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Welding Deformation Control Technology for Large Steel Structures

steel structure

Welding is a key process during the installation and construction of large steel structures in construction projects. In specific welding, many factors will have a negative impact on its welding quality, which will cause deformation problems. Therefore, the construction unit and technicians must take reasonable technical measures to control the deformation.

1.Project Overview

This time we are studying the laser smart town incubator project, with a total planned area of 69,500m2 and a new construction area of 150,399.7m2, including 105,286.4m2 of above-ground buildings and 45,113.3m2 of underground buildings. In this construction project, buildings 1#, 4#, and 5#  are steel frame structures, and their design service life is 50 years. The earthquake protection categories are all class C, and the safety level of the building structure is level two. The seismic fortification intensity of the area where they are located is 8 degrees, the design basic ground acceleration is 0.30, and the design earthquake is grouped into the second group. The foundation design grade is Class A, the site category is Class I, the characteristic period is Tg=0.665s, and the damping ratio of the building structure is 0.05.

In this project, Building 1# is located in the southwest corner, with a length of 84.90m, a width of 30.90m, and a height of 45.50m. There are 10 floors above ground and 1 floor underground. The height of one floor is 4.50m, the height of the rest is 4.0m, and the maximum inter-axis span is 10.0m. buildings 4# and 5# are located in the northern half, with a length of 54.10m, a width of 21.00m, and a height of 52.50m. There are 12 floors above ground and 1 floor underground. The height of one floor is 4.50m, and the height of the rest is 4.0m, and the maximum inter-axis span is 8.4m. In the specific construction, the steel beam is H-shaped, the steel column is box-shaped, and the main part is made of Q355GJC-shaped steel and the embedded parts are made of Q355B-shaped steel.

2. Installation Sequenceof Large Steel Structure

steel frame building

When installing and constructing the large-scale steel structure in this project, the overall installation principle is to install from bottom to top, from far to near, that is, starting from the first floor, followed by the second and third floors, until top floor. During construction, because the crane can only run on one side of the steel structure, it is necessary to install the distant components of each floor first, and then install the nearby components.

In the hoisting of components in building 1#, the sequence is ①hoisting of steel columns, ②hoisting of steel beams between steel columns, ③hoisting of steel beams between steel beams.

After completing the installation and construction of the standard floor steel structure of the above-ground part, the beam will be installed to ensure the size of the overall steel structure and achieve effective control of installation deviation. When installing the steel components in building 4# and building 5#, it is necessary to complete the installation of the overall components on one side of the building through the station car. The installation sequence is first far and then near, gradually moving from one side to the other.

Among them, building 1# can use the fifth and sixth axes as axes of symmetry, and divide the installation and construction of the overall steel structure into two flow sections. Building 4# and building 5# can carry out steel structure installation and construction according to two flow sections .

3. On-site Welding Process of Large Steel Structure

3.1 Welding Process Flow

In the process of on-site welding of large steel structures, the construction unit and technicians must strictly follow the established process to carry out welding construction, so that the welding quality can be well guaranteed.

3.2 Preparations Before Welding

Before the official start of welding construction, scientific and perfect preparation is the key to ensure the smooth progress of subsequent welding work, and it can also further ensure the welding quality. In this process, it is first necessary to check the assembly effect of the welding position and its surface cleaning effect. If it is unqualified, it needs to be repaired by welding, and the welding can only be carried out after it is qualified. The second is to do a good job of checking the groove assembly gap. If the deviation exceeds the regulations, it is necessary to do the groove welding treatment by surfacing welding on one side or both sides, and then grind it to pass. However, if the groove assembly spacing is greater than twice the thickness of the thin plate, or exceeds 20mm, the length of the component cannot be increased by surfacing welding. Finally, the technical disclosure work should be done according to the actual welding process, so that all welding technicians can fully clarify the specific welding content, technical specifications and precautions. In this way, scientific support can be provided for subsequent welding operations.

3.3 Welding Groove Form and Welding Sequence

When welding large-scale steel structures in construction projects, the welding groove form and welding sequence need to be reasonably controlled, so that the actual needs of large-scale steel structure projects for welding quality can be effectively met. In this project, the main control technical measures of welding groove form and welding sequence include the following aspects.

First, the stress and strain force control is taken as a criterion, and the welding sequence is formulated in detail, and it is strictly forbidden to set the closing weld joint at the stress concentration position of the rod.

Second, when welding rigid joints such as beams and columns in the overall steel frame, it is necessary to weld the middle of the overall structure first, and then expand the welding to the left and right sides after the frame is formed.

Third, in the process of butt welding construction between columns, two welding workers should be arranged to weld in different directions on both sides at the same time, and ensure the symmetry of the welding. The next weld can only be welded when the height of the previous weld reaches 40%, and 40% of the weld must be welded first, and then the remaining 60% of the weld must be welded. In addition, it is necessary to control the interlayer temperature during welding to avoid the adverse effects of excessive interlayer temperature on welding quality. If the length of the weld seam is too large, it should be welded by skip welding in sections, but the welding operations of the two welders must be synchronized, so as to reduce the problem of welding deformation. In the process of welding the last layer of steel structure, it must be welded at one time, and it cannot be welded in sections. At the same time, the shape control of the corner position of the column should be done well.

Fourth, when welding the connection between the beam and the column, the connection position between the beam web and the column should be welded first, and then the connection position between the beam flange and the column should be welded. In web welding, two welding workers should weld at the same time until the welding construction is completed; in wing plate welding, if the space is large enough, two welding workers can weld symmetrically and synchronously. If the space is not large enough, it need to first weld 30% of the upper flange weld, then weld 30% of the lower flange weld, then weld 70% of the upper flange, and finally weld 70% of the lower flange weld.  

3.4 On-site welding operation process

In the on-site welding construction of large steel structures, it is first necessary to do a good job of preheating and post-heat treatment, and take protective measures against laminar tearing at the plate parts, corner joint structures, and T-shaped joints with a thickness of more than 40mm, such as welding tongs preheating, such as slow cooling or post-heating after welding. During the preheating process, both sides of the welding groove need to be used as the heating area, and the preheating width should exceed 15 times the welding thickness, and not less than 100mm. The preheating temperature needs to be measured on the reverse side of the weld, and the distance between the measurement point and the welding point before the arc passes should be controlled at 75mm or more. If preheating is performed by a flame heater, it is necessary to take a temperature measurement on the front of the weld after stopping and heating.

The second is to do a good job of decompression treatment. In the specific treatment, the heating temperature should be controlled between 200 ° C ~ 250 ° C, and the holding time should be determined according to the actual thickness of the work plate. Usually, the thickness of the work plate increases by 25mm, and the heat preservation time needs to be extended 0.5h, and the total holding time should be controlled at 1h or more. After reaching the holding time, it should be cooled to normal temperature by slow cooling.

The last is to control shrinkage. In terms of specific control, the main measures are as follows: one is to minimize the angle and gap of the welding groove on the basis of ensuring penetration. The other is to improve the processing and manufacturing precision of steel components so that the length deviation is controlled at a reasonable level within the rang. Third, reasonably reduce the weld bead and heat input, and use multi-layer and multi-layer methods for welding.

4. Overview of welding deformation of large steel structures

In order to achieve good control of the welding deformation of large-scale steel structures, the construction unit must first fully clarify the main principles and causes of its welding deformation, so as to provide a scientific basis for its deformation control. 

4.1 Principle of welding deformation

During the welding process of large-scale steel structures, the welds will have corresponding deformations, but most of these deformations are temporary. After the welding construction is completed and the welds are completely cooled, most of the deformations will recover. However, there will be some deformation residues in it, and this part is the real welding deformation. In terms of its essence, there are two main principles of welding deformation formation. The first is that the weld metal in the molten state in the weld area shrinks due to cooling and solidification, and then forms deformation, including lateral deformation and longitudinal deformation of the weld. and angular deformation. The second is that the weld metal in the molten state transfers the high temperature in welding to other weld areas outside the weld area, thereby forming a heat-affected zone, which will be heated and subsequently cooled by the weld. This is a simple form of thermal deformation. Under the limitation of its body stiffness, the existence of this thermal deformation can easily lead to the deformation of the entire weld.

4.2 Main reasons for welding deformation

In the welding of steel structures, especially in the welding of large steel structures, there are many factors affecting the welding deformation problem. As far as the current welding deformation of large steel structures is concerned, the main influencing factors include the following aspects: First, all basic components composed of steel structures should fully meet the specified technical standards in terms of technology and line tolerances, but if some of the component structures are not up to standard, each component in the overall steel structure will have a congenital out-of-tolerance after the assembly construction is completed. In such a case, welding deformation is likely to occur in subsequent welding.

Second, in the process of assembling large-scale steel structures, if there is no strict control, the gap between some steel structure components will be too large, and large deformation will easily occur in the subsequent welding process Condition.

Third, if the weld along the section of the steel member cannot achieve a good symmetrical distribution effect, the corresponding member is prone to bending deformation during the welding process.

 Fourth, in the process of assembling and welding large-scale steel structures, if the welding groove form and welding sequence are not strictly controlled in accordance with specific requirements, or the welding method is not strictly controlled in accordance with the specifications, or improper selection of the size, quantity, and location of the welds can easily cause welding deformation of the steel structure.

5. Analysis of main control technologies for welding deformation of large steel structures

Because the steel structure in this construction project has a large span and many materials, the amount of welding work and welding construction procedures involved are also relatively large. In addition, the influence of various internal and external factors has increased its welding quality control difficulty. In order to effectively ensure the welding quality and prevent welding deformation problems, relevant units must take scientific and reasonable technical measures to control them.

5.1 Rational design of a welded joint structure

For the welding of large steel structures, in order to make its welding deformation to be well controlled, a key content is the reasonable design of the welding joint structure, so as to provide sufficient scientific reference and support for the subsequent welding, to the greatest extent to prevent welding deformation problems. In the specific design, the construction unit should pay enough attention to the following aspects. First, do a good job in controlling the size and quantity of welds. In the design process of welding joints for large steel structures, it is necessary to control the size and number of welds according to the actual situation, so that it is consistent with the actual steel structure components and the overall steel structure engineering, in order to minimize the probability of welding deformation.

Second, in strict accordance with the welding process used in construction, the shape and size of the weld groove should be reasonably selected, so that the bearing capacity requirements of the overall steel structure can be fully met. At the same time, ensuring the rationality of the groove shape and size can achieve a reasonable reduction in the welding cross-sectional area, so as to further reduce the welding deformation of the steel structure.

Third, the welded joints should be set at symmetrical positions on the steel member section, especially on the neutral axis of the steel structure. The welded joints should be set at the neutral axis symmetrical position on the steel member section as much as possible, or close to the neutral axis, and be sure to avoid stress concentration areas.

Fourth, in the process of selecting the joint form, it is necessary to choose a joint form with rigidity as much as possible, and it is not allowed to set the joint at the two-way intersection position and the three-way intersection position. Only in this way can the high-temperature concentration and welding stress concentration caused by the concentration of weld seams be effectively prevented, so as to further reduce the problem of welding deformation.

5.2 Correct the deformation after welding

In the rectification of large steel structures after welding, there are two main rectification methods, the first is mechanical rectification, and the second is flame rectification. Among them, mechanical rectification is to produce a plastic deformation of the steel member by means of external force, and its direction is opposite to the welding deformation, so as to offset the welding deformation and achieve a good deformation control effect. This method is very suitable for correcting welding deformation of steel structures such as T-beams and I-beams in large-scale steel structure projects. The flame straightening method is to reversely deform the steel structure through uneven heating, so as to compensate for or offset the original welding deformation. There are three ways of flame heating, the first is point heating, that is, the heating area is an origin. The second is linear heating, that is, moving along a straight line or swinging laterally in the width direction of the steel member. The third is triangular heating, that is, the heating area is triangular. For many welding deformation problems that cannot be solved by mechanical straightening, flame straightening can be solved.

5.3 Common welding deformation control measures

In the welding process of large steel structures, the most common deformation control measures include the following: First, for small components and embedded parts, deformation correction can be carried out by hammering after welding. Second, for steel roof trusses, especially along the span direction of steel frames, the welding process control must be done in strict accordance with actual requirements. Third, for the I-beam flange angle, deformation control can be carried out by anti-deformation, rigid fixation, mechanical rectification, and flame correction. Fourth, the longitudinal shrinkage deformation of the I-beam, can be realized through the reservation of weld shrinkage, and the shrinkage amount should be calculated according to the actual situation. Fifth, the side bending deformation of the I-beam, can be corrected by the flame heating method, and the heating should be carried out at the position where the side bending of the flange plate is the largest until the deformation meets the actual requirements.