Methods to reduce welding deformation and welding stress

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I. Methods to reduce internal stress 1. Hammering and forging – mechanical methods: When repairing longer cracks or applying weld overlays, and it is necessary to weld from one end to the other continuously, it is possible to use a hammer while the weld and the overlay are still hot during the welding process. This approach helps to reduce weld contraction and internal stress. When hammering, the best results are achieved when the temperature of the metal to be welded is 800°C. If the temperature drops, the striking force also decreases. The temperature must be high enough; hitting the material is not allowed at around 300°C to prevent cracks from forming. The principle of the forging method is basically the same as mentioned above, with the difference being that the welded parts must be heated completely before being hammered. 2. Preheating and slow cooling – thermal method: In this method, the workpiece to be welded is placed in a furnace before welding, heated to a certain temperature (100–600°C), and it is necessary to prevent the heated workpiece from cooling down rapidly during the welding process. The purpose of this approach is to reduce the temperature difference between the welded area and the base metal, thereby minimizing internal stresses. The method of slow cooling is to heat the welded workpiece to 600°C and then place it in an annealing furnace to cool down slowly. 3. The “break first, then rebuild” method: When welding cast iron parts with ordinary carbon steel electrodes, cracks tend to form easily, while using cast iron electrodes is not economical. Here is an introduction to a method of welding using carbon steel electrodes based on the principle of \"breaking first and then rebuilding\": first, cut along the weld seam with a low current, making sure to create only grooves without cutting through completely, and then weld while the area is still hot. Since the local stress around the cracks is eliminated during cutting, no new cracks are formed, resulting in excellent welding quality. There are the above three methods to reduce internal stress during welding; examples are given as follows: Welding of cracks in cast iron pump casings. (1) Drill crack-stopping holes (φ10mm) at both ends of the crack to prevent further expansion of the crack during welding. (2) Use a manual grinder to create a groove at the location of the crack; the top width of the groove should be 8–9 mm, with a slightly V-shaped profile, and its depth should be 32 mm (the wall thickness of this pump’s casing is 40 mm), so that welding flux can be inserted into it. (3) The welding is done manually, using a special φ3.2mm cast iron welding electrode and a DC welding machine set in reverse polarity; the current used is 150A. Intermittent welding is employed, meaning that after welding 15–20 mm of weld metal, the process is paused for a short while. During the welding pause, once the weld metal has solidified and transitions from a white-hot state to a red-hot state, a small hammer is used to strike the weld. The force applied should be light, the speed high, and the number of strikes numerous, in order to thin out the weld metal and cause it to stretch outward. This helps to counteract some of the welding contraction and reduce welding stress, thereby effectively improving the crack resistance of the weld metal. (Note that the hammer head should have an arc shape with a radius of around 10 mm.) Continue welding only after the weld pool has cooled down and the dark red color has disappeared. (4) For longer cracks, in order to prevent further cracking, welding repairs must be carried out in sections. The principle for segmentation is to weld the section that can stretch and contract freely first. If divided into three sections, the middle section should be welded first; once this section has cooled down to the point where the dark red color disappears, the next section should be welded immediately, and finally the last section. (5) Before welding, preheat the weld area and maintain heat after welding in order to reduce the cooling rate. Preheating and heat retention not only improve the crack resistance of the weld metal but also help to reduce the hardness in the area near the fusion line. II. Methods to reduce and prevent deformation during welding repairs 1. Preheating method Preheating the welded parts before welding not only reduces internal stresses but is also an effective way to minimize deformation. 2. Pre-deformation method: The pre-deformation method involves estimating, based on the properties of the metal to be welded, the direction and amount of deformation that will occur after welding, using experience. Before welding, the workpiece is mechanically deformed in such a way that the deformation that occurs after welding cancels out the pre-deformation. 3. Water cooling method: This method involves using cold water to spray the workpiece, thereby reducing the temperature of the base metal and preventing deformation. The workpiece can also be submerged in a tank of cold water, with only the area that needs repair exposed; this way, the temperature of the base metal does not rise, and thus no deformation occurs in the repaired area. 4. Clamping method: This method involves using fixtures with high rigidity to secure the welded parts, thereby preventing deformation of those parts during welding. However, this method leaves residual internal stresses within the welded joint; therefore, it is mainly used for welding thin sheets of low-carbon steel with good plasticity. 5. Selecting appropriate welding parameters – Choosing the right welding parameters before welding has a significant impact on reducing deformation in the welded parts. For example, as the current intensity increases, the deformation of the welded joint increases accordingly. The welding sequence of welds is of great importance in reducing deformation in the welded parts; for structural welds, the welds between the two components to be joined should be welded last. For columnar plate structures, the longitudinal (axial) welds should be welded first, followed by the circumferential welds; otherwise, it may cause bulging deformation or even cracks in the center of the structure. If the welded piece is a metal plate made up of several steel sheets, the transverse welds between these sheets should be welded first. Once a single sheet is formed, welding can be carried out in segments, with each segment being welded in a direction opposite to the overall welding direction – that is, using the reverse welding method. Furthermore, when welding the workpieces, fast and multi-layer welding methods should be used as much as possible if conditions permit; the shorter the interval between layers, the better the result.