Weld shape defects and preventive measures

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Defects in weld shape include: weld dimensions not meeting requirements, undercutting, incomplete penetration in the root layer, lack of fusion, burn-through, weld beads, cratering, arc scratches, spatter, etc. The causes and prevention methods are as follows: (1) Weld dimensions do not meet the requirements. Weld dimensions not meeting the standards refer to conditions such as the weld bead height and its variation, weld width and its variation, misalignment of the weld edges, and post-weld deformation, where the weld is uneven in height and width, with significant deformation, as shown in Figure 1. Inconsistent weld width not only results in an unsightly weld shape but also affects the strength of the bond between the weld and the base material. Excessive weld height leads to stress concentration, while a weld that is lower than the base material does not provide sufficient joint strength. Misalignment and excessive deformation can cause distortion in force transmission and stress concentration, thereby reducing strength. The causes include an improper groove angle, uneven root edges and fit gaps; unreasonable selection of welding process parameters; and low operational skills of the welder. Preventive measures include: selecting an appropriate groove angle and fitting clearance to improve the quality of assembly; choosing suitable welding process parameters; and enhancing the skill level of the welder, among others.                                                         Insufficient weld formation (2) Undercutting: Grooves or depressions that form in the base metal along the weld toe as a result of incorrect selection of welding process parameters or improper welding techniques are known as undercutting, as shown in Figure 2. Undercutting not only reduces the strength of the welded joint but also increases the risk of cracks due to stress concentration.                                                            The main causes of undercutting are excessive current, too long an arc, incorrect electrode angle, and improper welding technique. To prevent defects, when performing shielded metal arc welding, it is necessary to choose the appropriate welding current and welding speed; the arc should not be made too long, the electrode angle must be correct, and the method of moving the electrode must be proper. (3) Lack of penetration: Lack of penetration refers to the condition in welding where the bottom layer of the weld joint is not fully melted, as shown in Figure 3. Underwelding causes stress concentration and can easily lead to cracks; underwelding is not allowed in critical weld joints.                                               Underwelding in shielded metal arc welding occurs due to reasons such as an excessively small groove angle or gap, excessive root bevel, inappropriate selection of welding process parameters or poor assembly, as well as inadequate welding technique on the part of the welder. Preventive measures include: correctly selecting and processing groove dimensions, proper assembly to ensure proper spacing, choosing appropriate welding current and welding speed, and improving the welder’s technical skills. (4) Lack of fusion: Lack of fusion refers to the areas where, during welding, there is incomplete melting and bonding between the weld bead and the base material, or between weld beads themselves, as shown in Figure 4. Non-welding directly reduces the mechanical properties of the joint, and severe non-welding can render the welded structure completely unable to bear load.                                               The main causes of lack of fusion are too low welding heat input, an angled arc direction, rust and dirt on the groove sidewalls, and incomplete removal of slag between layers. Preventive measures include: correctly selecting welding process parameters, operating carefully, and ensuring thorough interpass cleaning. (5) Weld beads: Weld beads refer to metal protrusions that form when molten metal during welding flows onto the unmelted base material outside the weld seam, as shown in Figure 5. Weld beads not only affect the shape of the weld seam but also often result in slag inclusions and incomplete penetration in those areas.                                         Weld beads are formed due to an excessively high temperature in the molten pool, which results in slow solidification of the metal, leading to their formation under the effect of gravity. Preventive measures include: when using shielded metal arc welding, selecting appropriate welding process parameters based on the different welding positions, and strictly controlling the size of the molten pool. (6) Root crater: The recessed area that forms at the end of the weld is called a root crater. Cratering not only severely weakens the strength of the weld in that area, but it also causes crater cracks due to the concentration of impurities. The main causes are an excessively short arc extinction dwell time and excessive current during thin plate welding. To prevent this issue, when ending welding with shielded metal arc welding, the electrode should remain in the molten pool for a short while or move in a circular motion; once the molten pool is filled, the electrode should then be moved to one side to extinguish the arc.