Requirements: The flatness requirement is 8~10mm, and the material performance after welding of the steel plate must be guaranteed.

(1) The number and size of welded steel plates? It is made of 3 steel plates, with widths of 1 900mm, 1 900mm and 1 050mm respectively, and lengths of 4 830mm, which are butted in pairs. Considering the issue of welding shrinkage, a 9mm margin is reserved. After the production was completed, it was found that the shrinkage was 10~12mm. Since the workpiece processing allowance is 25~30mm, the maximum error is 3mm, so it does not affect the processing and use.

(2) Welding methods and groove forms? Generally, the welding methods of thick plates include electroslag welding, submerged arc welding, gas shielded welding and electrode arc welding. Considering the existing conditions of the enterprise and the efficiency of various welding methods, CO2 gas shielded welding is used for base welding and submerged arc welding for welding and covering.

Most thick plate groove forms are I-shaped, X-shaped and U-shaped. After comprehensive comparison, it was decided to adopt U-shaped groove. Considering the difficulty of root removal, an asymmetric U-shaped groove is adopted. In order to ensure the welding quality, the groove must be made by machining, and the size and surface roughness must be 12.5 μm.

(3) Trial welding in advance? In order to ensure the welding quality, a welding test was conducted with a 1m long, 200mm thick steel plate, which can not only train the welder, but also find deficiencies in the actual operation process. During the bottom welding test, it was found that when the welded end moved to the other end, the opening of the unsoldered end did not change significantly.

(4) Welding materials and parameters? First, a detailed analysis of the main causes of cracks: ① Hardening tendency: The material of the steel plate is Q345D steel, the upper limit of carbon content is 0.18%; wP, S≤0.03 %. Because the hardening tendency is small and the weldability is good, it is not the main cause of cold cracks. ②The role of hydrogen: The welding materials used have been strictly dried and the factory environment is dry. Even if a small amount of hydrogen remains in the weld during welding, the content is low and it is not the main cause of cold cracks. ③ Uneven temperature distribution in the thickness direction during welding will produce large lateral compression plastic deformation; uneven shrinkage in the thickness direction during post-weld cooling can easily lead to angular deformation between the two connecting parts.

Secondly, the principle for selecting welding materials is that the alloy composition and strength properties of the weld metal basically meet the lower limit specified by the base metal standards or meet the minimum performance indicators specified by the product technical conditions.

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