On single-sided welding for double-sided formation

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1 Definition and Classification of Single-Side Welding with Double-Side Forming 1.1 Definition of Single-Side Welding with Double-Side Forming This welding method of single-side welding with double-side forming is a technique and skill that welders working on pressure vessels, important silos, and boilers must master. It is also used in the manufacturing and installation of certain important welded steel structures, where full penetration is required but it is not possible to work on the back side of the component for re-welding; hence this welding technique is employed in such situations. In this welding process, no additional auxiliary measures are required; it is sufficient to leave appropriate gaps depending on the welding technique when welding at the root of the groove. When welding on the front side of the groove, uniform, well-shaped welds of high quality are obtained on both the front and back sides of the groove. This unconventional welding method is known as \"single-sided welding for double-sided formation\". 1.2 Classification of single-sided welding with double-sided formation 1.2.1 Arc intermittent welding: Place the end of the test plate with a smaller gap on the left side; initiate arcing by positioning it on the left side of the plate, and maintain the arc for a certain period of time to preheat the welded component. Then lower the arc and move the welding rod laterally between the two bevel edges to carry out welding. When the molten metal at the root edge merges with the molten metal from the electrode, and a \"hissing\" sound is heard, the arc should be extinguished at that moment. The characteristic of this welding technique is that, with each successive arc, the center of the welding electrode should be aligned with the 2/3 position of the welding pool, so that the arc can melt the areas on both sides of the root edge at the same time. Upon hearing the \"bubbling\" sound, quickly extinguish the arc so that each newly formed molten pool covers about 2/3 of the previously formed molten pool. 1.2.2 Continuous arc welding: Continuous arc welding is a technique in which the arc remains lit throughout the welding process; the arc light does not go out. A small gap is used at the joint, and a lower welding current is employed at the beginning, with continuous welding carried out using a short arc. The basic elements of this technique are as follows: after the welding electrode initiates the arc, the arc is first reduced to its minimum level. At the starting point of welding, a zigzag motion with a small tooth pitch is used, with the electrode moved horizontally; the purpose of this is to preheat the welded components. Also, make an effort to guide the welding rod toward the root area, performing a motion that causes the welding rod to penetrate the material; when a \"pop\" sound is heard, a molten hole is formed in the workpiece. At that point, immediately shift the arc to any position on the groove surface, and then, at an appropriate angle with respect to the groove surface, perform intermittent movements for about 2 seconds, so that the arc melts approximately 1.5 mm of the root area on both sides of the groove. After that, lift the welding rod by 1–2 mm and move it in a zigzag pattern with small tooth-like intervals, allowing the arc to melt the material ahead of the molten hole while continuing to weld forward. When performing welding, ensure that the center of the electrode is aligned with the front edge of the molten pool where it meets the base material, so that each new molten pool overlaps partially with the previous one. During arc extinction, slowly move the welding rod slightly to the right or left behind the molten pool, and raise the welding rod to extinguish the arc. When performing joint welding, the arc is first ignited at a distance of 10–15 mm from the weld pool. The welding rod is moved at a normal speed until it reaches halfway to the weld pool; then the rod is pressed downward. When a \"pop\" sound is heard, a slight movement of 1–2 seconds is made, after which the rod is raised by 1–2 mm so that it can melt the material in front of the weld pool while continuing to move forward for welding purposes. Analysis of the single-sided welding with double-sided formation process for 2 plates butted together 2.1 Pre-welding preparations (1) Selection of plate thickness: 12 mm, size 300×200 mm. The material is cut using oxygen-acetylene gas or a sheet cutting machine or cutter, after which a V-shaped groove must be created by mechanical means, such as with a planer or an angle grinder. There should be no chamfers on the back side of the groove; it is essential to ensure that the groove has a proper angle, is straight and smooth, without any burrs or protrusions. (2) Clean the test plate: use a file to remove the sharp corners of the groove, keeping the size of the bevel at 0.5–1.0 mm. Also, remove rust, oil, oxides, and other contaminants from the groove and within a 20 mm range on either side of it, so that the metal surface appears shiny. (3) Assembly and tack welding of the test plate. Perform tack welding at both ends on the back side of the test plate, leaving room for shrinkage. Keep the gap at the weld joint at 3.0–3.5 mm, and at the final weld end it should be 3.5–4.0 mm. The length of the temporary weld joint is approximately 10–15 mm; this temporary weld joint must be strong and secure, especially at the final weld end. To avoid a reduction in the gap at the groove of the unwelded area due to weld contraction, which could affect further welding or cause fractures during the welding process, it is necessary to leave a margin for counter-deformation when performing the temporary welding – this margin corresponds to a counter-deformation angle of 3°–4°. (4) Welding equipment: ZXG series DC arc welders. Type and polarity of power supply: DC welding electrodes must not become damp or deteriorated; the core must be free from rust, and the coating must not crack or peel off. Preheat to 350–400°C before use and maintain this temperature for 2 hours; the weld layers are formed by welding in four layers in a row. (5) Fix the test plate: Place the deformed test plate horizontally on the welding frame, ensuring it is at the proper height. The welder must not sit while welding; they are required to work in a crouching position, with the groove angle of the test plate being 60°. 2.2 Welding operations
2.2.1 Welding of the root pass
For welding the root pass, ordinary welding techniques can be used, or end arc welding techniques may also be employed. Welding is performed using the continuous arc welding technique. Igniting the arc: During welding, the arc should be initiated on the inside of the groove in the test plate. The arc should be kept low; the welding electrode should be gently oscillated back and forth between the two groove edges. When the molten metal from the edges combines with the droplets of filler metal from the electrode, and a “popping” sound is heard, this marks the formation of the first weld pool. At this point, there should be a penetration hole in front of the molten pool, with this hole extending 0.5–1.0 mm into each side of the groove on the test plate. Welding electrode movement method: Use a zigzag or crescent-shaped transverse oscillation welding technique. The length of the oscillation arc should be less than or equal to the diameter of the welding electrode. Use shorter arcs to deliver the molten metal through the root of the groove to the back side of the workpiece. When the welding electrode reaches both sides of the groove, pause for 1–2 seconds. This helps ensure proper fusion between the filler metal and the base material, prevents the formation of sharp corners at the junction between the weld and the groove, and facilitates slag removal. 2.2.2 Electrode angle: The angle with respect to the direction of welding progression is 70°–80°; an appropriate electrode angle helps to separate the slag from the molten iron, keeping the molten pool clear and bright and preventing the formation of slag inclusions. Key points for welding electrodes: look, listen, be precise, and keep it short. “When observing, one needs to pay attention to the shape of the molten pool and the size of the pores; the size of these pores determines the height of the back weld seam as well as the excess height. The shape of the molten pool should be circular or oval, and it must remain bright and clear to separate the slag from the iron. The size of the fusion hole should be such that the arc melts both the blunt edges, penetrating 0.5–1.0 mm into the base material on each side. When the size of the melt hole is too large, the weld on the back side becomes excessively tall; in severe cases, defects such as weld beads or even burn-through may occur. At this point, it is necessary to appropriately increase the welding speed, enlarge the oscillation amplitude of the electrode to both sides of the groove, and reduce the angle between the electrode and the direction of welding progression, etc. When the molten pool is too small, the root of the groove fails to be properly welded, resulting in poor fusion at the root. In such cases, it is necessary to lower the arc, increase the angle between the electrode and the direction of welding, slow down both the welding speed and the oscillation amplitude, and reduce the angle between the electrode and the welding direction. Maintaining the shape of the molten pool and the size of the molten hole is the key to successful welding. During welding, it is necessary to control the flow direction of the molten iron and the liquid. The arc should always burn in front of the molten iron; by utilizing the reverse blowing force generated by the gases produced when the arc and the flux melt, the molten iron can be pushed towards the back of the weld pool. This ensures an appropriate depth of penetration in the weld, as well as proper separation between the slag and the molten iron, thereby reducing the likelihood of defects such as slag inclusions and pores. Additionally, it is important to constantly monitor the fusion status of the groove after welding, ensuring that the rear edge of the weld pool fuses completely with both sides of the groove. ““Listening” means that during welding, the welder must have no distractions; they must listen to the “popping” sound produced when the arc penetrates the test plate. If this sound isn’t heard, it indicates that the root of the groove hasn’t been penetrated by the arc. Continuing to weld under such circumstances will result in incomplete penetration. Generally, keep the end of the welding rod 2 or 3 mm away from the root of the groove. “\"Precision\" means that the arc position must be accurately controlled; in other words, each new molten pool should overlap the previously formed molten pool by about 2/3, ensuring that 1/3 of the arc is located right in front of the molten pool. When the welder hears a \"popping\" sound, he should quickly extinguish the arc behind the molten pool, and the arc extinguishment must be done swiftly. This way, the protective effect of the arc on the molten pool can be fully utilized, preventing the formation of pores. “\"Short\" refers to the time required to extinguish the arc and re-establish it; this process must be completed in as short a time as possible. Otherwise, defects such as cold shrinkage pores can occur, and metal cracking is more likely to happen due to insufficient fusion between the molten pools. By increasing the arc extinguishing frequency and reducing the arc-on time, the next molten pool is formed before the previous one has solidified; as a result, the molten pools remain in a high-temperature state, which prevents the formation of cold shrinkage pores. Therefore, for the two-click breakdown method, an arc extinguishing frequency of 50–60 times per minute is appropriate, while for the single-click breakdown method, a frequency of around 80 times per minute is suitable. 2.2.3 Joints. Joints can be classified into hot joints and cold joints; these are described separately below. Hot welding: When the arc crater is still hot, the arc is ignited on the slope 10–15 seconds behind the crater, and welding is carried out at the previous arc termination point; this allows the temperature at the bottom of the crater to rise gradually. Then, the welding rod is pressed downward along the pre-existing molten hole. Once a \"popping\" sound is heard, a brief pause is made before lifting the welding rod to proceed with normal welding. The faster the electrode replacement is done, the better. Cold welding: Once the arc crater has cooled, for carbon steel, the arc crater and the slag within the first 10 mm are knocked off and cleaned thoroughly. Then, a new welding rod is used; at the lowest point of the slope, the welding rod is quickly pressed downward along the pre-made weld hole. Once a \"popping\" sound is heard, a pause of about 1 second is taken, after which the welding rod is raised and normal welding proceeds. Welding using the arc-breaking technique: The arc-breaking technique includes two methods – one click to penetrate the cartridge and two clicks to penetrate it. The two-click penetration technique involves igniting the arc alternately on both sides of the groove: first a drop of molten iron is dropped on the left blunt edge, then another drop on the right blunt edge, and this process is repeated gradually. This welding method is suitable for welding tasks with large assembly gaps. The one-click penetration method involves igniting the arc on both sides of the groove at the same time, thereby melting the edges on those sides as well. The basic method of this operation is as follows: ignite the arc. During welding, start the arc on the inside of the groove at the starting end of the test plate, use a long arc to preheat that area, then lower the arc and move the welding rod back and forth between the two bevels. When the molten iron from the root edge of the groove combines with the droplets of weld metal, and a \"popping\" sound is heard, the first molten pool is formed and the arc is extinguished. At this point, a melting hole is formed at the front end of the first molten pool; it is appropriate for this melting hole to extend 0.5–1.0 mm into each of the two sides of the test plate. Replace the weld rod joint. The jointing method for changing the welding electrode during arc interruption is basically the same as that during continuous arc welding. Before changing the electrode, in order to prevent cold shrinkage pores caused by arc interruption, the arc should not be extinguished too quickly; instead, two or three drops of molten iron should be quickly fed into the edge or the back side of the molten pool to fill it. At the same time, the temperature of the molten pool should be controlled so that it cools slowly, thereby forming a molten hole at the front of the pool. The arc is then lowered toward the groove side, and welding is carried out backward for about 10 mm before the arc is extinguished. This approach ensures that the molten iron in the posterior part of the pool remains sufficient, thus preventing the formation of cold shrinkage pores. After quickly changing the welding electrode, start the arc within 10–15 mm in front of the arc crater. Once the arc is lit, pull it back to lengthen it, using the long arc to preheat the metal that needs to be welded. Then, about 10 mm behind the arc crater, lower the arc and, using a continuous-arc welding technique, move the electrode toward the root of the arc crater. Once a \"popping\" sound is heard, pause for around 2 seconds before extinguishing the arc, and then continue welding using the original intermittent-arc welding method. Precautions: (1) The thickness of the root weld should be 1.5–2.0 mm on both the front and back sides of the groove. (2) The welder must use the flexibility of their wrist to move the welding rod. (3) It is better to complete a 300mm long weld in 4 passes. 2.2.4 Welding of the filler layer: After the slag from the previous weld layer has cooled, remove the slag and spatter thoroughly; ensure that all slag, especially that in the corners, is completely cleared away. After the slag is removed, clean it with a wire brush until a metallic shine appears. Start the arc 10 mm from the beginning of the weld, then move the arc back to the starting point to carry out welding. Use a crescent or serrated welding motion; when the electrode moves to both sides of the groove, pause briefly to stabilize the arc. Move quickly in the middle, keeping the arc as low as possible to help bring impurities in the deep grooves and sharp corners to the surface, thereby preventing slag inclusions. This also helps to ensure balance between the molten pool and the sides of the groove, avoiding the formation of slag inclusions at the junction between the filler metal and the base material, which would make it difficult to remove such slag. The angle between the welding electrode and the direction of welding progress is 75° to 85°. During welding, a short arc should be used; the weld pool should be circular or oval in shape, with its shape and size remaining consistent. The welding speed must be uniform to ensure consistent weld thickness. When welding joints, the thermal method is used; before changing the electrode and ending the arc, a slightly larger amount of molten iron should be added to the molten pool. After quickly changing the electrode, the arc should be ignited at a distance of about 10 mm in front of the arc pit. Next, pull the arc to 2/3 of the crater position, fill the crater first, and then proceed with normal welding. The joint locations of each layer of welds should be staggered. The filling height of the final layer should be 0.5 to 2.0 mm lower than the surface of the base material. It is preferable that the shape be higher on the two sides and lower in the middle. The weld seam within the groove should have a smooth transition, so as to allow clear visibility of the groove during the overlay welding, ensuring that the edges of the overlay weld seam remain straight. 2.2.5 Welding of the cover layer: The arc-starting method for the cover layer is the same as that for the filler layer. Use a crescent or transverse zigzag welding motion; the amplitude of the electrode’s movement should be slightly greater than that of the filler layer. It is important to maintain a consistent amplitude of movement as well as a uniform welding speed. The shape of the welding pool is oval; efforts should be made to maintain the shape and size of the pool roughly constant, and short-arc welding should be used. When the welding rod reaches the edge of the groove, stabilize the arc and pause briefly, ensuring that about 1.0–2.0 mm of the groove edge is melted, so that the edges on both sides of the weld fuse well and undercutting is prevented. The welding advance speed must be consistent to ensure a smooth and aesthetically pleasing weld surface. When joining electrodes, the hot welding method is used. Before changing the welding electrode to extinguish the arc, a small amount of molten iron should be added to the weld pool. After replacing the welding rod, ignite the arc about 10 mm in front of the crater, then pull the arc backward to the 2/3 position of the crater; once the crater is filled, proceed with normal welding. If the joint area is offset, then the joint area will be at a higher position ； If the joint area is positioned too far forward, a defect of weld separation will occur. 3 Common defects in single-side welding with double-side formation. As summarized above, during the butt welding of plates using single-side welding to achieve double-side formation, common defects that may occur include porosity, lack of penetration, lack of fusion, and weld beads. Below, each of these prone-to-occur defects will be analyzed in detail. 3.1 Porosity: Porosity occurs when gases present within the welding pool fail to escape from the metal before it cools and solidifies, remaining thus in the weld metal as pores. This gas may be absorbed by the molten pool from the surrounding environment; it is mostly hydrogen and carbon monoxide. It can also be generated as a result of reactions occurring during welding, for example, if the base material contains excessive sulfur, gases will be produced during the fusion with the weld metal. (1) The main reasons for pores arising during welding are as follows: The surface of the base material or filler metal used in welding is not clean, containing contaminants such as oil and rust, which need to be removed. The welding flux or electrodes were not dried, causing the moisture contained within them to decompose into gas during welding; since this gas had no time to escape, it led to an increase in the number of pores. The low line energy during welding results in a rapid cooling rate of the molten pool, preventing gases from escaping from the weld metal in time. Insufficient deoxidation of the weld metal also increases the formation of oxygen pores. The environmental conditions during welding have an impact; high humidity, such as on rainy or foggy days, can cause the weld to absorb moisture from the surrounding air, thereby increasing the likelihood of defects like pores. (2) The hazards of stomata. Under different standard requirements, there are various specifications regarding the number of pores. However, the formation of pores necessarily reduces the effective cross-sectional area of the weld, causing the weld to become porous and reducing the strength of the joint; it can also lead to production defects such as leaks. Pores can also cause structural defects due to stress concentration, posing a threat to the components. It reduces strength, thereby affecting the service life of the component. Therefore, by analyzing the causes of pores one by one and eliminating the external factors that lead to their formation, it is possible to reduce the occurrence of pore defects. 3.2 Under-welding, lack of fusion 3.2.1 Causes of under-welding: (1) A reduced bevel angle on the test plate, an excessively large root gap or too small assembly clearance, insufficient and too thin spot welding length; during welding, tensile and compressive stresses cause the weld spots at those locations to expand, thereby reducing the gap. Alternatively, it may be caused by using a wire diameter that is too large, preventing the welded metal from reaching the root of the bevel. (2) An excessively fast welding speed or too low welding current results in reduced arc penetration, causing the weld pool to become shallower and the edges of the workpiece to not be fully melted ； Or the arc ignition time during welding on both sides of the groove is too short, resulting in the failure to form a weld pool of the required size. (3) An improper electrode angle or magnetic blow caused by the arc leads to heat dissipation from the arc or its deflection to one side, resulting in lack of penetration in areas where the arc cannot reach. (4) During root pass welding, lack of penetration at the joints occurs most frequently in plate-type weldments. The process leading to this is as follows: after changing the welding electrode, the temperature generated by the arc decreases; there is a significant temperature difference between the already welded portion and the unwelded portion. Additionally, depending on the size of the gap between the test plates, welding is carried out before reaching the required preheating temperature following electrode replacement and arc initiation. As a result, the arc fails to quickly penetrate the root face of the test piece at the joint, leading to a section where there is no penetration—thereby causing a defect. 3.2.2 Measures to prevent under-welding defects: (1) Select an appropriate electrode angle; when performing the root pass, the welding speed must be controlled properly so that the arc can fully melt the weld root. (2) Thoroughly clean the groove weld of oil, rust, and other contaminants. (3) During welding, if it is detected that electrode eccentricity causes arc deviation, the angle of the electrode should be adjusted promptly by swinging it in the direction opposite to the arc deviation, so as to align the arc with the weld pool, or the electrode should be replaced. (4) During welding, attention should be paid to observing the melting process to ensure good fusion. 3.3 Weld tumors (1) Excessively large root edge of the groove, excessive assembly gap ； (2) During flat welding for the root pass, if the welding current is too high and the welding speed is too slow, this can result in a high temperature of the molten edge and a large volume of it; the liquid metal then falls due to its own gravity ； (3) During welding, improper angle of the welder’s torch, etc. Measures to prevent weld beads: (1) Select appropriate bevel dimensions and fitting clearances, and control the size of the molten hole during welding ； (2) Strictly control the current and interlayer temperature ； (3) Select an appropriate electrode angle; when swinging the electrode, move it more quickly in the middle and slightly more slowly on the sides ； (4) Pay attention to observing the state of the molten pool ； (5) When it is observed that many small sparks are ejected from the molten pool, the arc should be extinguished immediately, and welding should resume only after the temperature of the molten pool has dropped slightly.