When to use single-layer, multi-layer, and multi-layer multi-pass welding?

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This post was last edited by huanghong on 2021-1-29 08:43. When is single-pass welding used? For welds with leg sizes of less than 8 mm, single-pass welding (one weld layer at a time) is generally employed, and the diameter of the welding rod is selected to be between 3 and 5 mm depending on the thickness of the steel plate. For welds with a root bead size of less than 5 mm, the straight travel method and a short arc can be used for welding. The welding speed should be consistent; the angle of the electrode should be 45° relative to the horizontal surface, and 65° to 80° relative to the welding direction. If the electrode angle is too small, the root penetration depth will be insufficient; if the angle is too large, slag is likely to move to the front, resulting in slag inclusions. When using the straight-line welding technique to weld joints with small leg dimensions, press the edge of the sleeve at the tip of the electrode against the weld and hold it gently; as the electrode melts, it will gradually move in the direction of welding. This not only facilitates operation but also results in a greater penetration depth, as well as a more attractive appearance of the weld seam. When the weld leg size is between 5 and 8 mm, a slanted circular or reverse serrated welding technique can be used; however, the welding speed must be adjusted accordingly, as otherwise issues such as undercutting, slag inclusions, and poor edge fusion may occur. When should multi-layer welding be used? When the leg size is between 8 and 10 mm, a two-layer, two-pass welding method can be employed. When welding the first layer, a welding rod with a diameter of 3–4 mm can be used, and the welding current should be slightly higher to achieve a greater penetration depth. Using a linear welding technique, the crater at the end of welding should be filled or left slightly higher; this way, when welding again at the end, the crater will not become too low due to the increased temperature of the weld seam. Before welding the second layer, it is necessary to remove all slag from the first layer. If any inclusions are found, they must be repaired using a weld rod of small diameter before the second layer can be welded, so as to ensure a tight bond between the layers. When welding the second layer, a welding rod with a diameter of 4 mm can be used; the welding current should not be too high, as too high a current can cause undercutting. When should multi-layer, multi-pass welding be used? When welding joints with root gap sizes greater than 10 mm, if multi-layer welding is employed, the wide surface area of the weld and its steep slope make it easy for the molten metal to sag, thereby posing certain difficulties for the welding process. Therefore, multi-layer and multi-pass welding is used in actual production. When the weld leg size is 10–12 mm, it is generally completed using two layers with three passes. When welding the first layer (the first pass), a weld rod with a smaller diameter and a higher welding current can be used, along with a straight-line welding technique; the ending method is the same as that for the first layer in multi-layer welding. After welding, remove the slag completely. When welding the second weld seam, it should cover at least 2/3 of the first weld seam. The angle between the welding rod and the horizontal surface should be slightly larger, generally between 45° and 55°, to ensure good fusion of the molten metal with the horizontal surface. The angle between the welding rod and the welding direction remains at 65°~80°; a diagonal circular or reverse zigzag motion of the rod should be used. During welding, it is important to ensure good fusion between the molten metal and the horizontal surface. When welding the third bead, it should cover 1/3 to 1/2 of the second bead. The angle between the welding rod and the horizontal plate is 40° to 45°; an angle that is too large can lead to the weld foot drifting downward. Generally, a straight-line welding technique is used, and the welding speed should be consistent; it should not be too slow, as a slow speed can lead to the formation of weld beads, resulting in an unsightly weld appearance. 【Precautions】 When the second weld covers more than 2/3 of the first layer, the straight back-and-forth weaving method can be used when welding the third weld to prevent it from becoming too high. If the second pass covers too little of the first pass, the diagonal circular weaving technique can be used during the third pass; it is necessary to pause slightly on the vertical plate while weaving the weld bead in order to prevent undercutting, which will thus compensate for the offset of the weld root caused by insufficient coverage from the second pass. When welding joints with weld leg dimensions of over 12 mm, a configuration of three layers with six passes each or four layers with ten passes each can be used. The larger the weld leg size, the more welding layers and passes there will be.