Causes of porosity during welding with openings under pressure

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During welding in pressure-containing hole-opening projects, voids that form within (or on the surface of) the pipe weld metal as a result of bubbles remaining in the weld pool during solidification are known as pores. Based on their location of formation, they can be divided into internal stomata and surface stomata ； 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. The most common types of pores that appear in welds when performing overhead welding on pipes and fittings are hydrogen pores and nitrogen pores. Hydrogen pores are mostly single pores, distributed randomly. Since the hydrogen dissolved in the molten pool, during the cooling and crystallization of the pool, sees a sharp decrease in its solubility, it does not have time to escape and remains trapped within the solid metal ； Nitrogen pores are usually dense and honeycomb-shaped, appearing primarily at the arc initiation point of the welding rod; the poor shielding effect at the end of the welding rod allows nitrogen to enter the molten pool and remain in the weld, thereby 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 – commonly known as pinholes – pose a severe threat; in extreme cases, they can lead to leaks. 1. Causes of welding porosity resulting from opening a hole under pressure: 1.1 The electrode or flux is damp, or it has not been dried as required. The coating on the welding rod cracks, peels off, or deteriorates. 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 of the weldment and at the groove, these contaminants decompose under the high temperature of the arc to produce gases such as carbon monoxide, hydrogen, and water vapor. When these gases enter the molten pool, they can cause the formation of 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 a short time, preventing gases from escaping from the molten metal. 1.5 An excessively long arc length causes the molten pool to lose its protection from gases; air can easily penetrate the molten pool. An excessive welding current makes the electrode turn red and causes the coating to fall off, thereby eliminating its protective function. There may also be arc deflection and unstable electrode movement. 1.6 During submerged arc welding, using an excessively high arc voltage results in large fluctuations in the supply 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 are commonly found in the middle part of the weld. 1.7 Reasons for pores at the start of welding: ① Since the entire specimen is at a low temperature, the solidification rate of the molten pool is fast, and the gases that enter the molten pool do not have time to escape, remaining in the weld and thus forming pores. 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 magnetism 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 create 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, which easily leads to the formation of porosity ； Furthermore, the flux at the end of the welding rod has beveled edges that expose the core of the wire; this prevents proper gas-shield protection during welding, allowing nitrogen from the air to enter the molten pool and thus causing numerous pores to form. 1.9 Reasons for pores 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 test piece is already at a high temperature; the area of the molten pool increases and the temperature of the liquid metal rises. Under the force of the arc, the molten pool moves violently, which reduces the protective effect of the flux over the molten pool. Nitrogen enters the molten pool, and when the metal transitions from a liquid to a solid state, nitrogen does not have enough time to escape, resulting in the formation of numerous pores ； Furthermore, cracking of the welding rod coating can also allow nitrogen to enter the molten pool during welding, resulting in numerous pores. 1.10 Reasons for the formation of pores in the middle section of the weld when making openings in pipes under pressure: The pores that appear in the middle part of the weld are usually single pores, and based on their nature, they belong to hydrogen pores. This is mostly due to the influence of the moisture content in the flux; when the initial moisture content of the flux exceeds 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 used promptly when required. 2.2 Welding electrodes should be selected such that their coatings do not crack, peel off, deteriorate, or become eccentric, and they should have a low carbon content as well as strong deoxidizing capabilities. 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.