One. Methods to reduce internal stress
1. Hammering and forging - mechanical method
When welding and repairing long cracks and surfacing layers, it is necessary to continuously weld one end to the other end. During the welding repair, while the welding seam and surfacing layer are in a hot state, beating with a hand hammer can reduce the Shrinkage of welds and reduction of internal stress. When beating, the best effect is when the welding metal temperature is 800℃. If the temperature drops, the knocking force also decreases. If the temperature is too low, beating is not allowed at around 300°C to avoid cracks. The principle of the forging method is basically the same as the above, the difference is that all the weldments should be heated before beating.
2. Preheating and slow cooling - thermal method
This method is to place the workpiece to be welded in the furnace before welding and heat it to a certain temperature (100-600 °C), and prevent the heated workpiece from cooling rapidly during the welding process. The purpose of this treatment is to reduce the difference between the temperature of the repaired part and the temperature of the base metal, thereby reducing the internal stress. The method of slow cooling is to heat the welded workpiece to 600°C and put it in an annealing furnace to cool it slowly.
3. "Break first and then establish" law
When the cast iron is welded with ordinary carbon steel electrodes, cracks are easy to occur, and it is not economical to use cast iron electrodes. Now introduce a method of "breaking first and then standing" welding with carbon steel electrode: first cut along the weld with a small current, pay attention to only grooving but not cut through, and then weld while hot. Since the local stress around the crack is eliminated during cutting, new cracks will not be generated, and the welding effect is very good.
There are three ways to reduce internal stress during welding. Here is an example: Welding of cast iron pump casing cracks.
(1) Drill anti-cracking holes (φ10mm) at both ends of the crack to prevent the crack from further expanding outward during welding.
(2) Use a manual grinder to make a groove at the position of the crack. The top of the groove is 8-9mm wide, slightly V-shaped, and 32mm deep (the wall thickness of the pump casing is 40mm), so that the welding liquid can be welded.
(3) Welding is manual welding, using special cast iron electrode of φ3.2mm, using DC welding machine, reverse connection, current is 150A, and intermittent welding is performed, that is, each welding length of 15-20mm electric welding seam, stop for a while. In the welding stoppage, when the welding molten metal solidifies, from the white-hot state to the red-hot state, use a small sharp hammer to hit the electric welding seam. Thinning and elongating around, offset some weld shrinkage and reduce welding stress, which can effectively improve the crack resistance of the weld metal (note that the small hammer must be arc-shaped with a radius of about 10mm). After the welding pool cools down until the dark red color disappears, continue welding.
(4) For longer cracks, in order to avoid cracking, it must be repaired by segment welding. The principle of segmenting is to weld the segment that can expand and contract freely. If it is divided into three sections, the middle section should be welded first. When this section cools until the dark red disappears, the other section should be welded immediately, and then the last section will be welded.
(5) Before welding, preheat the weld area first, and keep warm after welding to reduce the cooling rate. Preheating and heat preservation can not only improve the crack resistance of the weld metal, but also help reduce the hardness of the area near the fusion line.
TWO. Methods to reduce and prevent deformation during welding repair
1. Preheating method
Preheating weldments before welding not only reduces internal stress, but is also a good way to reduce deformation.
2. Pre-add anti-deformation method
The pre-deformation method is based on the properties of the metal to be welded, and the direction and shrinkage of the deformation after welding repair are estimated in advance by experience. and pre-deformation offset.
3. Water cooling method
This method is to use cold water to spray the weldment to reduce the temperature of the base metal to prevent deformation, or immerse the weldment in a cold water tank to expose the part that needs to be welded, so that the temperature of the base metal will not rise, so welding. Repair parts will not cause deformation.
4. Clamping method
This method is to fasten the weldment with a rigid fixture to prevent the weldment from being deformed during welding. However, this method will have residual internal stress inside the weldment, therefore, this method is mainly applied to the welding of low carbon steel sheets with good plasticity.
5. Reasonable selection of welding specifications
Before welding, reasonable selection of welding specifications has a great influence on reducing the deformation of weldments. For example, as the current intensity increases, the deformation of the weldment increases accordingly. The welding sequence of the weld is of great significance to reduce the deformation of the weldment. The structural weld should make the weld between the two parts to be connected last. For the cylindrical plate structure, the longitudinal (axial) weld should be welded first, and then the annular weld should be welded, otherwise, the center of the structure will cause bulge deformation or even cracks. If the weldment is a metal plate composed of several steel plates, the transverse welds of the steel plates should be tailored first. When a single strip is formed, segmented welding can be used, and each segment is welded in the opposite direction to the general welding direction. , that is, the reverse welding method is used.
In addition, when welding weldments, if conditions permit, fast and multi-layer welding methods should be used as much as possible. The shorter the interval between layers, the better the effect.
