2.3 Transformation of metal structure Under the action of welding thermal cycle, the internal microstructure of the metal changes, and the density of various tissues is different, which is accompanied by the change of volume, and an internal stress called tissue stress appears. For example, the hardened steel is cooled by high temperature austenite (density 0.1275) and transformed into martensite (density of 0.1310) under the action of welding thermal cycle, and the volume changes by nearly 10%. 2.4 The rigidity of the weldment is constrained if the weldment itself The rigidity is very large or the welding is carried out under the tightening condition. The restraining condition limits the free elongation and shortening of the weldment under the thermal cycle, which can control the welding deformation, but the internal stress is formed in the weldment. .
Welding deformation and stress are also related to the welding method and welding process parameters. In the case of gas welding, the heat source is not concentrated, and the area of ​​the heat affected zone on the weldment is larger than that of the arc weld, so the resulting weld deformation and stress are also large. In the case of arc welding, a large current or a slow welding speed results in an increase in the heat-affected zone and an increase in weld deformation and stress.
3. Control measures of welding stress It is inevitable that internal stress remains in the welded part, but some measures can be found according to the mechanism and law to effectively control it to minimize the degree of damage.
3.1 The method of controlling internal stress has two basic requirements: it is to make the heat on the weldment as uniform as possible and to minimize the restriction on the free shrinkage of the weld. The usual process measures are as follows:
3.1.1 Reasonable assembly and welding sequence is mainly to make the weld seam shrink freely in the order of assembly and welding, so as to effectively control the welding stress.
3.1.2 Pre-heating pump technology before welding, pump technology, pump and valve technology, pump CFD, pump numerical simulation, the greater the temperature difference between the parts of the workpiece being welded, the faster the cooling speed of the weld, the greater the residual stress of the welded joint. Preheating can reduce the temperature difference between various parts of the workpiece and slow down the cooling rate, so it is one of the powerful measures to reduce the residual stress of the welding.
Preheating can be divided into local preheating or overall preheating. For workpieces with large rigidity and large thickness, the whole should be preheated, so that the effect of reducing residual stress is better.
In addition to the above methods of controlling stress, measures are also taken in the design of the welded structure, such as symmetrically arranging welds and avoiding closed welds. And heating the part that hinders the free shrinkage of the welded joint to make it stretch synchronously with the weld. This method is called "subtraction method".
4. Methods for eliminating welding stress The main methods for eliminating welding stress are: heat treatment method, mechanical method, and vibration method.
Post-weld heat treatment is an effective method to eliminate residual stress and is a widely used method. It can be divided into integral heat treatment and local heat treatment.
4.1 Generally, the workpiece to be welded is heated below the A1 line, and the temperature is kept warm, and the cooling is slowly performed to achieve the residual stress elimination. For example, the temperature of post-weld heat treatment of Q235B and 16MnR materials is generally 625±25°C.
4.2 Mechanical method, mechanically applied external force to extend the welded metal after cooling to achieve the purpose of eliminating stress, this method is called mechanical method to eliminate stress. Such as hammering the weld; pressing the weld on the coiler; controlling the overload of the weld structure is a mechanical method to eliminate stress.
4.3 Vibration pump technology, pump technology, pump valve technology, pump CFD, pump numerical simulation, vibration of the entire component at low frequency to achieve the purpose of eliminating stress. Generally, steel structural parts need to be relieved of stress, which is often used for welding stress and deformation.
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