The causes and prevention measures of porosity in pressure opening welding

The reasons for the porosity in the welding with pressure openings are as follows During the construction of engineering welding with pressure openings, the bubbles in the weld pool of the pipeline failed to escape during solidification, and the holes formed in the weld metal of the pipeline (or the surface) are called pores. It can be divided into internal stomata and surface stomata according to its producing position. According to the shape, it can be divided into spherical stomata, slime stomata and needle stomata. According to the distribution characteristics of stomata, it can be divided into dispersed stomata and dense stomata. The size of the porosity formed during welding is different, with the number of 0.5 ~ 1.5mm. Hydrogen and nitrogen pores are the most common types of pores in the welding seams of pipes and fittings. Most hydrogen pores are single pores and distributed randomly. The hydrogen dissolved in the molten pool is formed in the solid metal without time to precipitate because the gas solubility decreases sharply during the cooling and crystallization process of the molten pool. Nitrogen pores are dense pores, honeycomb-shaped, mainly in the arc position of the electrode, electrode end protection performance is poor, so that nitrogen into the weld pool and residual in the formation of pores in the weld. Due to the residual porosity in the weld, it is bound to reduce the effective cross-sectional area of the weld metal, so as to reduce the strength of the welded joint, especially the dense porosity will make the weld is not dense, reduce the plasticity and toughness of the weld. The harm of the pipe and pipe parts brought by the porosity: the porosity will reduce the effective cross-sectional area of the weld, reduce the mechanical properties of the weld, damage the density of the weld, especially the small diameter, deep cylindrical long porosity (commonly known as pinhole) is very harmful, serious directly cause leakage. 1. Reasons for the formation of welding pores with pressure openings: 1.1 The electrode or flux is damp or not dried as required. Welding electrode coating cracking, falling off, deterioration. 1.2 The carbon content of base metal and steel core of electrode is too high. The deoxidation ability of electrode coating is poor. 1.3 There are water, oil, rust and other dirt on the weldment surface and groove. Under the action of high temperature arc, carbon monoxide, hydrogen and water vapor decomposed from these dirt often form carbon monoxide and hydrogen pores after entering the molten pool. 1.4 The welding current is too low or the welding speed is too fast, and the molten pool exists for a short time, so that the gas cannot escape from the molten pool metal. 1.5 Arc length is too long, so that the molten pool lost the protection of gas, air is easy to invade the molten pool, welding current is too large, welding rod red, medicine skin off, and lost the protective effect, arc bias blowing, transport technique is unstable. 1.6 In submerged arc welding, the arc voltage is too high and the network voltage fluctuates too much. At the bottom of the pipe, there are dense pores in the beginning welding end, the end welding end and the joint. The middle part of the weld is a single porosity. 1.7 Reasons for porosity at the initial welding end: (1) Because the whole specimen is in a low temperature state, the solidification speed of the molten pool is fast, and the gas integrated into the molten pool has no time to escape to form in the weld. (2) Due to the influence of the earth magnetic field or the position of the specimen groove adsorbed magnetic particles, so that the specimen groove there is strong magnetism. It can be seen from the distribution of magnetic field lines that the distribution of magnetic field lines at the beginning of welding is uneven, and it is easy to generate magnetic bias blowing during welding, which makes the arc drift and destroys the gas protective layer around the molten pool, resulting in the nitrogen in the air being involved in the molten pool and forming dense pores. 1.8 The reason for the porosity at the pipe joint: when replacing the welding rod joint, the molten pool has solidified and the temperature has been reduced rapidly. The cooling speed of the molten pool at the joint is fast enough for the gas to escape and easily form the porosity; In addition, the coating at the end of the electrode is chamfered so that the welding core is exposed, which can not form a good gas slag protection during welding, so that the nitrogen in the air enters the weld pool, thus forming dense pores. 1.9 The causes of porosity in the final welding end are as follows: there is a large gap between the groups of the final welding end when the welding groups with pressure holes are paired in the pipeline; At the end of welding, the specimen was already in a high temperature state, the area of the molten pool increased, and the temperature of the liquid metal increased. Under the action of the arc thrust, the molten pool moved violently, making the protective effect of the coating on the molten pool worse. When nitrogen entered the molten pool and the metal changed from liquid to solid, the nitrogen had no time to escape and formed dense pores. In addition, the cracking of electrode coating can also make nitrogen enter the weld pool and form dense pores during welding. 1.10 Reasons for the porosity in the middle part of the pipeline with pressure opening weld: The porosity in the middle part of the weld is mostly a single porosity, which belongs to hydrogen porosity according to its nature. It is mostly due to the influence of the water contained in the drug skin. When the original water content of the drug skin is greater than 0.4%, the total amount of hydrogen entering the molten pool increases, which is easy to produce hydrogen pores. 2. Prevention Measures: 2.1 Before welding, the electrode or flux must be dried according to the specified temperature and time, and should be taken with use, or placed in the electrode insulation bucket with use. 2.2 The electrode with no cracking, falling off, metamorphism, eccentricity, low carbon content and strong deoxidation capacity should be selected. The surface of the welding wire should be clean and free from oil and rust. 2.3 Carefully clean the groove and both sides to remove oxides, grease, moisture, etc. 2.4 When using alkaline electrode for welding, the arc length should be kept low, and windproof measures should be taken when the outside wind is strong. 2.5 Select appropriate welding specifications to shorten the arc extinguishing stop time. After arc extinguishing, when the molten pool is not fully solidified, it is timely to send the arc to the molten drop, breakdown welding. 2.6 The Angle of the strip shall be appropriate, the operation shall be skilled, and the slag shall not be dragged away from the molten pool. Turn self welding cut online铜焊丝,Copper Welding Rods,铝焊丝,aluminium welding wire,镍焊条,Nickel electrode,药皮焊条,Flux Coated Brazing Welding Rods