Welding Deformation Mechanism And Various Influencing Factors of Large Storage Tank Bottom

With the development of the economy, the storage tank capacity is getting larger and larger, and now tens of thousands of cubic meters of storage tanks are everywhere, and the bottom area of the tank is also increasing with the capacity. The bottom plate of the storage tank is composed of a plurality of strip plates and a plurality of bow plates, which is the most stressed part of the whole storage tank. Its welding characteristics are: large diameter, thin plate, small ratio of steel plate thickness to the width of the bottom of the storage tank, poor stiffness, large number of welds, large welding stress, easy to produce welding deformation and deformation, and difficult to control. Therefore, it is important to analyze the mechanism of welding deformation and various influencing factors, master the law of change, take effective measures to reduce deformation, control the welding deformation of tank bottom, and ensure the production quality of tank bottom.

1. Mechanism of welding deformation of tank bottom plate

1.1 Uneven heating of welding parts and rigid constraints of components

The welding process is a local and uneven heating process of the weldment. During welding, the high-temperature area (weld and weld side) is heated and expanded, which cannot be freely extended due to the rigidity of the surrounding low-temperature area, resulting in thermoplastic deformation. During cooling, the high-temperature area shrinks greatly due to thermoplastic deformation, while the low-temperature area shrinks little. This unbalanced internal shrinkage leads to condensation stress and condensation deformation of the base plate.

1.2 Transformation of metallographic structure

During welding, the organization of the high-temperature region changes from pearlite to austenite, and after cooling, austenite is transformed into a mixture, such as pearlite plus sostenite, sostenite plus troosite, etc., and even martensite (as shown in Figure 2). In addition, the heating and cooling speed of welding are faster, and the organization after welding is very uneven, so the hardness and brittliness of weld and heat-affected zone increase accordingly. The elongation and section shrinkage also increase, and the structure stress and deformation of the bottom plate are generated.

The joint action of condensation deformation and tissue deformation makes the bottom plate produce longitudinal contraction deformation and transverse contraction deformation, which causes various deformation of the bottom plate, such as contraction deformation, Angle deformation, bending deformation, twisting deformation, wave deformation, etc., while the welding deformation of the bottom of the tank is mainly contraction deformation, Angle deformation and wave deformation.

2. Method for preventing and reducing deformation of tank bottom plate

The method to prevent and reduce the deformation of tank bottom plate: reduce the energy of welding line as far as possible under the premise of ensuring welding penetration; Reduce the temperature difference between the welding zone and the overall structure; Minimize the rigid constraints of the base plate during welding; Improve the stiffness of the component; The microstructure transformation is controlled, the hardened microstructure is reduced as much as possible, and the microstructure is refined and uniform. Reduce welding stress and distribute stress evenly.

3. Control of deformation of tank bottom plate

3.1 Develop a reasonable layout design scheme

3.2 Develop a reasonable process

Large storage tank is one of the indispensable equipment in petrochemical equipment, but the serious welding deformation of tank bottom will reduce the bearing capacity and stability of the tank, and even make the bottom of the tank scrap. Therefore, the bottom of the tank is the key part of the entire storage tank, which is related to the success or failure of the entire production and installation of the storage tank. Through the analysis of welding stress, welding deformation mechanism and various influencing factors of large storage tank, the change law is understood, reasonable design scheme is proposed, correct welding process measures are formulated, and correct correction methods are adopted after welding, so as to minimize the welding deformation of tank bottom, ensure the production quality of storage tank, reduce costs, shorten the construction period and improve benefits.