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General welding technique for CO2 welding

CO2 gas welding general welding technology mainly includes plate butt, tube sheet welding and pipe butt joint, etc., among them, the flat to apply welding is a welding plate butt joint welding, vertical, horizontal, and one-sided welding of the overhead welding forming the basis of welding technology, and flat, vertical, horizontal, overhead welding and welding tube plate joint and the foundation of the pipe joint.  In particular, we should master a series of operating techniques such as arc initiation, joint, arc closing, gun holding posture, gun holding Angle, arc on the middle position, welding gun swing, control hole size and so on, and seriously sum up experience, experience the operation essentials of single-sided welding double-sided forming.

CO2 gas welding Plate butt welding

The operation of welding plate butt flat welding mainly includes the basic operation of flat welding and the operation of flat plate butt flat welding, vertical welding, horizontal welding and inverted welding.  Relatively speaking, flat welding is a basic operation that is easy to master, while flat butt welding is a welding operation technology that requires higher requirements on the basis of flat welding, which is relatively easy to master. It is the guarantee to master the welding technology of various positions in space, and also the basic welding technology of boiler pressure vessel and pressure pipeline welders.

Flat welding

Flat welding is the welding basis of CO2 welding, and it is a welding skill that is easy to master. Other welding operations can only be carried out on the basis of flat lay welding.

Preparation of Weldments

Preparation of Weldments

One (or two) sheet is made of Q235A steel, and the size of the sheet is 300mm×120mm×12mm, as shown in Figure CO2 welding-1. If training is performed on a steel plate, a line should be drawn on the steel plate with a stone pen at intervals of about 30mm along the length, as the trajectory of the welding.

CO2 welding

Leveling of weldments

By visual method

First observe whether the weldment may be deformed during the cutting process. If it is found that the weldment has been deformed, it must be corrected. When correcting, you can use a hammer to compare and correct it on a flat workbench, or you can use it in the The deformed part is directly hammered on the bench vice to achieve the purpose of correction.

Cleaning and placement of weldments

Cleaning of weldments.

After the weldment has been corrected, it must be cleaned. Use cleaning tools (such as wire brush, emery cloth, file or angle grinder, etc.) to clean up oil, rust, moisture or other contaminants within 20mm on both sides of the weldment and expose the metal itself. luster.

Placement of weldments.

After the workpiece is drawn according to the welding requirements, it is placed flat on the welding bracket, and the welding ground wire (ground wire) is connected to the workpiece or bracket; Short circuit after the machine starts by mistake.

Welding materials of CO2 welding

When welding, choose H8Mn2SiA welding wire, the diameter of the welding wire is 1.0mm, and the surface of the welding wire should be cleaned before use. The CO2 gas purity is required to reach 99.5%.

welding equipment of CO2 welding

Welding was carried out with a semi-automatic co2 welding machine.

Operation process of CO2 welding

Welding parameters can be selected by referring to Table CO2 welding-1. 

Table CO2 welding
Table CO2 welding-1

welding operation of CO2 welding

Select the welding parameters to set the welding machine parameters, the setting of welding machine parameters is the key to flat welding, in general, after the welding parameters are selected, the welding is no longer changed.  When the preparation work is determined to be completed, close the power switch to start the welding machine flat welding generally squat type, the distance between the body and the welding part is relatively close, which is conducive to the operation of welding and observation of the welding situation, two feet into 70°~80° Angle, the distance between two feet in about 250mm.  

arc ignition

After the welding posture and position are determined, use one hand or both hands) to hold the welding torch. The arm holding the welding torch can rest on the bent knee and leg or can be suspended without support). Aim the welding wire at the beginning of the drawing line, the head of the welding wire is about 10mm away from the end of the welding piece, and immediately cover the entire face with a mask to prepare for ignition of the arc.

  • Arc striking is the beginning of welding. Use direct short-circuit method to strike the arc. Before arc striking, keep the distance between the end of the welding wire and the weldment 2-3mm (do not touch too close), and the distance between the nozzle and the weldment is 10~15mm

  • Press the torch switch to ignite the arc. At this time, the welding torch has a tendency to lift due to short circuit, and it must be controlled with a balanced force.After preparing the welding gun, press the welding gun downward to minimize the impact of the rebound of the welding gun, and maintain a suitable welding distance between the nozzle and the weldment. If it is the butt welding of two workpieces, the arc striking plate should be used to strike the arc(Figure CO2 welding-2), or the arc should be struck at a distance of 2-4mm from the end of the workpiece, and then slowly lead to the workpiece to be welded. When the weld metal is fused, it can be welded at the normal welding speed.

CO2 welding-2

Straight welding

A weld formed by straight line welding
A weld formed by straight line welding

 The weld formed by straight-line welding has a slightly narrower width, higher weld and shallower penetration, as shown in Figure CO2 welding-3 shown. In the process of operation, the formation of the entire weld is often the most likely to produce defects at the beginning of the weld, the connection of the weld, and the end of the weld, so the following special measures should be taken.

  • The weldment at the beginning of the welding end is at a lower temperature. After the arc is struck, the arc should be slightly elongated to properly preheat the end of the weld, and then the arc should be lowered for the beginning end welding (Fig. CO2 welding-4a, b), in this way, a weld with a certain penetration depth and a relatively neat formation can be obtained. As shown in Figure CO2 welding-4c, the welding seam is not formed neatly due to the use of too short arc starting. For the welding of important weldments, an arc strike plate can be added to the end of the weldment to keep the defects that are easy to occur during arc strike on the arc strike plate.

  • When the weld joint is connected, the quality of the joint directly affects the quality of the weld. The processing of the joint is shown in Figure CO2 welding-5.

    The method of straight-line weld connection is to start the arc at 10-20mm in front of the original molten pool, and then quickly guide the arc to the center of the original molten pool. A certain height and angle, and move forward at a steady speed, as shown in Figure CO2 welding-5a. The method of swinging weld connection is to start the arc at 10-20mm in front of the original molten pool, then lead the arc to the joint in a straight line, start to swing at the center of the joint, and gradually increase the swing while moving forward (maintaining The formed weld is the same width as the original weld), and finally transferred to normal welding, as shown in Figure CO2 welding-5b.

Influence of starting strip method on weld forming
CO2 welding-4 Influence of starting strip method on weld forming
Method of welding joint connection
CO2 welding-5 Method of welding joint connection

(a)Straight line welding with long arc preheating

(b)Swing welding with long arc preheating

(c)Straight line welding with short arc welding

(a)      Straight weld joint;

(b)      Oscillating weld joint

  • If there is a deep arc crater at the end of the weld, it will cause defects such as cracks and shrinkage holes at the end of the weld. If the filament CO2 shielding gas short-circuit transition welding is used, the arc length is short and the arc crater is small, and no special treatment is required. The pits are deep. Therefore, when the arc ends, if the welding machine does not have a current attenuation device, the arc crater should be filled by multiple intermittent arc strikes until the arc crater is filled.

    There are two moving directions of the linear welding torch: one is that the welding torch moves from right to left, which is called left welding method; the other is that the welding torch moves from left to right, which is called right welding method, as shown in Figure CO2 welding-6.

Direction of movement of the welding torch
Figure CO2 welding-6 Direction of movement of the welding torch

a.Left welding method. When the left welding method is used, the blowing force of the arc acts on the molten pool and its front, and pushes the molten pool metal forward. Since the arc does not directly act on the base metal, the penetration depth is shallow, the weld bead is flat and widened, the spatter is larger, and the protection effect is good. Although it is more difficult to observe the molten pool using the left welding method, it is easy to grasp the welding direction and is not easy to weld deviation.

b.Right welding method. When the right welding method is used, the arc directly acts on the base metal, the penetration depth is large, the weld bead is narrow and high, and the spatter is slightly smaller, but it is not easy to accurately grasp the welding direction, and the welding deviation is easy, especially in butt welding. Therefore, the general CO2 gas shielded welding adopts the left welding method, and the forward inclination angle is 10°-15°.

Swing welding

In semi-automatic CO2 welding, in order to obtain a wider weld, the lateral swing motion is often used. The commonly used swing modes include zigzag crescent shape, equilateral triangle oblique circle shape, etc., as shown in Figure CO2 welding-7. During swing welding, the horizontal swinging wire feeding angle and the wire feeding method at the starting end are the same as those for straight welding. In the lateral swing movement, pay attention to the following essentials:

Several swing ways of welding torch
CO2 welding-7 Several swing ways of welding torch
  • The amplitude of the left and right swings should be the same. When swinging to the center of the weld, the speed should be slightly faster, and when it is on both sides, it should be paused.

  • The swing amplitude should not be too large, otherwise, the high temperature part of the molten pool cannot be well protected. Generally, the swing amplitude is limited to 1.5 times the inner diameter of the nozzle.

Weld seam cleaning

Weld cleaning is a very necessary link after the completion of welding, if the welding is conducted by flux-cored wire, but also to do a good job of weld cleaning.  The specific method is to knock the slag from the side of the weld with slag hammer to make it fall off.  To prevent the hot slag from burning my face, we can use a welding mask to cover the slag, and a spray from both sides of the weld can be used for cleaning.