Thread Content
1. Failure to select the optimal voltage during welding 【Phenomenon】 During welding, whether it’s for root pass, filler pass, or cover pass, the same arc voltage is chosen regardless of the size of the groove. In this case, it may not be possible to achieve the required penetration and width, leading to defects such as undercutting, pores, and spatter. 【Measures】Generally, depending on the different situations, choosing either a long arc or a short arc accordingly will yield better welding quality and work efficiency. For instance, during root welding, a short arc should be used to achieve better penetration; whereas during filler or cap welding, the arc voltage can be appropriately increased to obtain higher efficiency and wider weld width. 2 Welding: Welding current is not controlled. [Phenomenon] During welding, in order to expedite the process, no groove is prepared for butt welds on medium and thick plates. The strength parameters decrease, sometimes not even meeting the standard requirements, and cracks appear during bending tests; this results in an unguaranteed performance of the welded joints, posing a potential risk to the structural safety. 【Measures】During welding, the welding current should be controlled in accordance with the values specified in the process evaluation; a variation of 10–15% is allowed. The size of the root bevel should not exceed 6 mm. During butt welding, when the plate thickness exceeds 6 mm, a groove must be made for welding. 3 Failing to pay attention to the welding speed and current, as well as to using the electrode diameter appropriately. [Symptom] During welding, no attention is paid to controlling the welding speed and current, nor is there proper coordination regarding the electrode diameter and the welding position. When performing a root pass on fully penetrated corner joints, due to the narrow size of the root area, if the welding speed is too high, there is not enough time for gases and slag to be expelled from that area, which can lead to defects such as lack of penetration, slag inclusions, and pores at the root ； When welding the cover, if the welding speed is too fast, pores are also likely to form ； If the welding speed is too slow, the weld bead height will be too high and its shape will not be regular ； When welding thin plates or welds with small root gaps, a too slow welding speed can lead to issues such as burn-through. 【Measures】The welding speed has a significant impact on welding quality and production efficiency. When selecting it, an appropriate welding speed should be chosen in conjunction with the welding current, the position of the weld (root pass, fill pass, cap pass), the thickness of the weld, and the groove dimensions. A higher welding speed can be utilized to improve productivity, provided that full penetration is achieved, gases and slag can be easily removed, the weld does not suffer from burn-through, and good weld shape is maintained. 4 Failing to control the arc length during welding 【Phenomenon】The arc length is not adjusted appropriately based on factors such as the groove shape, number of welding layers, welding method, and electrode type during welding. Due to improper use of the welding arc length, it is difficult to obtain high-quality welds. 【Measures】To ensure weld quality, short-arc welding is generally used during welding. However, the appropriate arc length can be selected depending on the specific situation in order to achieve the best welding quality. For example, in V-groove butt joints and fillet joints, a shorter arc should be used for the first layer to ensure full penetration and prevent undercutting; a slightly longer arc can be used for the second layer to fill the weld. When the weld gap is small, a short arc should be used; when the gap is large, the arc can be slightly longer to increase the welding speed. The overhead welding arc should be as short as possible to prevent molten iron from flowing downward ； When performing vertical or horizontal welding, low current and short arc welding are also used to control the pool temperature. Furthermore, regardless of the welding method used, it is important to maintain a basically constant arc length during the welding process, in order to ensure that the weld width and depth remain consistent throughout the entire weld. 5 Failure to control welding deformation during welding 【Phenomenon】During welding, no attention is paid to controlling deformation by considering factors such as the welding sequence, personnel arrangement, groove design, selection of welding parameters, and operating methods; as a result, significant deformation occurs after welding, making correction difficult and increasing costs. This is especially true for thick plates and large workpieces, where correction is challenging, and mechanical correction methods may cause cracks or layered tearing. Flame correction is costly, and improper use can easily overheat the workpiece. 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. 【Measures】Adopt a reasonable welding sequence, select appropriate welding specifications and procedures, and also implement counter-deformation and rigid fixation measures. 6. Multi-layer welding is carried out discontinuously, with no attention paid to controlling the interlayer temperature. [Symptom] When welding thick plates in multiple layers, if proper control of the interlayer temperature is not ensured – for example, if too much time passes between layers and welding continues without re-preheating – cold cracks are likely to form in those layers ； 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 poses potential risks to the joint. 【Measures】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 to ensure that the interlayer temperature remains 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 appropriately higher than the initial preheating temperature. 7 When multi-layer welding is performed on thick plates, if the slag is not removed and defects exist on the weld surface before starting the next layer of welding, it is easy for defects such as slag inclusions, pores, and cracks to occur in the weld. This reduces the strength of the joint, and it also causes spatter during the welding of the next layer. 【Measure】When welding thick plates in multiple layers, welding should be carried out continuously for each layer. After welding each layer, slag, surface defects of the weld, and spatter should be promptly removed. Any defects such as inclusions, pores, or cracks that may affect the welding quality must be completely eliminated before further welding is carried out. 8 Insufficient weld leg dimensions in butt or fillet joint combinations requiring full penetration 【Phenomenon】In butt or fillet joint combinations such as T-joints, cross joints, and fillet joints that require full penetration, if the weld leg dimensions are insufficient, or if the weld leg dimensions of the connections between the web and the upper flange edges in crane beams or similar components for which fatigue analysis is required are insufficient, then both the strength and stiffness of the welds will not meet the design requirements. 【Measures】For butt joint welds that require full penetration, such as T-joints, cross joints, and corner joints, 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 thinnest plate at the joint). For crane girders or similar webs designed with fatigue checks, 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. 9 Welding: Inserting electrode stubs or iron blocks into the joint gap 【Phenomenon】 Since it is difficult to fuse the electrode stubs or iron blocks with the workpiece during welding, this can lead to welding defects such as lack of fusion and incomplete penetration, thereby reducing the strength of the joint. If rusty 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 these situations will **reduce** the quality of the weld at the joint, failing to meet the quality requirements for welds as specified in the design standards. 【Measures】 (1) When the assembly gap of the workpiece is large but remains within the specified allowable range, and when the assembly gap exceeds twice the thickness of the thin plate or is greater than 20 mm, the recessed areas should be filled using surfacing methods to reduce the assembly gap. It is strictly prohibited to use the method of filling with welding rod ends or iron blocks for repair in the joint gap. (2) When marking parts for machining, it is necessary to leave sufficient cutting allowance as well as welding shrinkage allowance after cutting; the dimensions of the parts should be controlled properly, and increasing gaps should not be used to maintain the external dimensions. 10 Failure to pay attention to the welding sequence for components with intersecting welds 【Phenomenon】For components with intersecting welds, proper consideration is not given to arranging the welding sequence by analyzing the effect of welding stress release and welding stress on component deformation; instead, welding is carried out in a random manner in both vertical and horizontal directions. As a result, the vertical and horizontal welds restrict each other, generating significant thermal contraction stresses that cause the plate to deform and become uneven, and it is also possible for cracks to appear in the welds. [Measures] For components with intersecting welds, a reasonable welding sequence should be established. When welding multiple intersecting welds, the transverse welds that experience greater contraction deformation should be welded first, followed by the longitudinal welds. This way, when welding the transverse welds, they are not constrained by the longitudinal welds, allowing the contraction stresses in those transverse welds to be released without restriction, which helps reduce welding deformation and ensure weld quality. Alternatively, the butt welds can be welded first, with the fillet welds done afterward.