CO2 gas shielded welding is the most commonly used welding method in fusion welding. It can be divided into semi-automatic welding and full-automatic welding. Semi-automatic CO2 gas shielded welding (CO2 welding for short) has simple equipment, convenient operation and flexibility, and can adapt to welding under various conditions. Therefore, it has become the most widely used welding method in production. And with the continuous improvement of welding technology, for some complex structures, small parts, short welds or bending welds, CO2 gas shielded welding can also be used to complete. CO2 gas shielded welding is one of the main welding methods at present.
A joint joined by welding is called a welded joint. The welded joint consists of three parts: weld seam, fusion zone and heat affected zone.
c——weld width；h——weld bead height；s——The depth of the weld
In CO2 gas shielded welding, the joint form and groove form are different because of the thickness, structure form and use of welding parts. There are many forms of welding joint, the main basic forms can be divided into butt joint, T joint, Angle joint, lap joint four. Sometimes there are other types of joints in the welding structure, such as cross joints, end joints, oblique butt joints, lock butt joints and so on.
The joint where the end faces of two welds are relatively parallel is called a butt joint. Butt joint is one of the most widely used joint forms in welding structure. According to the thickness of the weldment, welding method and groove preparation, butt joint can be divided into two types: non-opening groove and opening groove.
Butt joint without groove opening. When the thickness of the steel plate is below 6mm, the groove is generally not opened and only 1-2mm joint gap is left. But this is not absolute, in some important structures, when the plate thickness is greater than 3mm, it is required to open the groove. The so-called groove is according to the design or process needs, welding parts to welding parts processing of a certain geometric shape groove.
Sloped butt joint. Bevel opening is the process of machining bevel by means of mechanical, flame or arc. The joint is opened into a certain Angle called groove Angle, its purpose is to ensure that the arc can go deep into the root of the joint, so that the root of the joint is fully welded, and facilitate the removal of welding slag to obtain a better weld forming, and the groove can play a role in adjusting the proportion of the base metal and filling metal in the weld metal. Blunt edge (when the weldment is beveled, the part of the end that is not beveled along the thickness of the weldment) is to prevent burning through, but the size of the blunt edge should ensure that the first layer of weld can be welded through. Root clearance (the gap reserved between the joint roots before welding) is also to ensure that the joint root can be welded through. For steel plates with plate thickness greater than 6mm, in order to ensure welding penetration, the groove must be opened before welding. Groove forms are divided into the following types:
(a) U bevel; (b) single U bevel; (c) double U bevel
U-shaped groove is characterized by the minimum amount of welding metal, small deformation caused by welding parts, and small proportion of the base metal in the weld metal. However, this groove processing is difficult, and it is generally applied to the more important welding structure.
When butt welding steel plates of different thickness, if the thickness difference (δ-δ1) does not exceed that specified in Table(The thickness difference range of steel plate with different thickness), the basic form and size of the joint should be selected according to the size data of the thick plate. If the thickness difference of butt steel plates exceeds that specified in Table(The thickness difference range of steel plate with different thickness), the thickness of the thick plate should be cut by one side (a) or two sides(b) with the length L≥3(δ-δ1)
The joint where the end face of one weldment and the other weldment surface form a right Angle or approximately right Angle is called a T-joint. T-joints are widely used in welded structures, especially in shipbuilders where about 70% of the welds are in this form. According to the thickness of weldment and different groove preparation, t-shaped joint can be divided into four forms: no groove, unilateral V-groove, K-groove and double U-groove.
(a） no groove (b)unilateral V-groove (c)K-groove (d) K-groove and double U-groove
T-joint as a general contact weld, steel plate thickness of 2-30mm, can be used without opening groove, it does not need more accurate groove preparation. If the weld of the T-shaped joint is required to bear load, it should be in accordance with the thickness of the steel plate and the requirements of structural strength. Unilateral V-shaped, K-shaped or double-U-shaped groove forms can be selected respectively, so that the joint can be welded through and ensure the joint strength.
The joint with an Angle greater than 30° and less than 135° formed between the end faces of two welds is called Angle joint. According to the thickness of weldment and groove preparation, the Angle joint can be divided into four forms: no groove, unilateral V-groove, V-groove and K-groove, but the Angle joint with groove opening is seldom used in the general structure.
(a) no groove (b) unilateral V-groove (c)V-groove （d） K-groove
The joint formed by the overlap of two welds is called lap joint. Lap joints can be divided into three forms, non-beveling, round hole plug welding and long hole fillet welding, according to their different structural forms and requirements for strength.
（a）non-beveling （b）round hole plug welding （c） long hole fillet welding
The lap joint without groove opening is generally used for steel plates below 12mm, the overlapping part is 3-5 times the thickness of the plate, and the double-sided welding is adopted. The assembly requirements of this joint are not high, and it is easy to assemble, but its bearing capacity is low, so it is only used in unimportant structures.
The lap joint without groove opening is generally used for steel plates below 12mm, the overlapping part is 3-5 times the thickness of the plate, and the double-sided welding is adopted. The assembly requirements of this joint are not high, and it is easy to assemble, but its bearing capacity is low, so it is only used in unimportant structures.
During welding, the space position of the welds is called welding position. Welding position is mainly divided into flat welding position, horizontal welding position, vertical welding position and inverted welding position four form.
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