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Leakages in the semi-tubes of reaction vessels tend to occur in the heat-affected zones of the butt joints between the semi-tubes, as well as at the fillet joints where the semi-tubes meet the head. Reason 01: Welding sequence – First, weld the butt joints between the half-tubes; only after they pass inspection is welding to the tank body carried out. If the fillet welds connecting the two sides of each half-pipe to the tank body are welded first, and then the butt welds between the half-pipes are welded, the half-pipes will have no room for contraction after welding, resulting in high assembly stresses between the butt welds. This assembly stress generally manifests as tensile stress, which can easily cause welding cracks and reduce the tensile strength of the weld. 02 Manufacturing Process: The spiral half-tubes in the head section exhibit both variations in spiral radius and spiral lead angle, making their manufacturing and welding much more difficult than those of the cylindrical sections; even minor errors can lead to problems with proper fitting. If the uneven grooves are not treated during manufacturing. During the welding process, abnormal methods such as applying external force and increasing the height of the weld leg are used to ensure a tight fit between the half-pipe and the end cap, which in turn increases the internal stress in the half-pipe. 03 Welding process: The length of the half-tube sections is short, there are many weld joints, the welding and cooling rates are fast, and the weld beads are tall. Due to the different diffusion rates of elements C and Cr, the chromium content decreases in the heat-affected zone. The higher carbon content reduces the toughness and plasticity of the material, as well as its weldability, increasing the tendency to develop welding cold cracks. If the preheating and post-heating temperatures are insufficient and the time allocated for these processes is inadequate during welding, the residual welding stresses will be high, making weld cold cracks very likely to occur. Countermeasure 01: The diameter of the half-tubes should be increased appropriately, which helps to reduce the number of welds required for joining the half-tubes together as well as the length of the fillet welds connecting them to the tank body, thereby decreasing the likelihood of weld leaks. 02 Reduce assembly stress during the manufacturing process. There must be limits on the diameter tolerance and roundness of the half-tubes, in order to reduce the amount of misalignment at the joints; the welds at these joints must be fully penetrated, and thorough non-destructive testing must be carried out. 03 Improve the semi-tube manufacturing process. In accordance with the dimensional requirements of the pattern, the half-tube can be stamped from sheet metal; this avoids material waste and allows for mass production using molds, thereby reducing processing costs. 04 Considering the differences in the linear expansion coefficients and thermal conductivity of various materials, it is advisable to use the same material for the semi-tube as that used for the reactor vessel, in order to avoid increased thermal stress due to material differences, which could damage the welds of the semi-tube.