Q345 steel contains inevitable alloying elements with a carbon equivalent value of 0.39%, and has a certain tendency towards quenching and hardening. When welded in the production and manufacturing process of seamless stainless steel decorative pipes in thick plates and environments with excessively fast cooling rates, it is prone to cold cracking. In addition, during welding with large process parameters, the heat input during the New Year's Eve can easily lead to thermal cracking or thermal strain embrittlement in the weld seam. From the layout of the welded components, the length to section ratio is large, belonging to the category of slender rods. During the welding process, they are prone to deformation and the deformation environment is complex. Therefore, controlling welding deformation has become a difficult problem. 

1. The seamless stainless steel decorative pipe welding scheme is almost certain

To meet the strength requirements, the cross-sectional size of this seamless stainless steel decorative tube is 300mm × 200mm, with a plate thickness of 3mm. The design requires distortion and parallelism errors to be ≤ 1.2mm, making the construction process difficult. Therefore, after conducting research and investigation, Yun finally determined to manipulate the symmetrical half bending of the sheet metal into a groove shaped half shell, and then use the manufacturing process of CO2 gas shielded welding to form it. 

2. Welding process

(1) Two symmetrical groove shaped half pipes with V-shaped grooves are formed by pressing with a large bending machine, and the butt welding grooves are processed according to the process requirements, with reserved gaps for splicing. The design of welded joints is a relatively weak link in welding engineering. The groove form has a significant impact on controlling the internal quality of the weld seam and the manufacturing quality of the welding layout. Groove design must consider issues such as the fusion ratio of the base material, welding space, welding position, and comprehensive economic benefits. The horizontal shortening value Δ B should be calculated first according to the following formula. 

Δ B=5.1A ω/δ+1.27 d, where A ω - cross-sectional area of the weld seam, mm; δ - plate thickness, mm; D - root gap of weld seam, mm. After finding the relationship between Δ B and A ω, the data can be analyzed and optimized according to the relationship list, and the groove form of seamless stainless steel decorative pipe butt weld seam can be determined

(2) Controlling welding deformation is extremely difficult for this seamless stainless steel decorative pipe due to its elongated shape. The primary deformations of welding include bending (positive bending), lateral bending, angular deformation, and twisting deformation. For this seamless stainless steel decorative pipe, the primary deformation is horizontal shortening, which affects the size of the rectangular section and requires a 90% reduction in the reserved gap on each side; After the transverse shortening of the weld seam, the two ends of the vertical plate bend inward, forming a waist drum shape in the component; Due to the large cross-section of the weld seam and the high amount of heat input, it will inevitably cause significant longitudinal shortening, resulting in flexural deformation of the component at the specified length; It is difficult to correct the distortion and deformation caused by unreasonable welding, and sometimes it is necessary to cut and re weld or scrap the entire piece. 

According to the theory of welding deformation, the primary factor affecting the size of welding deformation is that the larger the weld size, the more deposited metal, and the greater the deformation; When the weld sizes are equal, the greater the heat input to the weld, the greater the deformation caused; When welding long welds, the deformation of segmented welding is smaller than that of longitudinal welding; Welding seam insertion is incorrect or symmetrical but incorrect. The more severe the deviation of the weld seam, the greater the deformation; The smaller the rigidity of the component, the greater the deformation. 

The welding specification has been determined through process testing and process analysis that the seamless stainless steel decorative pipe butt weld adopts double-layer CO2 gas shielded welding. H08Mn2SiA and 1.2mm welding wire are used for welding materials; The shielding gas is pure CO2 gas. The welding current for the first layer of the weld is 200-250A, and for the second layer it is 240-320A; The arc voltage is 24-26V. The process requirements are: the first layer of weld must be fully penetrated, and the back of the package must have excellent forming; The welding current, arc voltage, wire feeding speed, and welding speed can be adjusted according to the equipment model. 

To reduce deformation, the welding sequence of rectangular butt welding should follow the following principles: adopt symmetrical skip welding from the center to both sides in layers and sections, resulting in less welding deformation than longitudinal welding, which is conducive to stress separation and release, and avoids complex stress in the welded parts. During longitudinal swing welding, the narrow plastic deformation zone formed at the beginning of the welding only appears once, and due to continuous swing welding, the heat input is large, the heated area is large, and the plastic deformation zone caused by compression is large, resulting in significant post weld shortening deformation. When performing layered and segmented skip welding, each layer has a small cross-section and requires less heat. Each layer is also divided into several sections for skip welding, and each section is based on a temperature field established from scratch on the cold steel plate, presenting a narrow plastic deformation zone each time. Therefore, the average width of the plastic deformation zone (i.e. the size of the transverse shortening) is smaller than that of the response layered longitudinal welding, and the longitudinal shortening is also smaller, which is smaller than the continuous filling swing welding deformation of the longitudinal filling. 

According to the above explanation, the first layer of weld seam 1 is segmented and welded from the center to both ends, and the first layer of weld seam 2 is segmented and welded from the center to both ends. Weld seam 2 should use a larger welding specification than weld seam 1 to generate a greater reverse force and correct the original deformation. Then weld the second layer of weld seam 1 and the second layer of weld seam 2 in the same way.