■Plug the electrode or iron in the joint gap.

Phenomenon and harmfulness: Because it is difficult to fuse the electrode head or iron block into the welded piece, it will cause welding defects such as non-fusion, non-penetration and so on, and reduce the connection strength；If rusted electrodes or iron blocks are filled, it is difficult to ensure that the material is consistent with the base material. For example, oil and dirt on the electrode and iron will cause defects such as blowhole, slag and crack. These conditions will greatly reduce the weld quality of the joint, which can not meet the quality requirements of the design and specification.

Preventive measures: （1）When the assembly clearance of workpiece is large, but it does not exceed the allowable range, and when the assembly clearance exceeds 2 times the thickness of thin plate or more than 20 mm, the overlaying welding method is used to fill the depression or reduce the assembly clearance. It is strictly prohibited to use filler welding electrode or iron patch to repair welding joints. (2) When marking parts, attention should be paid to reserving enough cutting allowance and welding shrinkage allowance after cutting, controlling the dimension of parts well, and not increasing clearance to ensure the shape size.

■Plate butt welds of different thickness and width have no smooth transition

Phenomenon and harmfulness: When butt welds of plates with different thickness and width are used, it is not noticed whether the thickness difference of plates is within the allowable range of the standard. If not within the allowable range and without smooth transition treatment, such welds will easily cause stress concentration and non-fusion defects at the thickness of the sheet, which will affect the welding quality.

Preventive measures: When the thickness difference between the two butt weld plates exceeds the relevant regulations, the weld should be welded into a slope shape, and the maximum allowable slope should be 1:2.5；One or both sides of thick steel plate are processed into slopes before welding, and the maximum allowable gradient is 1:2.5. The gradient of structural slopes which bear dynamic loads directly and need fatigue checking should not be greater than 1:4. When the plates with different widths are butted, the transition should be smooth by thermal cutting, mechanical processing or grinding wheel according to the factory and site conditions, and the maximum allowable slope value of the joints is 1:2.5.

■Do not pay attention to the welding sequence of components with cross weld

Phenomenon and harmfulness: For components with cross welds, the welding sequence is not reasonably arranged by analyzing the effect of welding stress release and welding stress on the deformation of components, but, the longitudinal and transverse seams are welded randomly, resulting in mutual restraint between the longitudinal and transverse seams, larger temperature shrinkage stress, plate deformation, uneven surface, and possible cracks in the weld.

Preventive measures: A reasonable welding sequence shall be established for the component with cross-weld. When there are several kinds of longitudinal and transverse cross welds, the transverse weld with larger shrinkage deformation should be welded first, and then the longitudinal weld should be welded, In this way, the transverse weld will not be constrained by the longitudinal weld, so that the shrinkage stress of the transverse weld can be released without restraint, which can reduce the welding deformation, ensure the quality of the weld, or weld the butt weld first and then the fillet weld.

■When the circumference welding of lap joint of beam structural components, the corner of the lap plate shall be welded continuously

Phenomenon and harmfulness: When the lap joint between the section steel rod and the connecting plate is circumferentially welded, the welds on both sides of the rod are welded first, and the welds on the end are welded later. Discontinuous welding is beneficial to reduce welding deformation, but stress concentration and welding defects are easy to occur at the corner of the rod, which affects the quality of the welded joint.

Preventive measures: When the lap joints of profiled steel members are welded by circumferential welding, they should be welded continuously at the corner, not at the corner and then to the other side.

■Require equal strength material butt welding seam, no arc-triggering plate and lead-out plate at both ends being set for crane girder flange and web welding .

Phenomenon and harmfulness: When welding butt welds, fully penetrated fillet welds, flange plates and web welds of crane beams, There is no arc-initiating plate and lead-out plate at the starting and ending points, so when welding the starting and ending ends, the current and voltage are not stable enough, and the temperature of the starting and ending points are not stable enough. It is easy to lead to defects such as non-fusion, non-penetration, cracks, slag inclusion and gas holes in the starting and ending welds, which reduces the strength of the welds and fails to meet the design requirements.

Preventive measures: When welding butt weld, full penetration fillet weld and welding seam of crane beam flange and web, arc-initiating plate and lead-out plate should be installed at both ends of the weld, Its function is to introduce the defective parts at both ends to the workpiece, and then cut off the defective parts to ensure the quality of the weld.

■Not paying attention to the coordination of welding speed, welding current and electrode diameter.

Phenomenon and harmfulness: No attention should be paid to the coordination of welding speed and welding current, welding rod diameter and welding position when welding, For example, when the full penetrating corner seam is underwelded, because of the narrow size of the root, if the welding speed is too fast, the gas and slag in the root can not be discharged enough time, which will easily cause the defects such as impermeability, slag inclusion and pore in the root. When cover welding, if welding speed is too fast, it is also easy to generate pores; If the welding speed is too slow, the residual weld will be too high and the appearance will be irregular. When welding the thin or small root face weld, if the welding speed is too slow, and it is easy to burn and wear.

Preventive measures: Welding speed has a great influence on welding quality and productivity. When selecting welding speed, it is necessary to match welding current, weld position (bottom welding, filling welding, cover welding), weld thickness and groove size with appropriate welding speed, On the premise of guaranteeing penetration, clean discharge of gas and welding slag, no burning through and good appearance, a faster welding speed is selected to improve productivity

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