Causas de porosidade na soldagem protegida com gás CO2: medidas para re-soldagem

Causes of porosity in welding protected with CO2 gas: measures for re-welding

Introduction

Carbon dioxide ( CO2 ) gas shielded welding, a semi-automatic arc welding technique that utilizes CO2 as the shielding gas and wire as the electrode and filler material, has distinct advantages over manual arc welding, including greater efficiency production, less welding deformation and superior quality.

It is the preferred method for welders. However, improper selection of current and voltage can lead to weld defects, particularly pores in the weld seam.

Therefore, in daily operations, it is crucial to correctly apply CO2 gas shielded welding to improve welding quality and weld again after identifying and removing substandard weld beads with gas defects.

Characteristics of Pore Distribution in CO 2 Shielded Welding

The characteristics of pore distribution are often closely related to their causes and conditions. Depending on their location, some may be on the surface, within the weld seam or at its root. Some even penetrate the entire weld seam.

From the distribution status, there may be single pores, clusters of multiple pores, or pores that run in a chain-like pattern along the length of the weld seam.

Pore ​​formation process

Although different gases form pores that not only have unique appearance and distribution, but also vary in their metallurgical and process factors, any gas bubble formed in the molten pool follows the general rule of phase transformation from liquid to gas, involving nucleation stages. and growth.

3.1 Complete Pore Formation Process

The molten pool absorbs a large amount of gas and reaches a saturated state — under certain conditions, the gas aggregates and forms a nucleus — the bubble nucleus grows until it becomes a bubble of a certain size — the bubbles rise, become obstructed and remain in the solidified weld seam, forming a pore.

Therefore, pore formation is the result of several stages: gas absorption by the molten metal, bubble nucleation, growth and emergence. Each stage has its own influencing factors.

3.2 Factors that influence each stage

The presence of supersaturated gases (or gases that cannot be dissolved) in the liquid metal is the material basis for gas nucleation and growth. During welding, the molten pool presents suitable conditions to form gas bubbles.

Furthermore, the greater the degree of saturation in the molten pool, the less energy will be required for the gas to precipitate from the dissolved state.

Two conditions are necessary for gas growth: first, the internal pressure of the gas must be sufficient to overcome the external pressure to which it is subject.

Second, the growth must be fast enough to ensure that it reaches a certain macro size before the molten pool solidifies.

The rise of gas consists of two processes. First, the bubble must separate from the surface where it forms, and the difficulty of this depends on the contact situation between the bubble and the surface.

The rate of bubble increase is related to the following factors: pore radius, liquid metal density, and liquid metal viscosity.

Welding wire diameter.
(mm) Arc voltage
(V) Welding current
(A) 0.8 18 100~110 1.2 19 120~135 1.6 20 140~180

7.7.2 Welding torch angle

  1. Welding torch angle during flat welding:

Generally, the angle between the welding torch and the weld plane should be maintained at about 65°. The welding operation must be constant, with the torch moving neither too high nor too low, neither too fast nor too slow.

If the welding site is exposed to strong winds, a thin steel plate can be used to block the wind. It is best to use a thin steel plate 2mm thick and 200mm wide folded into a U-shaped frame and placed close to the welding area.

The U-shaped structure can block the wind coming from various directions to avoid interference with the welding area, and can also prevent the arc light from harming the eyes of surrounding workers.

  1. Welding torch angle during horizontal welding:

The angle between the welding torch and the base material must be maintained within a range of 45°. The operating speed during horizontal welding should not be too fast, and the rotating range of the welding torch should not be too large, generally between 10 ~ 15 mm.

If there is a lot of wind, a steel plate or U-shaped steel frame can be placed near the welding area to block the wind. However, the placement of the steel plate must not obstruct the welder's line of sight or interfere with the movement of the welding torch.

  1. Welding torch angle during vertical welding:

The angle between the welding torch and the welding seam of the base material should be about 15°. The welding current should not be too large, generally about 20% lower than flat welding.

During vertical welding, due to the influence of the upward air flow below the welding area, the CO 2 flow rate may be slightly increased during the welding operation (depending on the situation).

The higher the vertical welding position is in relation to the ground, the greater the upward airflow. If this situation is encountered, a 200mm thin steel plate can be placed under the welding torch to effectively block the impact of increased air flow on the welding area.

  1. The speed of air movement in the welding area directly affects the quality of the weld. When using CO 2 gas shielded welding, it is strictly prohibited to blow air directly into the welding area with a fan.

7.2 Correct on-site operation methods

Due to long-term welding, spatter may block the welding torch nozzle, reducing the flow of CO2 gas and impairing the protective performance, which can easily lead to the formation of nitrogen pores.

Spills must be removed immediately in this situation. Over time, the nozzle may become deformed and narrowed with use, reducing the range of protection and increasing the likelihood of gas pores.

When this situation is encountered, a new nozzle must be installed before welding operations can continue.

When all welding operations are completed, the welder must turn off the welding machine and the CO 2 valve to prevent overheating and possible damage to the heating wire.

Conclusion

The main reasons for the formation of pores in CO 2 gas shielded welding are cleanliness of the welding surface from the base material (presence of oil, oxides), insufficient CO 2 gas flow to protect the welding area, excess moisture in the gas affecting its purity; too large a distance between the nozzle and the workpiece, allowing air to enter; many splashes adhered to the internal wall of the nozzle, affecting the protection effect; and wind at the operation site breaking the protective gas curtain.

In addition to choosing the correct welding process parameters and ensuring the good condition of the welding equipment, corrective measures must also include ensuring the quality of the welding wire and the purity of the CO 2 gas and choosing the correct welding angle, among other things.

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