrode to pause for a short period or make several circular motions with it; do not abruptly stop the arc so as to ensure sufficient metal fills the weld pool. During welding, ensure an appropriate current level; for main components, a starting tab can be used to direct the weld crater outside the workpiece. 5. Inclusions 5.1 Phenomenon: Non-metallic inclusions such as oxides, nitrides, sulfides, phosphides, etc., are detected in the weld through non-destructive testing; these inclusions come in various irregular shapes, with conical and needle-shaped forms being the most common. Inclusions in metal welds reduce the plasticity and toughness of metal structures, and they also increase stress, leading to cold and hot brittleness that facilitates crack formation and results in the destruction of the components. 5.2 Reasons: 5.2.1 The base metal of the weld was not cleaned properly, and the welding current was too low, causing the molten metal to solidify too quickly so that the slag had no time to rise to the surface. 5.2.2 If the chemical composition of the base metal and welding electrodes is impure, and elements such as oxygen, nitrogen, sulfur, phosphorus, and silicon are present in the weld pool during welding, non-metallic inclusions are likely to form. 5.2.3 The welder is not skilled in operation, and the wire feeding method is improper, causing the slag to mix with the molten iron and become inseparable, which hinders the upward movement of the slag. 5.2.4 The groove angle at the weld joint is small, causing the flux coating on the electrode to fall off in chunks and not to be melted by the arc ； During multi-layer welding, incomplete removal of slag and failure to remove it in a timely manner during operation are both causes of slag inclusions. 5.3 Control Measures 5.3.1 Use electrodes with good welding properties, and the steel to be welded must meet the requirements specified in the design documents. 5.3.2 Select appropriate welding process parameters through welding procedure qualification. Pay attention to cleaning the welding groove and the surrounding area; the electrode groove should not be too small ； For multi-layer welds, be sure to remove the slag from each layer carefully. 5.3.3 When using acidic electrodes, it is necessary to ensure that the slag is located at the back of the molten pool ； When welding vertical fillet joints with alkaline electrodes, in addition to selecting the correct welding current, short-arc welding should be employed, and the electrode must be moved properly so that it swings appropriately to allow the slag to rise to the surface. 5.3.4 Preheating before welding, heating during the welding process, and maintaining heat after welding to allow for slow cooling are employed to reduce slag inclusions. 6. Porosity
6.1 Phenomenon: During the welding process, gases absorbed by the molten weld metal fail to escape from the melt pool before cooling, thus remaining within the weld and forming pores. Based on the location where stomata are formed, they can be divided into internal and external stomata ； Depending on the location and shape of the porosity defects, their presence in the weld reduces the strength of the weld, as well as causing stress concentration, which increases susceptibility to low-temperature brittleness and hot cracking. 6.2 Reasons 6.2.1 The quality of the welding rod itself is poor; the rod was damp and not dried as required ； The coating on the welding rod is deteriorated or has peeled off ； Rust on the welding core, etc. 6.2.2 Residual gases present in the melting of the base material ； Impurities such as rust and oil on the welding electrodes and workpieces vaporize due to the high temperatures during welding, generating gases. 6.2.3 The welder lacks proficiency in operating techniques, or has poor eyesight, making it difficult to distinguish between molten metal and the flux coating; this results in the gases within the flux mixing with the metal melt. An excessive welding current causes the electrode to turn red, thereby reducing the protection effect ； The arc length is too long ； Excessive fluctuations in the power supply voltage can cause unstable arc combustion, etc. 6.3 Control Measures 6.3.1 Use qualified welding electrodes; do not use electrodes with cracked or peeling flux coats, deteriorated conditions, eccentric shapes, or severely rusted cores. Clean any oil stains or rust spots on the area around the weld joint as well as on the surface of the electrodes. 6.3.2 The magnitude of the selection current should be appropriate, and the welding speed must be well controlled. Preheat the workpiece before welding. When welding is completed or paused, the arc should be withdrawn slowly; this helps to reduce the cooling rate of the molten pool and facilitates the expulsion of gases from within it, thereby preventing the formation of pore defects. 6.3.3 Reduce the humidity at the welding site and increase the temperature of the working environment. When welding outdoors, in conditions such as wind speeds of 8 m/s, rainfall, dew, or snow, effective measures such as using wind shields or rain canopies must be taken before proceeding with the welding operation. 7. Failure to remove spatter and slag after welding 7.1 Symptoms: This is one of the most common problems; it not only looks unsightly but also poses significant hazards. Intergranular spalling increases the hardened structure on the surface of the material, leading to defects such as hardening and local corrosion. 7.2 Reasons 7.2.1 The welding materials become damp and deteriorated due to improper storage, or the welding rods selected are not compatible with the base material. 7.2.2 The selection of welding equipment does not meet the requirements; AC and DC welding equipment as well as welding materials are inappropriate; the polarity of the welding secondary circuit is incorrect; the welding current is too high; the edges of the weld grooves are contaminated with debris and oil; and the welding environment does not meet the requirements for welding. 7.2.3 The operator lacks skill and failed to follow the procedures and take proper precautions. 7.3 Control Measures 7.3.1 Select appropriate welding equipment based on the welded base material. 7.3.2 Welding electrodes shall be equipped with drying and temperature-controlled facilities; the drying room should have dehumidifiers and air conditioners, and be placed at a height of no less than 300 mm from the floor and walls. Systems for the receipt, use, and storage of welding electrodes shall be established (especially for pressure vessels). 7.3.3 Clean the edges of the weld joint to remove moisture, oil, debris, and rust. During the winter rainy season, overlapping protective sheds are erected to ensure a suitable welding environment. 7.3.4 Before welding non-ferrous metals and stainless steel, a protective coating can be applied to the base metal on both sides of the weld as a form of protection. Methods such as selecting electrodes, flux-cored wires, and argon shielding can also be used to eliminate spatter and reduce slag. 7.3.5 Welders are required to clean slag promptly and take protective measures. 8. Arc scars 8.1 Phenomenon: Due to careless handling, the welding rod or welding torch comes into contact with the workpiece, or the ground wire has poor contact with the workpiece, which causes an arc to appear temporarily and results in arc scars on the surface of the workpiece. 8.2 Cause: The welder was careless, failed to take protective measures, and did not maintain the tools properly. 8.3 Control measures: Welders should regularly check the insulation of the welding cables and grounding wires they use, and repair any damage found promptly. The grounding wire must be installed firmly and reliably. Do not strike an arc outside the weld bead during welding. The welding torch should be kept separate from the base material or properly hung. Turn off the power in time when not welding. If arc scarring is detected, it must be promptly polished with an electric grinding wheel. Because on workpieces such as stainless steel that require corrosion resistance, arc scars can become starting points for corrosion, reducing the performance of the material. 9. Weld beads
9.1 Phenomenon
Failing to remove weld beads after welding affects the overall quality of the equipment; improper handling may also lead to surface cracks. 9.2 Cause: It is caused by the positioning welding fixtures used during the manufacturing and installation of custom equipment, which remain in place after the work is completed. 9.3 Control measures: The lifting tools and fixtures used during assembly should be ground with a grinder to make them level with the base material after they are removed; it is not advisable to use a sledgehammer to remove these fixtures, as this could damage the base material. Any deep arc pits or scratches resulting from welding must be rewelded, and then ground smooth to match the level of the base material. By being careful during operations, such defects can be eliminated. 10. Lack of penetration 10.1 Phenomenon: During welding, the root of the weld does not fully fuse with the base material, or there is a partial lack of penetration between adjacent sections of the base material; this defect is known as lack of penetration or non-fusion. It reduces the mechanical properties of the joint, and at the same time causes stress concentration in that area, leading to cracks. In welding, no weld shall have any lack of penetration. Image 10.2 Reasons 10.2.1 The groove was not prepared as specified, the thickness of the root margin is too large, or the angle of the groove or the gap between the components is too small. 10.2.2 During double-sided welding, if the back side is not thoroughly cleaned, or if the areas on both sides of the groove as well as the interfaces between layers are not properly cleaned, oxides and slag can prevent proper fusion between the metals. 10.2.3 The welder lacks proficiency in welding techniques; for example, if an excessive welding current is used, the base material has not yet melted while the welding rod has already melted, resulting in a failure to fuse the base material with the metal deposited by the welding rod ； When an excessively low current is selected ； Too fast welding speed can prevent proper fusion between the base metal and the weld metal produced by the electrode ； In welding operations, an incorrect angle of the electrode, melting that occurs on one side, or arcing issues can all result in incomplete welding in areas where the arc does not reach. 10.3 Control Measures 10.3.1 Machine and align the gaps according to the groove dimensions specified in the design drawings or standard specifications. 10.3.2 Before welding, it is necessary to thoroughly clean off rust and oil from the areas surrounding the weld joint; in particular, the root of the weld groove must be cleaned completely. For multi-layer welds, an angular grinder should be used during welding to remove the oxides that form between the various weld layers. 10.3.3 Select the appropriate current level and welding speed, and always pay attention to the correct angle of the welding rod. 10.3.4 For weldments with greater thickness or higher thermal conductivity and rapid heat dissipation, the welding area can be preheated before welding or heated during welding, so that the base metal and filler metal can fuse together.