Welding operations failed to pay attention to voltage selection.
【Observation】
During welding, whether it's for root or fill passes, regardless of the bevel size, the same arc voltage is chosen. This may result in insufficient weld penetration and width, leading to defects such as undercut, porosity, and spatter.
【Action】
For different situations, it is advisable to select the appropriate long or short arc to achieve better welding quality and efficiency. For instance, during打底 welding, to achieve a deeper weld penetration, short-arc operation should be used. When welding or covering, to enhance efficiency and wider fusion, it is appropriate to increase the arc voltage slightly.
2. Welding without controlling the welding current
[Event]
During welding, in a rush to meet deadlines, bevels are not opened on the weld joints of medium and thick plates. This results in a decrease in strength indicators, even failing to meet standard requirements. Cracks appear during bend tests, which compromises the joint performance of the weld and poses potential hazards to the structure.
【Action】
During welding, the welding current as specified in the process evaluation should be controlled, allowing for a fluctuation of 10% to 15%. The bevel edge size of the groove should not exceed 6mm. When welding lap joints, if the plate thickness is over 6mm, a groove should be opened for welding.
3. Failure to pay attention to welding speed and welding current, as well as the coordinated use of welding rod diameters.
[Event]
When welding, it is important to control the welding speed and current, and to coordinate the use of the welding rod diameter and position. When performing a root weld on fully penetrated corner joints, due to the narrow root size, if the welding speed is too fast, there is not enough time for the root gases and slag to be expelled, which can lead to defects such as incomplete fusion, slag inclusions, and pores. During face welding, a fast welding speed can also result in the formation of pores. A slow welding speed can cause the weld bead to be too high, resulting in an uneven appearance. When welding thin plates or joints with small钝edges, a slow welding speed can easily lead to burn-through and other issues.
[Action]
Welding speed significantly impacts welding quality and production efficiency. When selecting it, coordinate with welding current, weld position (root weld, welding, face weld), weld thickness, and bevel size to choose an appropriate welding speed. Under the premise of maintaining sufficient penetration, easy expulsion of gas and slag, avoiding burn-through, and achieving good shape, opt for a higher welding speed to enhance production efficiency.
4. Failure to control the arc length during welding
[Phenomenon]
Improper adjustment of arc length during welding, not accounting for factors such as groove shape, number of welding layers, welding method, and electrode type. Due to improper use of welding arc length, achieving high-quality welds becomes difficult.
[Measurements]
To ensure weld quality, short-arc welding is generally used during welding, but the appropriate arc length can be chosen based on different conditions to achieve optimal welding quality. For example, when performing V-groove butt joints or corner joints, a shorter arc should be used to ensure full penetration without edge burns, while the second layer can be slightly longer to fill the weld. For small gaps, a short arc is preferable, while a longer arc can be used for larger gaps to increase welding speed. For overhead welding, the arc should be short to prevent the downward flow of molten metal; for vertical and horizontal welding, small current and short-arc welding are also used to control the molten pool temperature. Furthermore, regardless of the welding method, it is important to maintain a consistent arc length during movement to ensure uniform weld width and depth throughout the entire seam.
5. Lack of control over welding deformation
[Observation]
Failure to control distortion during welding, including factors such as welding sequence, personnel arrangement, bevel shape, selection of welding specifications, and operation methods, can lead to significant distortion post-welding, making correction difficult and costly, especially for thick plates and large workpieces. Mechanical correction is prone to cause cracks or lamellar tearing. Flame correction is expensive and can easily lead to overheating of the workpiece. Without effective distortion control measures for precision parts, the installation dimensions may not meet the requirements, potentially resulting in rework or scrapping.
【Action】
Implementing a reasonable welding sequence, selecting appropriate welding specifications and techniques, and employing anti-deformation and rigid fixation measures.
6. Multi-layer welding is not performed continuously; insufficient attention is paid to controlling the temperature between layers.
[Observation]
During multi-layered thick plate welding, failing to control the inter-layer temperature can lead to cold cracks forming between layers if the interval between layers is too long and welding is performed without re-heating. Conversely, if the interval is too short and the inter-layer temperature exceeds 900°C, it will affect the properties of the weld and heat-affected zone, resulting in coarse grain structure, reduced toughness and plasticity, and leaving potential hazards at the joint.
[Action]
During multi-layer heavy plate welding, strict control over inter-layer temperatures is essential. During the continuous welding process, inspect the base material temperature to ensure the inter-layer temperature is as consistent as possible with the preheating temperature. Temperature control between layers is also critical. The welding time should not be excessive. In case of welding interruptions, appropriate post-heating and insulation measures should be taken. When resuming welding, the re-preheating temperature should be slightly higher than the initial preheating temperature.
7. Multi-layer welds are焊接 without slag, and defects on the weld surface are addressed before proceeding to the next layer.
[PHENOMENON]
During multi-layer thick plate welding, if slag and defects are not removed after each layer is completed, welding the next layer directly can easily lead to defects such as slag inclusions, gas holes, and cracks in the weld, reducing the connection strength. Additionally, it can cause spatter during the welding of the lower layer.
【Measures】
During multi-layer heavy plate welding, each layer should be welded continuously. After the welding seam of each layer is completed, slag, surface defects of the seam, and spatter should be promptly removed. If defects such as slag, porosity, and cracks, which may affect the welding quality, are found, welding should be performed again after they are addressed.
8. The weld angles of the combination welds for penetration-seamed or corner joints do not meet the required dimensions.
[Observation]
Insufficient weld leg dimensions in fusion or corner joint combinations, such as T-joints, cross-joints, and corner joints, or in the weld connections between the web and upper flange of crane beams or similar structures with fatigue calculations, will result in焊接 strength and stiffness failing to meet design requirements.
[Measures]
Tee joints, cross joints, corner joints, and other fusion-welded joints requiring full penetration should meet the design requirements with sufficient weld leg dimensions, generally not less than 0.25t (where t is the thickness of the thinner plate at the connection). For crane beams or similar webs with fatigue calculations required for the weld joints connecting to the upper flange, the weld leg dimensions should be 0.5t, and should not exceed 10mm. The allowable deviation for welding dimensions is:0～4 mm。
Welding a weld rod head or iron block into the joint gap
【Observation】
Due to the difficulty in melting the welding rod tip or iron block into the base material during welding, defects such as incomplete fusion and incomplete penetration can occur, which reduces the strength of the joint. Using rusty welding rod tips or iron blocks makes it hard to ensure consistency with the base material's composition; and using welding rod tips or iron blocks with oil, impurities, etc., can lead to defects like gas pores, slag inclusions, and cracks in the weld. These conditions significantly degrade the quality of the weld joint, failing to meet the design and specification requirements for weld quality.
【Action】
When the assembly gap of the workpiece is significantly large, yet does not exceed the specified allowable range, and the gap is thinner than twice the thickness of the plate or greater than 20mm, the welding method should be used to fill in the concave areas or reduce the assembly gap. It is strictly prohibited to use the method of filling the joint gap with welding rod tips or iron blocks for repair welding.
(2) When marking parts for machining, ensure sufficient cutting allowance and welding shrinkage allowance after cutting. Control the part dimensions properly and do not compensate for the outer dimensions by increasing the gaps.
When joining sheets of different thicknesses and widths, an uneven transition is observed.
【Trend】
When joining boards of different thicknesses and widths, it is crucial to ensure that the thickness difference is within the allowable standard range. If it is not, and if a smooth transition is not properly handled, the weld seam at the point where it exceeds the thickness of the thinner board is prone to stress concentration and焊接 defects such as incomplete fusion, which can affect the quality of the weld.
【Action】
When applicable, weld seams should be formed into a slope with a maximum slope ratio of 1:2.5; or the face or both faces of the thickness should be machined into a slope before welding, with a maximum slope ratio of 1:2.5. For structures that directly bear dynamic loads and require fatigue calculations, the slope should not exceed a ratio of 1:4. When joining plates of different widths, the method of thermal cutting, mechanical processing, or grinding with a sanding wheel should be used according to the factory and construction site conditions to ensure a smooth transition, with a maximum allowable slope ratio of 1:2.5 at the connection point.
11. Failure to pay attention to the welding sequence for components with interlocking welds.
[Observation]
For components with intersecting weld seams, failing to properly arrange the welding sequence by analyzing the effects of stress relief and welding stress on component deformation, and instead arbitrarily welding in both directions, can result in mutual restraint between the longitudinal and transverse seams. This can generate significant temperature contraction stress, causing the plate to deform and become uneven, and potentially leading to cracks in the welds.
[Action]
For components with intersecting weld seams, a reasonable welding sequence should be adopted. When multiple transverse and longitudinal intersecting welds are being welded, the transverse welds with greater shrinkage deformation should be焊接 first, followed by the longitudinal welds. This ensures that the transverse welds are not constrained by the longitudinal welds, allowing the shrinkage stress of the transverse welds to be released without constraint, thereby reducing welding deformation and ensuring weld quality. Alternatively, one may first weld the butt welds and then the corner welds.
When using encircling welds for steel bar joint connections, continuous welding should be applied at the turning corners.
[The Phenomenon]
When using encircling welding for the junction of steel rod members and continuous plates, the welding process involves first welding the seams on both sides of the rod members, followed by welding the end head seams, without continuous welding. Although this method is beneficial for reducing welding deformation, it is prone to stress concentration and welding defects at the rod member's corner, which may affect the quality of the welding joint.
【Measures】
When using surrounded welding for steel bar connection joints, the welding should be completed in one continuous run at the corner, do not stop at the corner and then move to the other side for welding.
13. Requirements for strong alignment: No leading and trailing plates are installed at the ends of the crane beam's wing plates and web plates.
[Observation]
During the welding of lap joints, full penetration fillet welds, and the welds between the flange and web of crane beams, no additional striking and extinguishing plates are installed at the striking and extinguishing points. This results in unstable temperatures at the start and end points of the welding due to insufficient stability in current and voltage, which easily leads to defects such as incomplete fusion, under fusion, cracks, slag inclusions, and gas pores at the start and end points of the welds. This reduces the strength of the welds and fails to meet the design requirements.
【Action】
When welding lap joints, full penetration corner joints, and the welds between crane beam flanges and webs, it is necessary to install leading and trailing plates at both ends of the weld. Their purpose is to guide the prone-to-defect sections away from the workpiece, and then cut off these defective sections to ensure the quality of the weld.
