According to the design or process needs, the welding parts to be welded into a certain geometric shape, the groove formed after assembly is called the groove. The process of machining groove by mechanical, flame, arc and other methods is called bevel opening. The bevel opening enables the arc to go deep into the root of the groove, ensuring the root penetration, facilitating the removal of welding slag to obtain a better weld forming, and also adjusting the proportion of base metal and filling metal in the weld metal.
The CO2 welding groove form should be selected according to the structure form, welding piece thickness and technical requirements. The most common groove forms are I, V, double V, double V with blunt edge, double U with blunt edge and so on. Choosing groove form, on the premise of guarantee the weldment penetration, should consider the shape of the groove easy processing, high productivity of welding and after welding deformation of the workpiece as small as possible factors such as double groove than single-sided groove, U groove less than the consumption of V groove welding, after welding deformation of small, but U groove machining is difficult, only commonly used in important structure.
CO2 welding uses a large current density, so it can be welded thoroughly under the condition of small welding Angle and large blunt edge. And because the diameter of the welding gun nozzle is much thicker than the diameter of the welding rod, the arc radius of the U-shaped groove used by the thick plate is larger, so as to ensure the root penetration. The type and size of groove recommended for CO2 gas shielded welding can be referred to the technical guidance document for the mechanical industry.
The process of machining beveling using machinery (shearing, planing and turning), flame or electric arc (carbon arc gouging) is called beveling. Groove processing can be carried out by a variety of methods, according to the size and shape of the component during welding and different processing conditions, there are mainly the following commonly used processing methods:
1) All kinds of straight bevel can be used edge planer or head planer processing.
2) When the length of v-shaped groove, Y-shaped groove, double Y-shaped groove and I-shaped groove is not large, it is better to process on high-speed milling machine.
3) nc flame cutting or semi-automatic gas cutting, cutting out i-shaped, V, Y, double V groove can be cut out of the I, V, Y, double V groove, usually single v-shaped slope used in training the oral exam board is not a blunt edge, semi-automatic gas cutting machine to cut out test plate with good cutting Angle grinder can use polish.
4) Manual processing If there is no processing equipment, you can use manual gas cutting, Angle grinder, electric grinding head , pneumatic tools or file to process the groove.
5) The groove of the end face of the pipe and the hole on the tube plate are usually processed on the lathe.
6) Drilling machine processing can only process the hole on the tube sheet, because the hole is large, must use a large drilling machine for drilling.
7) The bevel on the end face of the pipe can be processed by the special machine , which is very convenient. Figure (Pneumatic pipe beveling machine) shows the two most used pneumatic pipe beveling machines.
At present, the welding groove of seamless steel pipe is mostly processed by end milling or turning (grinding is only used when the pipe diameter is large). When processing, the tool and workpiece relative rotation, and rotation center distance to maintain a certain. The oil, rust, scale and other dirt on the edge of the groove that has been processed should be cleaned and eradicated before welding, so as to obtain better weld quality during welding. Sometimes necessary cleaning is also done with degreasing agents such as gasoline, acetone and carbon tetrachloride.
Before co2 welding, the relative position of the welding parts is fixed to ensure the correct geometry and size of the whole structural parts. The welding operation is called positioning welding, commonly known as spot solid welding. The formation of positioning welding is relatively short, not removed in the welding process, but the part of the formal weld is retained in the weld, so the quality, position, length and height of the positioning weld is appropriate, will directly affect the quality of the formal weld and the deformation of the weld. According to experience, some major quality accidents occurred in production, such as large structural deformation, incomplete welding and cracks and other defects, are often caused by unqualified positioning welding. Therefore, sufficient attention must be paid to positioning welding. The requirements of the welding rod and welder operation skill proficiency should be exactly the same as the formal weld, or even higher. When defects are found in the positioning weld, it should be shoveled or polished off and re-welded, not allowed to remain in the weld.
The following points should be paid attention to when conducting positioning weld co2 welding:
(1) The weld must be positioned according to the requirements of the welding process. The welding rod of the same grade and specification as stipulated in the formal welding process shall be used for welding with the same welding process parameters; If the process requires preheating before welding and slow cooling after welding, the positioning weld shall also be preheated before welding and slow cooling after welding. The preheating temperature is the same as during formal welding.
(2) the arc starting and arc closing end of the positioning weld should be smooth and not too steep, to prevent the welding joint at both ends of the welding penetration. Position weld must ensure good fusion, weld pass can not be too high.
(3) The positioning welding is discontinuous welding, and the workpiece temperature is lower than normal welding. Due to the lack of heat, it is easy to produce incomplete welding, so the welding current application is 10%~15% higher than that of formal welding. After positioning welding, welding must be done as soon as possible to avoid stopping or storing for too long.
(4) The length, residual height, spacing and other dimensions of the positioning weld can generally be selected according to Table 4-6. However, the size and quantity of the weld can be appropriately increased in some parts that play an important role in ensuring the size of the weldment.
| The thickness of the weldment | surplus height of seam | weld length | weld spacing |
| <=4 | <4 | 5~10 | 50~100 |
| 4~12 | 3~6 | 10~20 | 100~200 |
| >12 | ~6 | 15~30 | 200~300 |
(5) The positioning weld cannot be welded at the intersection of the weld or the place where the direction of the weld changes sharply. Usually, it should be at least 50mm away from these places to weld the positioning weld.
(6) In order to prevent workpiece cracking during welding, forced assembly should be avoided as far as possible. If the structure is assembled by force, the length of the positioning weld shall be increased according to the specific situation and the time of positioning weld shall be reduced.
(7) Positioning welds are easy to crack when welding at low temperature in order to prevent cracking, positioning welding after forced assembly should be avoided as far as possible; The length of positioning weld should be appropriately increased; And special attention should be paid to welding and welding all joints as soon as possible after positioning welding, to avoid pauses and long time idle. The current used in position welding is generally 10% to 15% larger than that used in ordinary welding.
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