Heat treatment issues in large storage tanks
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According to clause 3.2.4 of GB50128-2005: \"In any of the following situations, overall stress-relief heat treatment shall be carried out on the attachment and the tank wall plates after welding\"------. The actual heat treatment process for large storage tanks involves rolling the steel plates, welding manholes, adding inlet pipes, installing reinforcement rings, etc.; thereafter, the entire steel plate is subjected to heat treatment, followed by the assembly and welding of the ring plates. The problem now is: the so-called \"overall heat treatment\" does not cover welds of types A and B (using the terms from GB150); are welds of types C and D therefore more important than those of types A and B? Why is this stipulated? In my opinion, even if true overall heat treatment is not possible (it’s not feasible to treat tanks in this way due to their large diameter; for example, 80-meter tanks with 100,000 compartments), local stress-relief heat treatment should still be applied to Class A welds. As for the requirement for \"overall heat treatment\" of Class C and D welds, this is questionable – in my view, local treatment is sufficient. Furthermore, in accordance with the same requirement, Q345R plates with a thickness greater than 32 mm and nozzles of >=300 mm also require \"full heat treatment\"; however, if the plate thickness is exactly 32 mm (as is the case with many 50,000-tank units), no heat treatment is necessary regardless of the size of the holes made in them. Is it reasonable? Discussion is welcome!
The original terms in GB150 are welding joints of classes B, C, and D. Local heat treatment is permitted for Class A welded joints where the spherical head is connected to the cylinder, as well as for areas that have been repaired due to defects. The aforementioned joints are all relatively convenient for local heat treatment. Overall heat treatment is the preferred type of post-weld heat treatment. The size of the opening has no inherent connection with whether heat treatment is applied or not. The purpose of post-weld heat treatment is to eliminate residual stresses. And when welding materials with a greater thickness. This situation is quite obvious. Therefore, post-weld stress relief must be carried out.
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Since large storage tanks are fabricated on-site, it is difficult to carry out overall heat treatment. Moreover, the C and D welds resulting from the welding of nozzles to the tank shell and reinforcement plates are most prone to stress concentration, which can lead to cracks and accidents. Perhaps in the process of formulating the standards, foreign standards and experiences were taken as a reference; such regulations can meet the requirements for using the tanks. At the same time, during design, first attention should be paid to material selection, and second, emphasis should be placed on welding procedure qualification and on-site inspections. It ensures the safe use of the tank.