Five common problems in welding thick plates in steel structures and their solutions

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01 When joining plates of different thicknesses, if the transition is not smooth, when using plates of varying thicknesses and widths, no attention is paid to whether the difference in thicknesses is within the limits permitted by the standards. If it is not within the allowable range and no gradual transition is provided, the weld will be prone to stress concentration and welding defects such as lack of fusion at areas that are thicker than the thickness of the thin plate, thereby affecting the quality of the weld. Solution: When the specified limits are exceeded, the weld should be made in a sloped shape, with the maximum allowable slope ratio being 1:2.5 ； One or both of the thick sides shall be tapered before welding, with the maximum allowable slope ratio being 1:2.5; for structures that are subjected to dynamic loads and require fatigue analysis, the slope ratio shall not exceed 1:4. When joining plates of different widths, heat cutting, mechanical processing, or grinding with a grinder should be employed, depending on the conditions at the factory and the construction site, to ensure a smooth transition; the maximum allowable slope at the joint is 1:2.5. 02 Discontinuities in multi-pass welding can lead to cold cracks. When performing multi-pass welding on thick plates, if the inter-pass temperature control is neglected—for instance, if too much time elapses between passes and welding is carried out without reheating—cold cracks are likely to occur between the passes ； If the interval time is too short and the interlayer temperature is too high (above 900 °C), it will also affect the properties of the weld and the heat-affected zone; this can lead to coarse grains, resulting in a decrease in toughness and ductility, and it creates potential risks for the joint. Solution: When welding thick plates in multiple layers, it is necessary to strengthen the control of the interlayer temperature. During continuous welding, the temperature of the base material being welded should be monitored, so that the interlayer temperature can remain as close as possible to the preheating temperature; moreover, the maximum interlayer temperature must also be controlled. The welding time should not be too long; in the event of a welding interruption, appropriate post-heating and holding measures should be taken. When welding resumes, the preheating temperature should be set slightly higher than the initial preheating temperature. 03 Deformation during welding: Failure to pay attention to controlling deformation during welding by considering factors such as the welding sequence, personnel arrangement, groove design, selection of welding parameters, and operating methods results in significant deformation after welding, making correction difficult and increasing costs. This is especially true for thick plates and large workpieces, where correction is challenging; mechanical correction methods can easily cause cracks or layered tearing. Flame correction is costly, and improper operation can easily cause the workpiece to overheat. For workpieces with high precision requirements, failing to implement effective measures to control deformation can result in the installation dimensions not meeting the required standards, which may even lead to rework or scrap. Solution: Adopt a reasonable welding sequence, select appropriate welding specifications and procedures, and also implement counter-deformation and rigid fixation measures. 04 Insufficient weld leg dimensions in joints requiring penetration: For butt or fillet welds of T-joints, cross-joints, fillet joints, and other types that require penetration, if the weld leg dimensions are insufficient, or if the weld leg dimensions of the joint between the web and the upper flange of crane beams or similar components, which are subject to fatigue analysis requirements, are inadequate, then both the strength and stiffness of the welds will not meet the design specifications. For butt joint combinations such as T-joints, cross joints, and corner joints that require full penetration, sufficient weld leg dimensions must be ensured in accordance with the design requirements; generally, the weld leg size should not be less than 0.25t (where t is the thickness of the thinner plate in the joint). For crane girders or similar webs designed with fatigue verification requirements, the root thickness of the welds connecting them to the upper flange is 0.5t, and shall not exceed 10 mm. The allowable deviation for welding dimensions is 0–4 mm. 05 Welding by stuffing electrode tips or iron pieces into the joint gap: It is difficult to fuse the electrode tips or iron pieces with the materials being welded during this process, which leads to welding defects such as lack of fusion and insufficient penetration, thereby reducing the strength of the connection. If rusted electrode tips or iron pieces are used for filling, it is difficult to ensure consistency with the material of the base metal ； Using welding rod ends or iron pieces contaminated with oil, impurities, etc. as fillers can cause defects such as pores, slag inclusions, and cracks in the weld. All of these conditions will **reduce** the quality of the weld at the joint, failing to meet the quality requirements for welds specified in the designs and standards. Solution (1): When the assembly gap of the workpiece is large but remains within the specified allowable range, and when this gap exceeds twice the thickness of the thin plate or is greater than 20 mm, the surfacing method should be used to fill in the recessed areas or reduce the assembly gap. It is strictly prohibited to use filled welding rod ends or iron pieces for patch welding in the joint gaps. (2) When marking parts for machining, it is necessary to leave sufficient cutting allowance as well as allowance for welding shrinkage after cutting. The dimensions of the parts should be controlled properly; increasing the gaps should not be used to maintain the external dimensions.