Summary: Large welded structures are becoming more and more widely used in the field of industrial production today, and higher requirements for their dimensional accuracy and geometric tolerance are put forward. In this article, the mechanism of welding deformation, causes and measures of welding deformation control are discussed, and 9 measures on reducing the welding deformation and how to reduce the welding stress and the correction of welding deformation after welding are put forward.

Key words: welding deformation, mechanism, control measures 

1 Mechanism of welding deformation

As we all know, welding process is to use arc heat, physical heat, chemical heat and other heat energy to melt mother metal and welding material to form welding pool, the solidification process of molten pool transforms from liquid phase to solid phase, which is actually a metallurgical process. Although welding solidification and casting solidification all undergo crystallization nucleation and growth process, the former is non-equilibrium solidification, the latter is equilibrium solidification, and the two are very different.
(1) The weld pool is small and the cooling speed is fast. Its average cooling speed is as high as 100 C/S, about 104 times of casting, therefore, defects such as pores, cracks, inclusions and segregation are easily formed in weld metal.

(2) Molten metal in the molten pool is overheated, and the temperature gradient of molten metal in the center and edge of the molten pool is 103-104 times higher than that in casting.

(3) The molten pool crystallized in the motion state, and the crystallization front moved with the heat source synchronously. The crystal spindle reversed the direction of heat dissipation and grew to the center of the heat source, and stopped growing in the center of the weld, this area is an easily aggregated area of impurities.

(4) There are a lot of ready-made surfaces on the fusion line of the parent material, and after the nucleation of the semi melting grain, the exogenous crystallization is formed, which shows the characteristics of the heterogeneous nucleation of the weld pool.

These characteristics of the welding process make the structure in the process of welding, that when it is heated locally, it cannot fully extend due to the constraints of its surrounding members, resulting in compressive stress; When it is cooled, tensile stress is generated due to the contraction of the weld, resulting in bending deformation of the welding. When the welding finished, both internal compressive stress and tensile stress exist, both the existence of elastic deformation and plastic deformation exist. Compressive stress is produced inside the weld due to weld shrinkage, and tensile stress is produced due to tensile stress on the base metal. It can be said that the stress and strain are inevitable. Therefore, we should use this mechanism to control the influence of the welding deformation from two aspects of structure design and welding process factors.

2  Questions that should be paid attention to in structural design

(1)  Minimize the number of welds seams in the case of structural permission.

The fewer of welding seams, and the smaller of input heat needed, and the welding deformation will be smaller.

Take the base of over head crane for example manufactured by our factory, before upper and lower surface of the base is fabricated by 4-6 pieces of steel plate according to its shape, the welding deformation is large and difficult to correct, later, the upper and lower bottom was replaced with the whole piece of plate and thus more than 10 welding seams have been reduced, after welding, the overall deformation is very small, and it also save working hours and welding materials, and the effect is very significant. The steel welded box, iff the thickness of the box body steel plate is below 10mm, the steel plate can be bent into a certain shape before welding, by this way, it will not only reduce the number of weld, make the weld symmetrical and beautiful appearance, but also improve the stiffness of components and reduce the deformation of components. The oil tanks and water tanks fabricated by our factory are previously welded with 6 pieces of steel plates, it needs 6 welding seams, which are of poor rigidity and large deformation. And later, we change the process, the one piece of bent part instead of the front and back plate and both ends plate to cover, by this way, only 4 pieces of plates are needed and 4 welding seams is required, and the rigidity is good and the welding deformation is small

(2)  Design the form and size of the weld

The butt welds are better than the fillet welds; therefore, butt welds are preferred when possible. We know that the larger the weld size, the more welding materials to be filled, the greater the input heat needed, and the greater the internal stress produced when the weld is contracted, and the greater the welding deformation. Therefore, the weld size should be minimized as far as the strength requirements are met.

(3)Trying to arrange welding seams symmetrical

During the welding, local heating and rapid cooling cause internal compressive stress and tensile stress respectively, and further it will cause welding deformation. When the weld louout is symmetrical, the stress and strain produced in cooling can cancel each other, and the smaller deformation can be obtained on the whole. The joint position should be arranged in a place where the stiffness of the component is larger. Under the same conditions, the greater the stiffness of the weld area, the smaller the welding deformation, which is beneficial to the overall deformation control.

(4) Avoiding centralization and overlap of weld position

When the welding is concentrated and overlapped, the heat affected zone affects each other, which not only makes the metal of the heat affected zone become coarser because of repeated heating, but also reduces the mechanical properties and increases the deformation and affects the size accuracy of the workpiece. Therefore, the weld should be staggered as far as possible. The distance between the welds should be above 100mm.

3 Attention on design of welding procedure

(1)  The accuracy of material preparation is directly related to the dimensional accuracy of components, and further directly affects the accuracy of component assembly and welding. When the size of the prepared material is too short, the welding gap is larger, the mount of welding materials needed is more, the heat of the welded parts get more, so eventually the welding deformation is relatively large.  Usually the gap between the welded parts should be less than 3mm. If the profile is cut by hot cutting, it is better to leave a little allowance and to cut the allowance after cutting.

(2)  The precision of the composite jig is the key to ensure the accuracy of welding parts size. If the deviation of the jig for assembly is too large, the guarantee of the size accuracy of the structure is ineffective. For large structural parts, it is necessary not only to control the size deviation of the longitudinal and horizontal axes, but also to control the diagonal deviation of the key sections. It is usually required that the linear size deviation of the large assembly welding fixture is less than 2/1000. In addition, the flatness of the platform for assembly should be controlled, the flat surface of the platform is too large and the workpiece will be deformed after the workpiece is pressed tight. If the flatness deviation of the platform is too large, and when pressing workpiece against the platform, it will cause the deformation of the workpiece. Usually, the flatness of the platform should be within the range of 3mm.

(3) The select of the proper welding method

As a common welding method, electrode arc welding is widely used in production because of its convenience and adaptability, but the disadvantage is that the efficiency is low. Because of the large input current and high deposition efficiency, the submerged arc automatic welding is widely used in the large workpiece with long weld continuous welding, but it is not suitable for the welding of the short welding seams, such as the derrick and the drilling platform.

CO2 gas shielded welding is popularized in more and more fields of welding because of its high current density, high welding wire penetration, small heat impact zone, small welding deformation, low welding cost, high production efficiency and so on. At present, the problem that needs to be solved is a large amount of welding spatter. When the purity of CO2 gas is less than 99.5%, it is easy to produce a large number of holes in the weld, which can be solved by using the high purity CO2 gas, using the flux cored wire, and using the mixed protective gas.

(4) Right selection of welding parameters and welding sequence.

Generally, the size of welding seam for large steel structure is bigger, the welding material to be filled needs more, and the input heat needs more too, as result the deformation of the welding parts is relatively large, and the defects in the weld seam occur often. Therefore, the welding deformation can be controlled by welding technology such as small diameter welding rod (wire), small current, large wire speed, multi pass welding and so on, but it should be noted that too small welding current can lead to defects such as incomplete penetration. The welding position should be selected in the place where the rigidity is large and the welding deformation is small so as to locate the welding accurately. When the welding sequence is determined, the symmetrical structure should be welded on both sides of the same parameters at the same time so that the welding thermal deformation can be offset to one another, and the purpose of reducing the welding deformation is to be achieved. For example, the beam of the dump truck fabricated by our factory is welded I-steel structure, we use special assembly and welding jig to position the workpiece by pressing against on the platform, and two welders use the same process parameters at the same time to jump welding at the same time. Because of the large weld size, multi-layer and multi pass welding is adopted, and the welding deformation is effectively controlled by the above technical measures. The flatness of workpiece with size more than ten meters and the parallelism of the upper and lower wing plates are controlled within the requirement of the drawing.

(5) Use of special welding jig

The larger the workpiece size is, the larger the accumulated value of the overall welding deformation is, therefore, the large welding steel structure are usually welded by special welding and assembly jig, and the workpiece is pressed tight and fixed avoid after positioning to avoid assembly and welding in the free state, and make the workpiece always is in the best welding position in the tooling, so that the workpiece have smallest deformation.