There are many things to note during the welding process of steel structures; otherwise, serious mistakes may occur. Today, we've compiled frequently overlooked aspects of steel structure welding. Please don't take it lightly!

 

Failure to select the optimal voltage during welding construction

Phenomena and hazards

　　Whether it's the root pass, filling, or capping, regardless of the groove size, the same arc voltage is used. This may not achieve the required penetration and width, resulting in defects such as undercut, porosity, and spatter.

Preventive measures

　　Generally, different situations require the selection of long or short arcs to achieve better welding quality and efficiency. For example, short arcs should be used for root pass welding to obtain better penetration, and longer arcs are recommended for filling or capping welds to improve efficiency and width.

 

Failure to control arc length during welding

Phenomena and hazards

　　Failure to adjust the arc length appropriately according to the groove shape, number of welds, welding method, and electrode type, leading to difficulties in achieving high-quality welds due to improper arc length.

Preventive measures

　　To ensure weld quality, short arcs are generally preferred, but the appropriate arc length can be selected based on different situations to achieve optimal welding quality. For instance, a shorter arc should be used for the first layer of V-groove butt and fillet welds to ensure penetration without undercut. The second layer can be slightly longer to fill the weld. Short arcs are recommended for small gaps, while longer arcs are suitable for larger gaps to increase welding speed. The arc length for overhead welding should be the shortest to prevent molten metal from flowing down. For vertical and horizontal welds, smaller currents and shorter arcs are used to control the molten pool temperature. Regardless of the welding method, maintaining a consistent arc length is crucial to ensure consistent weld width and penetration.

 

Insufficient weld size in butt or fillet welds requiring full penetration

Phenomena and hazards

　　Insufficient weld leg size in butt or fillet welds requiring full penetration, such as T-joints, cross-joints, and fillet welds, or in welds connecting the web and top flange of crane girders or similar components with fatigue design requirements, may result in insufficient strength and stiffness.

Preventive measures

　　Butt welds in T-joints, cross-joints, and fillet welds requiring full penetration should have sufficient leg size according to design requirements, generally no less than 0.25t (t is the thickness of the thinner plate). The leg size of welds connecting the web and top flange of crane girders or similar structures with fatigue design requirements should be 0.5t, and not greater than 10 mm. The allowable deviation in welding dimensions is 0-4 mm.

 

Multi-layer welds without slag removal or defects on the weld surface before proceeding with the next layer

Phenomena and hazards

　　In thick plate multi-layer welding, failure to remove slag and defects after each layer before welding the next layer can lead to inclusions, porosity, cracks, reduced joint strength, and increased spatter during subsequent welding.

Preventive measures

　　In thick plate multi-layer welding, each layer should be welded continuously. After each layer, slag, surface defects, and spatter should be removed promptly. Any inclusions, porosity, or cracks affecting weld quality should be thoroughly removed before welding.

 

Failure to control welding current

Phenomena and hazards

　　To accelerate progress, the butt welds of medium and thick plates may not be grooved. This leads to reduced strength, potentially failing to meet standards, and cracks may appear during bending tests, compromising the weld joint performance and potentially jeopardizing structural safety.

Preventive measures

　　The welding current should be controlled according to the welding process parameters, with a 10-15% fluctuation allowed. The blunt edge of the groove should not exceed 6mm. For butt welding, a groove is required for plate thicknesses exceeding 6 mm.

 

Non-continuous welding in multi-layer welds and failure to control interlayer temperature

Phenomena and hazards

　　 In thick plate multi-layer welding, neglecting interlayer temperature control, such as excessively long intervals between layers without preheating before welding, can easily cause cold cracks between layers. If the interval is too short, and the interlayer temperature is too high (above 900 degrees Celsius), this affects the performance of the weld and heat-affected zone, resulting in coarse grains, reduced toughness and ductility, and potential hidden dangers.

Preventive measures

　　In thick plate multi-layer welding, interlayer temperature should be carefully controlled. The base material temperature should be checked during continuous welding to maintain consistency with the preheating temperature, and the maximum interlayer temperature should be controlled. Welding time should not be too long. If welding is interrupted, appropriate post-heating and insulation measures should be taken. When welding is resumed, the preheating temperature should be higher than the initial preheating temperature.

 

Failure to control welding deformation

Phenomena and hazards

　　Neglecting to control deformation through welding sequence, personnel arrangement, groove shape, welding specification selection, and operating methods can lead to significant deformation after welding, making correction difficult and expensive, especially for thick plates and large workpieces. Mechanical correction is difficult and can easily cause cracks or delamination. Flame straightening is costly and may cause overheating if not properly handled. For workpieces with high precision requirements, failure to take effective measures to control deformation may result in failure to meet installation dimensions, necessitating rework or scrap.

Preventive measures

　　Use a reasonable welding sequence, appropriate welding specifications and operating methods, and anti-deformation and rigid fixing measures.

 

Welding with filler rod heads or iron blocks in the joint gap

Phenomena and hazards

　　Because it is difficult to melt the welding rod head or iron block with the workpiece during welding, it may cause welding defects such as lack of fusion and lack of penetration, reducing the connection strength. If rusty welding rod heads or iron blocks are used for filling, it is difficult to ensure the consistency of the material with the base metal; if the welding rod heads or iron blocks are oily or have impurities, the weld will have defects such as porosity, slag inclusion, and cracks. These situations will greatly reduce the quality of the weld joint, failing to meet the design and standard requirements for the quality of the weld.

Preventive measures

　　1) When the workpiece assembly gap is large but does not exceed the specified allowable range, and the assembly gap exceeds twice the thickness of the thin plate or is greater than 20mm, the buildup welding method should be used to fill the concave part or reduce the assembly gap. It is strictly forbidden to use the method of filling the welding rod head or iron block to weld in the joint gap.

　　2) When processing and marking parts, attention should be paid to leaving sufficient cutting allowance and shrinkage allowance after cutting, controlling the dimensions of the parts, and not increasing the gap to ensure the outer dimensions.

 

Uneven transition when using different thicknesses and widths of plates for butt welding.

Phenomena, Hazards

　　When butt welding plates of different thicknesses and widths, if the thickness difference is not within the standard allowable range and no smooth transition is made, the weld is prone to stress concentration and welding defects such as lack of fusion at the part exceeding the thickness of the thin plate, affecting the welding quality.

Preventive measures

　　When exceeding the relevant regulations, the weld should be welded into a slope shape, with the maximum allowable slope being 1:2.5; or one or both sides of the thickness should be processed into a slope before welding, with the maximum allowable slope being 1:2.5. For structures that directly bear dynamic loads and require fatigue calculation, the slope should not be greater than 1:4. When butt welding plates of different widths, thermal cutting, mechanical processing, or grinding should be used according to factory and site conditions to achieve a smooth transition, with the maximum allowable slope at the connection being 1:2.5.

 

Inattention to welding sequence for components with intersecting welds.

Phenomena and hazards

　　For components with intersecting welds, the welding sequence is not arranged reasonably by analyzing the release of welding stress and the influence of welding stress on component deformation, but the welding is carried out horizontally and vertically randomly. As a result, the horizontal and vertical seams will mutually constrain each other, causing large temperature shrinkage stress, causing plate deformation, uneven plate surface, and possibly cracks in the weld.

Preventive measures

　　A reasonable welding sequence should be formulated for components with intersecting welds. When several horizontal and vertical intersecting welds are welded, the transverse weld with larger shrinkage deformation should be welded first, followed by the longitudinal weld. In this way, when welding the transverse weld, it will not be constrained by the longitudinal weld, so that the shrinkage stress of the transverse weld can be released without constraint, reducing welding deformation and ensuring weld quality, or welding the butt weld first and then the fillet weld.

 

Continuous welding at corners when using fillet welding for lap joints of steel members.

Phenomena, Hazards

　　 When using fillet welding for lap joints of steel members and continuous plates, weld the two sides of the members first and then the end welds, and do not perform continuous welding. Although this is beneficial for reducing welding deformation, it is easy to cause stress concentration and welding defects at the corners of the members, affecting the quality of the welded joints.

Preventive measures

　　When using fillet welding for lap joints of steel members, continuous welding should be completed at the corners at one time. Do not weld to the corner and then move to the other side for welding.

 

Equal strength butt joint is required, but no arc starting and ending plates are provided at both ends of the flange plate and web plate of the crane girder.

Phenomena and hazards

　　When welding butt welds, full penetration fillet welds, and welds between the flange plate and web plate of the crane girder, if arc starting and ending plates are not provided at the starting and ending points, the current and voltage will not be stable enough at the starting and ending points, and the temperature will not be stable enough, easily leading to lack of fusion, lack of penetration, cracks, slag inclusion, and pores at the starting and ending points of the weld, reducing weld strength, and failing to meet design requirements.

Preventive measures

　　When welding butt welds, full penetration fillet welds, and welds between the flange plate and web plate of the crane girder, arc starting and ending plates should be provided at both ends of the weld. Their function is to guide the parts prone to defects at both ends to the outside of the workpiece, and then cut off the defective parts to ensure the quality of the weld.

 

Inattention to the coordination of welding speed, welding current, and electrode diameter.

Phenomena, Hazards

　　During welding, attention is not paid to controlling the welding speed, welding current, electrode diameter, and welding position in coordination. For example, when performing the root pass of a full penetration fillet weld, because the root is narrow, if the welding speed is too fast, the gas and slag in the root will not have enough time to escape, easily causing lack of penetration, slag inclusion, and pores in the root; when performing the cover pass, if the welding speed is too fast, it is easy to produce pores; if the welding speed is too slow, the weld reinforcement will be too high and the shape will be irregular; when welding thin plates or welds with small bevel sizes, if the welding speed is too slow, it is easy to cause burn-through.

Preventive measures

　　Welding speed has a significant impact on welding quality and welding productivity. When selecting welding speed, it should be matched with welding current, weld position (root pass, filling pass, cover pass), weld thickness, and bevel size. Under the premise of ensuring penetration, easy removal of gas and slag, no burn-through, and good formation, a higher welding speed should be selected to improve productivity.