Common problems frequently encountered in welding engineering quality

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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 referred to as internal defects. But what is common is the failure to remove slag and spatter after welding, as well as the failure to clean up 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 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, on the other hand, can cause the electrode to stick together during arc initiation, resulting in incomplete welding or weld defects. 1.2.3 The welder lacks sufficient skill in operation; 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 the final layer of multi-layer welds on the welding surface, under the condition of ensuring fusion with the underlying layer, a welding current lower than that used for the intermediate layers should be employed, and surfacing welding should be performed using electrodes with a small diameter (φ2.0mm–3.0mm). The welding speed must be consistent, advancing longitudinally in a rhythmic manner while making lateral movements of a certain width, which helps to achieve a neat and attractive 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 weakens the strength of the welded joint, leading to local stress concentration; cracks may form at the area of undercutting under load. 2.2 Causes: Undercut is mainly caused by excessive welding current, an overly long arc, improper handling of the electrode angle, inappropriate travel speed of the electrode, and a too-short remaining length of the electrode at the end of welding. It is generally a common defect in vertical welding, horizontal welding, and overhead welding. 2.3 Control measures 2.3.1 The current should not be too high during welding; the arc should not be made too long or too short, and short-arc welding should be preferred as much as possible. 2.3.2 It is necessary to master the appropriate angle of the welding rod as well as skilled welding techniques; the rod should be moved more slowly at the edges so that the molten metal from the rod can fill those areas, while it should be moved faster in the middle. 2.3.3 The depth of weld undercut shall be less than 0.5 mm, its 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 fragmentation occurs in the welded area; this can take place inside or outside the weld seam, and it may also occur in the heat-affected zone. Based on the location where they arise, cracks can be classified as longitudinal cracks, transverse cracks, crater cracks, root cracks, etc.; furthermore, they can 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 more hardened structures, making it prone to cracking after cooling. 3.2.3 There is a relatively high hydrogen concentration in the weld. And other harmful elemental impurities, etc., can easily lead to 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 plate needs 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 electrodes 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, in order to improve the microstructural condition of the weld and the 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 gradual 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 extinguishing time at the end of welding, or the use of too high a current when welding thin sheets. 4.2 Control measures: At the end of welding, keep the electrode in place for a short period or make several circular movements with it; avoid stopping the arc suddenly to ensure sufficient metal is poured into the weld pool. Ensure an appropriate current during welding; for the main components, an arc starting plate can be used to guide the arc pit outside the welded part. 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 thus damages 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 break off in pieces and remain unmelted 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 its surrounding area; the electrode groove should not be too small ； For multi-layer welds, it is necessary to carefully remove the slag from each layer of weld. 5.3.3 When using acidic electrodes, it is necessary to keep the slag behind the molten pool ； When welding vertical fillet joints with alkaline electrodes, in addition to selecting the appropriate welding current, it is necessary to use a short arc, and the electrode must be moved correctly 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: Gases absorbed in the molten weld metal during welding do not have time to escape from the weld pool before it cools, remaining inside 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 welding rod was damp and not dried as required ； The coating on the welding electrode is deteriorated or 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 Inexperienced welding techniques or poor eyesight prevent the welder from clearly distinguishing between the molten iron and the flux, resulting in gases contained in the flux mixing with the metal melt. An excessive welding current causes the electrode to turn red, thereby reducing the protective effect ； The arc length is too long ； Excessive fluctuations in supply voltage can cause unstable arc combustion and other issues. 6.3 Control Measures 6.3.1 Use qualified welding electrodes; those with cracked or peeling flux coats, deteriorated condition, eccentric shape, or severely rusted cores should not be used. Oil and rust deposits around the weld area as well as on the surface of the electrodes must be removed thoroughly. 6.3.2 The magnitude of the current selected should be appropriate; control the welding speed properly. 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. Failing to remove spatter and slag after welding 7.1 Phenomenon: This is the most common issue; it not only looks unsightly but also poses significant hazards. Welding spatter increases the hardened structure on the surface of the material, leading to defects such as hardening and localized corrosion. 7.2 Causes 7.2.1 The coating of welding materials becomes damp and deteriorates during storage; or the selected electrodes are incompatible with the base metal. 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 groove are contaminated with debris and oil; and the welding environment does not satisfy the requirements for welding. 7.2.3 The operator lacks proficiency and failed to follow the procedures and take necessary precautions. 7.3 Control Measures 7.3.1 Select appropriate welding equipment based on the welding 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 to remove moisture, oil, dirt, and rust. During the winter and rainy seasons, overlapping protective sheds are used to ensure a suitable environment for welding. 7.3.4 Before welding **** and stainless steel, a protective coating can be applied to the base metal on both sides of the weld as a form of protection. Options such as using electrodes, thin-flux-coated electrodes, and argon shielding can also be chosen 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, failing to take protective measures and maintain the tools. 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. Upon discovering arc burns, they must be promptly ground down using a wire 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 scars 9.1 Phenomenon: Failing to remove weld scars after welding affects the overall quality of the equipment; improper handling can also cause surface cracks. 9.2 Cause: It is caused by the positioning welding fixtures used during the manufacturing and installation of non-standard equipment, which remain in place after the work is completed. 9.3 Preventive and corrective measures: The lifting 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 to be level with 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. 10.2 Reasons 10.2.1 The groove was not prepared as specified, the thickness of the root edge 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 areas between layers are not cleaned properly, 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 too low a 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 the 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 to remove the oxides between the various weld layers during the welding process. 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 that are thick or have high thermal conductivity and rapid heat dissipation, the welding area can be preheated before welding or heated during welding, so as to enable fusion between the base metal and the electrode metal.