Introduction:

Welding is one of the most important processes in mechanical manufacturing. However, in the process of welding manufacturing and welding production, it is easy to cause transient deformation and transient thermal stress in the welded joint due to the constraint of field operation conditions and the uneven of metallurgical structure change, cooling and heating in the welded structure. Especially the transverse contraction of welded joints will seriously affect the welding deformation of welded structures. The damage caused by welding stress is nearly 10 million yuan per year, and it may even cause serious casualties, this is the problem we have to face.

1.  Types of welding deformations produced by welded structures
There are many factors affecting the deformation of the welding structure, such as whether If there is a constraint in the process of welding, cooling time, welding sequence, filling material, material, welding parameters and welding methods in the process of welding. The factors that mainly affect the deformation of the welding structure are: 1) Welding method. This is mainly reflected in the role of V and I, the line energy of the submerged arc welding (SAW) is large, and the line energy of the hand arc welding (SMAW) is small, and the positive inverse is more remarkable on the rotation deformation. 2) The bevel size or root gap. The amount of weld metal W increases with the increase of the gap between the slope or the root, and the transverse shrinkage increases or the shrinkage deformation appears, and the angle ratio of the two sides of the slope has an influence. 3) The length of deposited metal per unit length is W or line energy. The amount of shrinkage deformation is directly proportional to the line energy, and the larger the lgW, the greater the shrinkage deformation ij the welding structure. 4) Shrinkage deformation includes transverse contraction (perpendicular to the weld direction) and longitudinal contraction (occurring in the weld direction).

2. Technical requirements for controlling welding deformation for welding structures
1）In order to effectively reduce the shrinkage stress and welding deformation of welding structural, position and spot welding should be adopted, and in order to greatly reduce the constraint on the structural deformation and ensure that the first welded parts can be freely contracted, the welding sequence of welded structures should start with the parts with larger constraints and support, and then gradually push forward to smaller parts.  

2）If the welding area needs to be preheated according to the requirements of the welding process procedure, it should be preheated for both at position welding seam and main welding seam at the same time, and the preheating temperature at the main welding seam area should be higher 20-30º than at position welding seam area. The main seam and positioning weld seam are preheated by uniform heating of the heating plate, the width of the preheating zone is not less than 100 mm, which is usually about 3 times of the thickness of steel plate along the center line of the weld seam, and the measuring points of each weld are not less than 3. The welding temperature must conform to the rules of the WPS. The heating energy must meet the requirements of the standard. If there is no specific regulation, it should be referred to the recommended value of DL5017-93 6. 3.12.

3）Once welding work start for welding structures, continuity must be maintained and no interruption can be made in the middle. If it has to be temporarily interrupted for a special reason, the corresponding measures should be taken to prevent the cracking of the weld. And before the original process will be continued, the surface of the weld should be cleaned up, and make sure this is no any defect in the welding.

4) The technical personnel participating in welding work of steel structures fabrication should be able to work in accordance with the relevant laws and regulations of the state and industry and obtain the corresponding qualifications. If someone with a qualified certificate, but if it has not been engaged in the welding work for more than 6 months, he(she)  cannot be allowed to take the welding work immediately, it is necessary to take re-examination and re-get qualified before the take welding work for steel structure.

3 How to effectively control the welding deformation of welding structures

3.1 The selection of proper welding method

In mechanical manufacturing, it is very important to choose a suitable welding method, if the welding line energy is low, the welding compression zone can be reduced greatly, and the welding deformation of the welded structure can be effectively controlled. At present, the argon arc welding, manual arc welding and CO2 gas shielded arc welding can effectively reduce the amount of welding distortion.

3.2  Reduction of deformation
3.2.1 Reducing line energy

The wave deformation, angular deformation, longitudinal bending (longitudinal contraction) and transverse shrinkage of the welded joint can be effectively prevented by reducing the line energy, especially the effect of the angle deformation and the transverse contraction of the T joint. Therefore, during the construction of the welding structures, we should focus on the control of the wave deformation, angular deformation, longitudinal bending (longitudinal contraction) and transverse contraction of the welding joint. Especially for roundness control, wave deformation and angular deformation are very important. When the line energy is at a certain value, if the thickness of the plate is increased, then the shrinkage deformation will decrease, however, there is a difficult problem, that is the welding must be full penetration in the welding structures, with the increase of plate thickness, the line energy will increase too, that is, the amount of deposited metal will increase, which will also lead to the increase of shrinkage deformation. In addition, in the actual welding process, the welding is very likely to need repair partially, because it is difficult to finish fully penetration welding and meet our requirements by once welding, as a result, the thickness of the plate will increase, so the shrinkage deformation will inevitably increase. Therefore, in the design process, we should optimize design and take into account the thickness of the plate as far as possible. At the same time, the influence of groove or root gap in the manufacturing process must be controlled. The production should be carried out on a professional platform with a flatness of below 2 mm. Material preparations and drawing line must be precision. Special attention should be paid to the perpendicularity of the center line of rollers of rolling machine to longitudinal center line when rolling steel plate. If the groove of the longitudinal welding seam has been prepared in advance, then probably the welding preparation will be repaired and adjusted again, then the gap will increase. If it is not vertical, then it is easy to cause too much gap between longitudinal joints and too much repair work for longitudinal joints.

3.2.2 Free shrink

Before welding the welded structure, the welding shrinkage deformation and the reverse deformation of the structure should be offset each other, thus greatly reducing or eliminating the welding deformation. Therefore, we should first weld the inside of the longitudinal weld, and then carry out the appropriate assembly, so that the radius of the reference circle is slightly smaller than the radius of the assembly component, and the two circle will reach the state of the basic coincidence after the welding contraction, and the angular deformation will be better eliminated, It is worth noting that the radius of the two circle can be drawn from the practical experience in construction practice because of the different thickness of the plate and the diameter of the pipe.

3.3 Elimination of deformation

At present, the mechanical pressure method and the flame heating can effectively eliminate the existing residual welding deformation, which is proved by a lot of practical and theoretical research.

4. Concluding remarks

In a word, it is very important to effectively control the welding deformation of welded structures; it has great economic and social significance. In our daily production activities, we should improve the welding process as much as possible so that the welding deformation of welding structures can be controlled to the maximum extent.