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The inspector should be aware of the repair stress and prevention of defects
Release time：2023-09-26
1、 Methods for reducing internal stress
1. Hammering and forging - mechanical method
When repairing long cracks and surfacing layers, it is necessary to continuously weld from one end to the other end. During the welding process, while the weld seam and surfacing layer are in a hot state, use a hammer to strike them, which can reduce the shrinkage of the weld seam and reduce internal stress. When striking, the effect is good when the welding repair metal temperature is 800 ℃. If the temperature drops, the tapping force also decreases. If the temperature is too low, it is not allowed to strike around 300 ℃ to avoid cracking. The principle of forging and smelting method is basically consistent with the above, but the difference is to heat all the welded parts before striking them.
2. Preheating and slow cooling - thermal method
This method involves placing the workpiece to be welded in the furnace before welding repair, heating it to a certain temperature (100-600 ℃), and preventing rapid cooling of the heated workpiece during the welding process. The purpose of this treatment is to reduce the difference between the temperature of the welding repair section and the temperature of the base metal, thereby reducing internal stress. The method of slow cooling is to heat the welded workpiece to 600 ℃ and slowly cool it in an annealing furnace.
3. The method of 'breaking first and then standing'
When welding cast iron parts with ordinary carbon steel electrodes, cracks are easily generated, and using cast iron electrodes is not economical. This article introduces a method of using carbon steel welding rods for welding, which is "first break and then stand": first cut along the weld seam with a small current, pay attention to only grooving without cutting through, and then weld while it is hot. Due to the elimination of local stress around the cracks during cutting, no new cracks will be generated, and the welding effect is very good.
There are three methods to reduce internal stress during the welding process, as follows: welding of cracks in the cast iron pump casing.
(1) Drill stop holes at both ends of the crack（ φ 10mm) to prevent further outward expansion of cracks during welding.
(2) Use a manual polishing machine to make a groove at the crack location, with a width of 8-9mm at the top of the groove, slightly V-shaped, and a depth of 32mm (the wall thickness of this pump casing is 40mm), allowing for the welding of welding fluid.
(3) Welding is manual welding, using φ 3.2mm special cast iron welding electrode, using a DC welding machine, reverse connected, with a current of 150A, is used for intermittent welding, that is, each welding seam is 15-20mm long, with a pause for a moment. In the gap between welding stops, when the welding melt solidifies and changes from a white hot state to a red hot state, use a small pointed hammer to hammer the electric welding seam. The hammer force should be light, the speed should be fast, and the number of times should be more, so as to reduce the thickness of the welding seam metal and extend it around, offset some welding seam shrinkage, and reduce welding stress. This can effectively improve the crack resistance of the welding seam metal (note that the small hammer head must be a circular arc with a radius of about 10mm). Wait until the welding pool cools down until the dark red color disappears before continuing welding.
(4) For longer cracks, segmented welding repair is necessary to avoid cracking. The principle of segmentation is to first weld the section that can be freely retracted. If divided into three sections, the middle section should be welded first. When this section cools to the point where the dark red color disappears, the other section should be welded immediately, and then the last section should be welded.
(5) Before welding, preheat the weld area and maintain insulation after welding to reduce the cooling rate. Preheating and insulation can not only improve the crack resistance of the weld metal, but also help reduce the hardness of the area near the fusion line.
2、 Methods to reduce and prevent deformation during welding
1. Preheating method
Preheating the welded parts before welding can not only reduce internal stress, but also be a good method to reduce deformation.
2. Pre added anti deformation method
The pre added reverse deformation method is based on the properties of the welded metal and estimates the direction and shrinkage of deformation after welding repair based on experience. Before welding repair, the workpiece is pre deformed using mechanical methods to make the deformation after welding repair exactly offset the pre deformation.
3. Water cooling method
This method is to use cold water spray welding to prevent deformation by reducing the temperature of the base metal. Alternatively, the welding part can be immersed in a cold water bath to expose the part that needs to be welded, so that the temperature of the base metal does not rise, and therefore the welded part will not cause deformation.
4. Clamping method
This method is to use a fixture with greater rigidity to fasten the welded part, preventing deformation of the welded part during welding. However, this method will residual internal stress inside the weldment, so it is mainly applied to the welding of low-carbon steel sheets with good plasticity.
5. Reasonable selection of welding specifications
Reasonable selection of welding specifications before welding has a significant impact on reducing deformation of the welded part. As the current intensity increases, the deformation of the weldment correspondingly increases. The welding sequence of the weld seam is of great significance in reducing the deformation of the weldment. The structural weld seam should ensure that the weld seam between the two connected components is finally welded. For columnar plate structures, longitudinal (axial) welds should be welded first, followed by circumferential welds. Otherwise, it may cause protrusion deformation or even cracks in the center of the structure. If the weldment is a metal plate composed of some steel plates, the transverse weld of the steel plate shall be welded first. When a single Flat noodles 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 used.
In addition, when welding weldments, if conditions permit, quick and multi-layer welding methods should be used as much as possible. The shorter the interval time between each layer, the better the effect.
1. Hammering and forging - mechanical method
When repairing long cracks and surfacing layers, it is necessary to continuously weld from one end to the other end. During the welding process, while the weld seam and surfacing layer are in a hot state, use a hammer to strike them, which can reduce the shrinkage of the weld seam and reduce internal stress. When striking, the effect is good when the welding repair metal temperature is 800 ℃. If the temperature drops, the tapping force also decreases. If the temperature is too low, it is not allowed to strike around 300 ℃ to avoid cracking. The principle of forging and smelting method is basically consistent with the above, but the difference is to heat all the welded parts before striking them.
2. Preheating and slow cooling - thermal method
This method involves placing the workpiece to be welded in the furnace before welding repair, heating it to a certain temperature (100-600 ℃), and preventing rapid cooling of the heated workpiece during the welding process. The purpose of this treatment is to reduce the difference between the temperature of the welding repair section and the temperature of the base metal, thereby reducing internal stress. The method of slow cooling is to heat the welded workpiece to 600 ℃ and slowly cool it in an annealing furnace.
3. The method of 'breaking first and then standing'
When welding cast iron parts with ordinary carbon steel electrodes, cracks are easily generated, and using cast iron electrodes is not economical. This article introduces a method of using carbon steel welding rods for welding, which is "first break and then stand": first cut along the weld seam with a small current, pay attention to only grooving without cutting through, and then weld while it is hot. Due to the elimination of local stress around the cracks during cutting, no new cracks will be generated, and the welding effect is very good.
There are three methods to reduce internal stress during the welding process, as follows: welding of cracks in the cast iron pump casing.
(1) Drill stop holes at both ends of the crack（ φ 10mm) to prevent further outward expansion of cracks during welding.
(2) Use a manual polishing machine to make a groove at the crack location, with a width of 8-9mm at the top of the groove, slightly V-shaped, and a depth of 32mm (the wall thickness of this pump casing is 40mm), allowing for the welding of welding fluid.
(3) Welding is manual welding, using φ 3.2mm special cast iron welding electrode, using a DC welding machine, reverse connected, with a current of 150A, is used for intermittent welding, that is, each welding seam is 15-20mm long, with a pause for a moment. In the gap between welding stops, when the welding melt solidifies and changes from a white hot state to a red hot state, use a small pointed hammer to hammer the electric welding seam. The hammer force should be light, the speed should be fast, and the number of times should be more, so as to reduce the thickness of the welding seam metal and extend it around, offset some welding seam shrinkage, and reduce welding stress. This can effectively improve the crack resistance of the welding seam metal (note that the small hammer head must be a circular arc with a radius of about 10mm). Wait until the welding pool cools down until the dark red color disappears before continuing welding.
(4) For longer cracks, segmented welding repair is necessary to avoid cracking. The principle of segmentation is to first weld the section that can be freely retracted. If divided into three sections, the middle section should be welded first. When this section cools to the point where the dark red color disappears, the other section should be welded immediately, and then the last section should be welded.
(5) Before welding, preheat the weld area and maintain insulation after welding to reduce the cooling rate. Preheating and insulation can not only improve the crack resistance of the weld metal, but also help reduce the hardness of the area near the fusion line.
2、 Methods to reduce and prevent deformation during welding
1. Preheating method
Preheating the welded parts before welding can not only reduce internal stress, but also be a good method to reduce deformation.
2. Pre added anti deformation method
The pre added reverse deformation method is based on the properties of the welded metal and estimates the direction and shrinkage of deformation after welding repair based on experience. Before welding repair, the workpiece is pre deformed using mechanical methods to make the deformation after welding repair exactly offset the pre deformation.
3. Water cooling method
This method is to use cold water spray welding to prevent deformation by reducing the temperature of the base metal. Alternatively, the welding part can be immersed in a cold water bath to expose the part that needs to be welded, so that the temperature of the base metal does not rise, and therefore the welded part will not cause deformation.
4. Clamping method
This method is to use a fixture with greater rigidity to fasten the welded part, preventing deformation of the welded part during welding. However, this method will residual internal stress inside the weldment, so it is mainly applied to the welding of low-carbon steel sheets with good plasticity.
5. Reasonable selection of welding specifications
Reasonable selection of welding specifications before welding has a significant impact on reducing deformation of the welded part. As the current intensity increases, the deformation of the weldment correspondingly increases. The welding sequence of the weld seam is of great significance in reducing the deformation of the weldment. The structural weld seam should ensure that the weld seam between the two connected components is finally welded. For columnar plate structures, longitudinal (axial) welds should be welded first, followed by circumferential welds. Otherwise, it may cause protrusion deformation or even cracks in the center of the structure. If the weldment is a metal plate composed of some steel plates, the transverse weld of the steel plate shall be welded first. When a single Flat noodles 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 used.
In addition, when welding weldments, if conditions permit, quick and multi-layer welding methods should be used as much as possible. The shorter the interval time between each layer, the better the effect.
