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Welding Deformation Of Steel Structure

Mar 08, 2018

Welding of steel structure welded steel structure is widely used, and full penetration welding is used for some important welds.In the process of local heating and melting during metal welding, the temperature of the metal in the heating zone is very different from that of the surrounding parent material, resulting in instantaneous stress in the welding process.

After cooling to the original temperature, the tensile stress zone of the whole joint area and the near seam area is in balance with the value of the mother material in the compressive stress zone, resulting in the welding residual stress of the structure itself.

At this point, the welding joint structure takes many forms of deformation under the action of welding stress.The existence and deformation of residual stresses are mutually transformed, and it is not difficult to find a way to prevent and correct the deformation. 

1. Forms and causes of welding deformation:

The deformation of steel structure after welding can be divided into two kinds: the deformation of the whole structure and the deformation of the structure.The deformation of the whole structure includes the longitudinal and transverse shortening and bending of the structure.The local deformation is convex, wavy, angular deformation and so on. 

1.1 common basic forms of deformation.

The basic forms of common welding deformation are as follows: the length of the plate groove is shortened (longitudinal shrinkage) and the width is narrow (transverse shrinkage);The Angle deformation after butt welding of plate groove;

The distortion of the angular deformation along the longitudinal direction of the component along the longitudinal axis of the component and the longitudinal contraction of the component flange and the web is distorted.

The wave distortion formed by the buckling of the plate surface due to instability due to the instability of the pressure zone of the laminates.Due to the longitudinal and transverse contraction of the weld, the integral bending of the component is caused by the asymmetry of the neutral axis of the component, and this deformation is bent.

1.2 reasons for welding deformation:

In the welding process, the partial and uneven heating of welding parts is the cause of welding stress and deformation.Near the welding seam and the weld metal is heated area expansion, due to a cold metal stop this expansion, occurs in welding area compression stress and plastic shrinkage deformation, has a different degree of transverse and longitudinal shrinkage.Due to the contraction of these two directions, various deformation of welding structure is caused.

Ii. Factors affecting the deformation of welding structure:

There are many factors affecting the welding deformation, and sometimes the same factors have opposite effect on longitudinal deformation, transverse deformation and angular deformation.It is necessary to take reasonable measures to control the deformation of various factors.Otherwise it is difficult to achieve the desired effect.

1) the influence of the cross-sectional area of the weld: the cross-sectional area refers to the metal area within the range of the fuse line, and the larger the weld area is, the larger the plastic deformation caused by shrinkage during cooling. 

2) the influence of the welding heat input: in general, the large heat input, large range of heat of high temperature and cooling speed slow, make joint plastic deformation area increases, both of the vertical, horizontal, or Angle of the influence of deformation are increased.However, when the thermal input is increased to a certain extent, the temperature of the whole plate is close to the surface, so the Angle deformation will not increase but decrease even if the thermal input continues to increase.

3) influence of preheating and interlayer temperature of the workpiece: the higher the preheating temperature and the temperature between the layers, the higher the heat input, the slower the cooling rate and the increase of shrinkage deformation. 

4) the influence of welding methods: several methods commonly used in construction steel structure welding, besides electroslag welding, submerged arc welding heat input, the largest in the case of other conditions such as weld area of the same, the biggest contraction deformation.The manual arc welding heat input center, the shrinkage deformation is smaller than the submerged arc welding.CO2 gas shielded welding heat input is minimal, and the contraction deformation response is minimal.

5) influence of welding position on deformation: due to the asymmetry of the position of the weld in the structure and the asymmetric position of the welding seam, various deformation will be caused.

6) the effect of rigid structure on welding deformation: the rigidity of the structure depends mainly on the shape of the structure and the size of its section, the structure with small rigidity and large welding deformation;Large rigid structure, less deformation after welding.

7) the influence of assembly and welding specifications on welding deformation: due to the different assembly methods adopted, the deformation of the structure also has an effect.The whole assembly is finished and welded, and its deformation is generally less than that of side assembly.

In engineering welding time, due to various conditions, and all kinds of factors of welding deformation law of residual is more complex, the understanding of the factors to separate effects on engineering the comprehensive analysis of the specific situation in details.

Iii. Measures to prevent and reduce structural deformation:

1) reduce the cross-sectional area of the weld: as far as possible, the smaller groove size (Angle and clearance) should be adopted as far as possible.

2) under the yield strength of 345MPa, the hardened steel is adopted with a smaller thermal input, and the preheating and interlayer temperature is not preheated or properly reduced.Heat input is preferred for smaller welding methods such as CO2 gas shielded welding. 

3) thick plate welding shall replace single layer welding as much as possible.

7) rigid fixed method: also known as compulsory method.In the actual production, there is less deformation after welding of rigid and large components, and the rigidity of the components can be strengthened before welding, and the deformation response is reduced after welding.When using this method, must wait for cooling after welding fixture and support to be lifted up. Several common methods have a jig, support method, method of clamping fixture, temporary fixed method (such as welding nail fixed and fixed clamping method), positioning welding method. 

8) hammer weld method: this method is mainly applicable to the welding of thin plates.When the weld of the sheet and its thermal impact zone are not completely cooled, hammer the area immediately, and the thick plate is hit with the air gun.

9) the longitudinal shrinkage deformation of the welding seam is compensated by using the component reserved length method. 

10) to minimize the number and size of welds in design;Reasonable layout of the weld, in addition to avoid the weld density, should also make the weld position as close to the axis of the components as possible, and make the weld layout and components.

Relative to the axis. 

11) select the welding sequence correctly.In the steel structure, when there is a weld and fillet weld, in principle, the weld shall be welded to the weld and the arc weld.For the cruciform and t-type welds, the correct order should be taken to avoid welding stress concentration and ensure the welding quality of joints.It is advantageous to reduce deformation by adopting the welding of the neutral axis of the whole steel structure and taking the welding from the middle phase.Welding of important parts in steel structure with high strength should be free to contract and not be restrained.