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Matters needing attention in welding process

2019-06-20

In addition, welding is a local rapid heating and cooling process. The welding zone can not expand and shrink freely due to the constraints of the workpiece body around it. After cooling, welding stress and deformation will occur in the weldment. Important products need to eliminate welding stress and correct welding deformation after welding.


Modern welding technology has been able to weld seams without internal and external defects, with mechanical properties equal to or even higher than those of the joints. The positions of welded bodies in space are called welded joints. The strength of welded joints is not only affected by the quality of welds, but also related to their geometry, size, stress and working conditions. The basic forms of joints are butt joint, lap joint, T-joint (orthogonal joint) and corner joint.


The cross-sectional shape of butt joint weld depends on the thickness of welded body before welding and the groove form of both sides. When welding thicker steel plates, various shapes of grooves are made at the edge for penetration, so that welding rods or wires can be easily fed in. The groove forms are one-sided groove and two-sided groove. When choosing groove form, besides guaranteeing penetration, factors such as convenient welding, less filling metal, small welding deformation and low groove processing cost should also be considered.


When two steel plates with different thickness are butted, in order to avoid serious stress concentration caused by sharp change of section, thicker edges are often thinned gradually to equal thickness at both edges. The static strength and fatigue strength of butt joint are higher than those of other joints. The butt joints are usually preferred for joints working under alternating, impact loads or in low temperature and high pressure vessels.


Pre-welding preparation of lap joints is simple, assembly is convenient, welding deformation and residual stress are small, so it is often used in site installation of joints and unimportant structures. Generally speaking, lap joints are not suitable for working under alternating load, corrosive medium, high temperature or low temperature.


The use of T-joint and corner joint is usually due to structural requirements. The working characteristics of incomplete fillet weld on T-joint are similar to those of overlap joint. When the weld is perpendicular to the direction of external force, it will become a front fillet weld. At this time, the surface shape of the weld will cause different degrees of stress concentration. The stress of the penetrating fillet weld is similar to that of the joint.


The bearing capacity of corner joint is low, and it is not used alone. It can be improved only when penetration or fillet welds are available both inside and outside. It is mostly used at the corner of closed structure.


Welded products are lighter than riveted parts, castings and forgings, which can reduce self-weight and save energy for transportation. Welding has good sealing property and is suitable for manufacturing all kinds of containers. By developing joint processing technology and combining welding with forging and casting, large-scale, economical and reasonable cast-welding structure and Forging-welding structure can be made, with high economic benefits. Welding technology can effectively utilize materials. Welding structure can use different materials with different properties in different parts, give full play to the advantages of various materials, and achieve economic and high quality. Welding has become an indispensable and increasingly important processing method in modern industry.


In the modern metal processing, welding technology develops later than casting and forging technology, but it develops rapidly. The weight of welded structure accounts for 45% of steel output, and the proportion of welded structure of aluminium and aluminium alloy is increasing.


In the future, on the one hand, new welding methods, welding equipment and welding materials should be developed to further improve the welding quality and safety, such as improving the existing welding energy sources such as arc, plasma arc, electron beam and laser, and using electronic technology and control technology to improve the technological performance of arc and develop reliable and lightweight arc tracking method.


On the other hand, it is necessary to improve the level of welding mechanization and automation, such as the realization of program control and digital control of welding machines; the development of special welding machines for automation of the whole process from preparation process, welding to quality monitoring; and the promotion and expansion of numerical control welding manipulators and welding robots in automatic welding production lines can improve the level of welding production and improve welding hygiene and safety conditions. (Source: Welding Information)


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