Causes and treatment methods of welding porosity defects

1. Phenomenon

Porosity is the cavity formed in the weld when the gas in the molten pool does not have time to escape during the solidification process. Porosity can be divided into external pores and internal pores according to the location of occurrence; it can be divided into single pores, continuous pores and dense pores according to the distribution.

2. Cause Analysis

The surrounding atmosphere during welding, the gases dissolved in the base material, welding wire and the steel core of the electrode, the gases produced by the decomposition of the electrode coating when it melts, the gases produced by the thermal decomposition of oil, moisture, rust and other impurities on the welding wire and base material, the gases produced by welding metallurgical reactions, etc., all factors that can cause a large amount of gas to be produced are the causes of pores. The main reasons are welding materials and welding processes.

(1) There is water, oil, rust and other dirt on the surface of the parent material and the groove.

(2) The welding rod is damp, the coating deteriorates or falls off, and the steel core is rusted.

(3) The welding current is too large, causing the coating to turn red and fall off, resulting in protection failure and allowing air to invade.

(4) When the alkaline electrode strikes the arc, the temperature of the base material and the electrode head is low, the shielding gas produced by the electrode is insufficient, the molten pool exists for a period of time, and air invades the molten pool, which easily produces dense pores.

(5) Improper electrode operation, the arc of the alkaline electrode is too long, the welding speed is too fast, the molten pool exists for a short time, and the gas has no time to escape.

(6) Arc blowing off track.

3. Hazards

Pores in the weld reduce the effective cross-section of the metal, thereby reducing the strength of the welded joint; stress concentration may occur at the edges of the pores, and dense pores will make the weld structure loose, reducing the plasticity of the joint; penetrating pores destroy the density of the weld and cause leakage. Hydrogen pores in the weld may also cause the generation and expansion of cracks.

4. Precautionary measures

(1) Oil, paint, oxides, moisture and other contaminants should be removed from the groove and within 15 mm of the groove.

(2) The welding rod should be baked strictly in accordance with the requirements of the welding rod manual. It cannot be replaced by a lower drying temperature or a longer baking time. After drying, it should be kept at a constant temperature and taken out when needed. Unused welding rods should be promptly recycled and re-baked. The number of baking times shall not exceed two times, otherwise they cannot be used for welding.

(3) Do not use welding rods that are cracked, peeling, deteriorated, have excessively eccentric coatings or have rusted cores.

(4) For alkaline electrodes, the long arc re-welding method should be used to strike the arc. The long arc serves as a preheating method, and the re-welding method can re-melt the arc striking point to form a good protective atmosphere and eliminate defects.

(5) Avoid adverse environmental influences such as wind and rain when welding. A wind shield should be set up when performing gas shielded welding outdoors. When welding pipes, pay attention to the influence of airflow inside the pipes. When performing gas shielded welding, pay attention to the purity and water content of the gas must meet relevant standards.

(6) Select appropriate welding process parameters. The polarity of alkaline electrodes should be reversed by direct current, and the welding angle should be adjusted in time when arc blow occurs.

5. Treatment Methods

(1) The cause of the welding quality defects should be found out before rework, and the rework should be carried out by a welder who has passed the assessment and holds the corresponding qualifications.

(2) For weld surface defects, if the depth does not exceed 0.5mm, only grinding with a grinding wheel is required, and no repair welding is required; if the depth exceeds 0.5mm, repair welding should be performed, and then grinding should be performed. The surface after grinding should be smooth and flat, with uniform transition. For defects exceeding the standard inside the weld, the defect location should be determined based on the flaw detection results, and the repair location should be determined.