Causes of porosity in welding during hot tapping under pressure – keep these 6 points in mind

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During welding operations in pressure-containing opening works, pores are voids that form within (or on the surface of) the weld metal of a pipeline; they occur because bubbles in the weld pool fail to escape during solidification. Based on their location of formation, they can be classified into internal pores and surface pores ； Based on shape, they can be divided into spherical stomata, ribbon-shaped stomata, and needle-shaped stomata, etc ； Based on the distribution pattern of stomata, they can be further divided into scattered stomata and dense stomata. The pores formed during welding vary in size, with the majority being between 0.5 and 1.5 mm in diameter. When performing overhead welding on pipes and fittings, the two most common types of pores that appear in the weld are hydrogen pores and nitrogen pores. Hydrogen pores are mostly single pores, distributed randomly. Due to the hydrogen dissolved in the molten pool, during the cooling and crystallization of the pool, the solubility of gases drops sharply, leaving insufficient time for it to be released; as a result, it remains trapped within the solid metal ； Nitrogen pores are usually dense and honeycomb-shaped, appearing primarily at the point where the welding rod starts to arc. The poor shielding effect at the end of the welding rod allows nitrogen to enter the molten pool and remain in the weld, thus forming pores. The presence of pores in the weld inevitably reduces the effective cross-sectional area of the weld metal, thereby lowering the strength of the welded joint. In particular, a high density of pores makes the weld less dense, which reduces its plasticity and toughness. The hazards caused by pores in pipes and fittings: Pores reduce the effective cross-sectional area of the weld, diminish its mechanical properties, and impair its density. In particular, cylindrical pores that are small in diameter but deep are extremely harmful; in severe cases, they can lead to leaks. 1. Causes of pores in welding during hot tapping under pressure: 1.1 The welding electrode or flux is damp, or has not been dried as required. Cracking, peeling, and deterioration of the electrode coating. 1.2 The carbon content in basic metals and the steel cores of welding electrodes is too high. The deoxidizing capacity of the welding rod coating is poor. 1.3 If there are contaminants such as water, oil, and rust on the surface and groove of the weldment, these contaminants, under the high temperature of the electric arc, decompose to produce carbon monoxide, hydrogen, water vapor, etc. Upon entering the molten pool, these gases often result in carbon monoxide pores and hydrogen pores. 1.4 If the welding current is too low or the welding speed is too fast, the molten pool exists for only a short time, preventing gases from escaping from the molten metal. 1.5 The arc length is excessively long, causing the weld pool to lose its gas protection. Air can easily penetrate the weld pool. The welding current is too high; the electrode becomes red-hot and its flux coating peels off, thereby losing its protective effect. Additionally, there may be arc blow and unstable electrode manipulation. 1.6 During submerged arc welding, using excessively high arc voltage results in excessive fluctuations in network voltage. The areas in the base coat of the pipe where pores appear are mainly the start and end welding points, as well as the joints, where dense pores form ； Single pores mostly appear in the middle portion of the weld. Causes of porosity at the starting end of welding: ① Since the entire specimen is at a low temperature, the solidification rate of the molten pool is fast; thus, the gas incorporated into the molten pool fails to escape in time and remains within the weld, resulting in porosity. ②Due to the influence of the Earth’s magnetic field or the presence of magnetic particles adhering to the groove of the test piece, a strong magnetic field exists at that area. Based on the distribution of the magnetic field lines, it can be seen that at the starting point of welding, the distribution of these lines is uneven; this makes it easy for magnetic deflection to occur during welding, causing the arc to wander. As a result, the gas shield surrounding the weld pool is disrupted, allowing nitrogen from the air to enter the weld pool and form numerous pores. 1.8 Reasons for porosity at pipe joints: When replacing the weld rod joint, the molten pool has already solidified and the temperature drops rapidly; the fast cooling rate of the molten pool at the joint prevents gas from escaping, leading to the formation of porosity ； Additionally, the flux coating at the end of the electrode is chamfered, exposing the core wire. During welding, this prevents the formation of adequate gas-slag protection, allowing nitrogen from the air to enter the molten pool, thereby resulting in numerous pores. 1.9 Reasons for porosity at the final welding end: The alignment gap at the final welding end is large when welding is carried out on pipes with pressure still present ； By the time welding reaches the final end, the specimen is already at a high temperature; the area of the molten pool increases, and the temperature of the liquid metal rises. Under the action of the arc thrust, the molten pool undergoes intense movement, which weakens the protective effect of the flux coating on it. Nitrogen then enters the molten pool, and since it fails to escape in time as the metal transitions from a liquid to a solid state, numerous pores are formed ； Furthermore, cracking of the electrode coating during welding can also allow nitrogen to enter the molten pool, resulting in numerous pores. Causes of pores appearing in the middle portion of welds during pressurized hole drilling in pipelines: Most pores that occur in the middle section of such welds are single pores; by nature, they should be classified as hydrogen pores. This is mostly due to the influence of the moisture content in the flux; when the initial moisture content of the flux is greater than 0.4%, the total amount of hydrogen entering the molten pool increases, which makes it easier for hydrogen pores to form. 2. Preventive measures: 2.1 Before welding, the electrodes or flux must be dried at the specified temperature and for the specified time; they should be used as needed, or stored in an electrode insulation container so that they can be retrieved and used promptly. 2.2 Welding electrodes with a coating that does not crack, peel off, deteriorate, or become eccentric, and which have low carbon content and strong deoxidation capability should be selected. The surface of the welding wire should be clean, free of oil and rust. 2.3 Thoroughly clean the groove and its surrounding areas, removing oxides, grease, moisture, etc. 2.4 When welding with alkaline electrodes, a low arc length should be maintained, and windproof measures should be taken in cases of strong external winds. 2.5 Select appropriate welding parameters to reduce the arc extinction downtime. After arc extinction, before the molten pool has fully solidified, an arc is re-established in a timely manner to supply droplets and complete the welding. 2.6 The welding angle should be appropriate, and the operation must be skillful to prevent slag from being dragged away from the molten pool.