What is Welding Stress And Deformation For Kids Amusement Equipment?
During the welding process, the internal stress and deformation of the workpiece caused by the uneven heating of the arc heat are temporary.When the workpiece is cooled, the internal stress and deformation that still remain inside the workpiece are called residual internal stress and residual deformation.Generally speaking, welding stress and deformation refer to the residual internal stress of welding and the residual deformation of welding.
Due to the type of welded connector , the thickness and shape of the workpiece, the length and position of the weld, various forms of deformation will occur during welding, which can be roughly divided into longitudinal deformation, transverse deformation, bending deformation, angular deformation, wave deformation and Distortion, etc.
1.The formation of welding deformation and stress Welding deformation and stress are caused by the simultaneous action of many factors.Among them, the main factors of the volume response are:：Uneven temperature distribution on the weldment；Deposited metal shrinkage；Metal structure transformation of welded joints and rigid constraints of weldments.
1）Amusement Equipment The temperature distribution on the weldment is not uniform.Due to the action of the arc, the weldment is locally heated to the melting temperature, and a large temperature gradient is formed between the weld and the base metal.According to the principle of thermal expansion and cold contraction, the object needs to be elongated when heated, and the elongation is different at different temperatures. The high temperature area of the joint requires a large elongation and is hindered, forming a compressive stress.The lower temperature region has a small elongation due to the resistance to the elongation of the high temperature region, resulting in tensile stress.
During the cooling process of the joint, the volume of the molten metal shrinks, and the base metal outside the joint limits its shrinkage, so tensile stress is formed in the weldment in the weld zone, and the base metal near the weld zone bears compressive stress. .
The weld seam and the near seam area almost lose the yield strength at high temperature, and plastic deformation will occur under the action of stress. After cooling, residual stress and residual deformation will be formed in the weldment.
2）The deposited metal shrinks.During the solidification and subsequent cooling of the weld metal, the volume shrinks, causing deformation and stress in the weldment. The magnitude of the deformation and stress depends on the shrinkage of the deposited metal, which in turn depends on the melting the amount of metal.For example, the angle deformation of the V-shaped groove is due to the large amount of deposited metal on the upper part of the weld and the large amount of shrinkage, while the lower cross-sectional area of the weld is small, the amount of deposited metal is small, the amount of shrinkage is small, and the upper and lower shrinkage is inconsistent.
3）Metal structure transformation.Under the action of the welding thermal cycle, the internal microstructure of the metal changes, and the density of various structures is different, which is accompanied by a change in volume, and an internal stress called structural stress appears. After cooling from high-temperature austenite (relative density 0.1275), it transforms into martensite (relative density 0.1310), with a volume change of nearly 10%.
4）Weldment rigid constraints.If the rigidity of the weldment itself is very large, or the welding is performed under tightening conditions, the restraint conditions limit the free elongation and shortening of the weldment under the action of thermal cycles, which can control the welding deformation, but form a relatively large amount of deformation in the weldment by the large internal stress.
Welding deformation and stress are also related to welding method and welding process.For example, Amusement Equipment during gas welding, the heat source is not concentrated, and the area of the heat affected zone on the weldment is larger than that of arc welding, so the welding deformation and stress are also larger.Another example is arc welding, high current or slow welding speed lead to an increase in the heat affected zone, and the resulting welding deformation and stress also increase.
2.Welding stress control measures Residual internal stress in the weldment is inevitable, but some measures can be found to effectively control it according to its generation mechanism and law, so as to minimize the degree of harm.
There are two basic points for controlling internal stress：Make the heat on the weldment as uniform as possible；Minimize restrictions on free shrinkage of welds.The commonly used process measures are as follows:
（1）Reasonable assembly and welding sequence. Try to make the welding seam shrink more freely in the sequence of assembly and welding.；Take measures in the design of welded structures, for example, symmetrically arrange welds, avoid closed welds, etc.；Heating the part that hinders the free shrinkage of the welded joint to make it expand and contract synchronously with the weld, this method is called “strain reduction method”, which can effectively control the welding stress.
（2）The greater the temperature difference of each part of the workpiece to be preheated before welding, the faster the cooling rate of the weld, and the greater the residual stress of the welded joint.Preheating can not only reduce the temperature difference of each part of the workpiece, but also slow down the cooling rate, so it is one of the powerful measures to reduce the residual stress of welding.The preheating can be partial preheating or overall preheating. For workpiece with high rigidity and large thickness, it should be preheated as a whole, so that the effect of reducing residual stress is better.
（3）Methods of eliminating welding stress There are three main methods for eliminating welding stress: heat treatment method, mechanical method and vibration method.
1）Heat treatment method.Post-weld heat treatment is an effective method to eliminate residual stress, and it is also a widely used method. It can be divided into overall heat treatment and local heat treatment.Generally, the workpiece to be welded is heated below the A1 line, kept at an even temperature, and then slowly cooled to achieve the purpose of eliminating residual stress.
2）Mechanical method.Applying external force mechanically to expand the cooled weld metal to achieve the purpose of stress relief, this method is called mechanical stress relief, such as hammering the weld.
3）Vibration method.Vibrating the entire component at a low frequency to achieve the purpose of stress relief, which is often used in general steel structure components to eliminate stress.
