Easily reduce welding deformation and welding stress

1、 Methods for reducing internal stress 1. Hammering and forging – mechanical method When long cracks and surfacing layers need to be welded continuously from one end to the other end, during the welding repair, knock with a hammer while the weld and surfacing layer are hot, so as to reduce the shrinkage of the weld and reduce the internal stress. When striking, the effect of welding repair is best when the metal temperature is 800 ℃. If the temperature drops, the knocking force also decreases. If the temperature is too low, it is not allowed to knock at about 300 ℃ to avoid cracking. The principle of forging and smelting method is basically the same as the above, but the difference is that the weldment should be completely heated before knocking. 2. Preheating and slow cooling – thermal method This method is to put the workpiece to be welded in the furnace before welding repair and heat it to a certain temperature (100-600 ℃), and prevent the heated workpiece from cooling sharply during welding. The purpose of this treatment is to reduce the difference between the temperature of the repaired part and the temperature of the base metal, so as to reduce the internal stress. The slow cooling method is to heat the welded workpiece to 600 ℃ and put it into the annealing furnace to cool slowly. 3. Law of “breaking first and establishing later” When the cast iron is welded with ordinary carbon steel welding rod, it is easy to produce cracks, and it is uneconomical to use cast iron welding rod. This paper introduces a welding method of “breaking first and then standing up” with carbon steel welding rod: first cut the welding seam with small current, pay attention to only slotting and not cutting through, and then weld while hot. Since the local stress around the crack is eliminated during cutting, no new crack will be generated, and the welding effect is very good. There are three methods to reduce the internal stress during welding. Here are some examples: welding of cast iron pump shell crack. (1) Drill crack stop holes at both ends of the crack（ φ 10mm) to prevent further outward expansion of cracks during welding. (2) A manual polishing machine is used to make a groove at the crack position. The groove top is 8-9mm wide, slightly V-shaped, and 32mm deep (the wall thickness of the pump casing is 40mm), so that the electric welding fluid can be welded. (3) Manual welding is adopted φ 3.2mm special cast iron welding rod is connected reversely by DC electric welding machine with a current of 150A. Intermittent welding is carried out, i.e. each welding length is 15-20mm electric weld, and wait for a moment. In the stop welding gap, when the welding melt solidifies and changes from white hot state to red hot state, hammer the electric weld with a small pointed hammer. The hammering force should be light, the speed should be fast, and the number of times should be more. The weld metal should be thinned and stretched to the surrounding, offset some weld shrinkage and reduce welding stress. This can effectively improve the crack resistance of weld metal (note that the small hammer head must be circular arc with a radius of about 10mm). The welding shall be continued after the welding pool is cooled to dark red and disappears. (4) For long cracks, in order to avoid cracking, it must be repaired by welding in sections. The principle of segmentation is to weld the section that can expand and contract freely first. If it is divided into three sections, the middle section shall be welded first. When this section is cold to dark red, the other section shall be welded immediately, and then the last section shall be welded. (5) Before welding, the weld area shall be preheated first, and thermal insulation shall be conducted after welding to reduce the cooling rate. Preheating and heat preservation can not only improve the crack resistance of weld metal, but also reduce the hardness of the area near the fusion line. 2、 Methods for reducing and preventing deformation during welding repair 1. Preheating method Preheating the weldment before welding can not only reduce the internal stress, but also is a good method to reduce the deformation. 2. Prestressing inverse deformation method The pre added inverse deformation method is to estimate the deformation direction and shrinkage amount after welding repair in advance according to the nature of the metal to be welded. Before welding repair, the workpiece is pre deformed by mechanical method, so that the deformation after welding repair is just offset with the pre deformation. 3. Water cooling method This method is to spray the weldment with cold water to reduce the temperature of the base metal to prevent deformation, or immerse the weldment in the cold water bath to expose the part to be repaired, so that the temperature of the base metal will not rise, and therefore the weldment will not cause deformation. 4. Clamping method This method is to fasten the weldment with a rigid fixture to prevent the weldment from deforming during welding. However, this method will leave internal stress in the weldment. Therefore, this method is mainly applied to the welding of low-carbon steel sheet with good plasticity. 5. Reasonable selection of welding specifications Before welding, reasonable selection of welding specifications has great influence on reducing the deformation of weldments. For example, with the increase of current intensity, the deformation of weldment increases correspondingly. The welding sequence of the welds is of great significance to reduce the deformation of the weldment. For the structural welds, the welds between the two parts connected should be finally welded. For cylindrical plate structure, longitudinal (axial) welds shall be welded first, and then circumferential welds shall be welded. Otherwise, convex deformation or even cracks will be caused in the center of the structure. If the weldment is a metal plate composed of some steel plates, the transverse welds of the steel plates shall be welded first. When a single strip is formed, it can be welded in sections. Each section is welded in the direction opposite to the general direction of welding, that is, the reverse welding method is adopted. In addition, when welding weldments, if conditions permit, fast and multi-layer welding methods shall be used as much as possible. The shorter the interval between each layer, the better the effect.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