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Welding repairs: Welding repairs during the manufacturing process are impossible to completely avoid. To ensure the quality of welding repairs, the following requirements apply. Rework conditions: If the welded joint has defects that exceed the specified limits, welding repair is often employed after those defects are removed. For surface defects located at the weld joint, it is advisable to remove these defects using methods such as grinding with a grinding wheel; if the depth of grinding does not exceed the standard allowable value, rework of the weld is not necessary. This is particularly important for pressure vessels in use, as the welding conditions at the site of operation (such as the welding location, the welding environment, as well as preheating and slow cooling) and the skill level of the welders often differ from those at the manufacturing facility, which may lead to other potential hazards during welding repairs. Requirements for rework: Thoroughly analyzing the causes of defects that exceed specifications and studying preventive measures are prerequisites for effective welding rework. Generally, welding repairs consist of the following steps: (1) First is the removal of defects, which can be accomplished using either cold methods (such as grinding with a grinding wheel) or hot methods (such as gas metal arc cutting). The choice of removal method depends on the material strength grade and alloy content. When using hot working methods to remove defects in steel materials with a minimum standard tensile strength of Rm≥540 MPa and Cr-Mo low-alloy steel, care must be taken to avoid the expansion of existing defects or the formation of new defects due to the presence of a surface hardened layer. A combination of cold and hot processing can also be used, that is, carbon arc gouging followed by grinding the hardened layer on the surface with a grinder. (2) Inspection for defect removal: After the defects are removed, non-destructive testing methods such as magnetic particle testing and penetrant testing are generally used to confirm whether the defects have been completely removed; this is crucial for ensuring the quality of welding repairs. (3) Process qualification before welding repair: Since the process used for repair may differ from that used during initial welding – for example, if automatic welding was used initially but manual shielded metal arc welding is employed for repair – it is necessary to conduct a welding process qualification before proceeding with the repair. If the existing qualified process can cover the rework process in accordance with the relevant provisions of NB/T 47014, it is deemed that a process evaluation has been conducted prior to the rework. (4) Inspection after welding repair: To ensure the quality of the repair, the same non-destructive testing methods (internal RT or UT, surface MT or PT) and acceptance criteria as those used during welding shall be employed to inspect the repaired welds, in order to confirm the absence of any defects exceeding acceptable limits. Timing for rework: The purpose of post-weld heat treatment is to eliminate excessive welding stresses; therefore, containers that are to undergo post-weld heat treatment should generally have their welds repaired prior to such treatment. If welding repairs are required after a pressure test, and the depth of the repair exceeds 1/2 of the wall thickness, then in accordance with China’s relevant regulations and standards (such as GB/T150), a new pressure test must be conducted. Number of repairs: There was once concern that repeated repairs to the same weld area might cause grain growth due to repeated heating, thereby affecting the safe operation of the container. However, numerous tests have shown that there is no significant tendency for grain growth as a result of repeated repairs, and this does not pose a threat to the safe operation of the container. Therefore, it is unfounded to declare a weld defective when the number of repairs at the same location of the weld exceeds 2 times. For the above reasons, China’s standards do not make it mandatory to limit the number of times a weld can be repaired in the same area; therefore, the phrase “should not exceed 2 times” is used. For cases where more than 2 repairs are carried out, the following warning measures are specified: “Each repair must be approved by the technical supervisor of the manufacturing unit, and the number of repairs, the locations involved, as well as details of the repairs should be recorded in the container’s quality certificate.” This is done to ensure strict adherence to welding procedures. Weld repair of stainless steel containers: Corrosion that occurs along the boundaries of metal grains is known as intergranular corrosion. Intergranular corrosion is one of the main forms of corrosion in stainless steel. Intergranular corrosion also often occurs in the welds and heat-affected zones of welded joints. This is because, when stainless steel is exposed to temperatures between 500°C and 700°C for a period of time, the precipitation of chromium carbide leads to a reduction in chromium content at the grain boundaries, thereby causing intergranular corrosion. Intergranular corrosion is an extremely dangerous form of degradation in stainless steel containers; it is characterized by almost no change in external dimensions or metallic luster, but a significant decline in the material’s mechanical properties. To prevent intergranular corrosion, it is necessary to select an appropriate method from the relevant standards for testing the susceptibility of stainless steel to intergranular corrosion, and to conduct strict screening of the materials (including welding materials) to ensure that the requirements for resistance to intergranular corrosion are met. During welding repairs, improper material selection or handling may undermine the original resistance to intergranular corrosion; therefore, standards such as GB 150 require that \"for containers or pressure components with special corrosion resistance requirements, the repaired areas must still maintain a corrosion resistance level no lower than that of the original material.\" Therefore, during repair, the lowest possible welding heat input should be used; when multiple passes of welding are performed, the interpass temperature must be strictly controlled – the next pass should only be carried out after the previous pass has cooled to around 60°C – in order to reduce the time that the welded joint spends within the sensitization temperature range ； Furthermore, under the condition of ensuring strength, ultra-low carbon austenitic stainless steel electrodes should be preferred as much as possible to reduce the likelihood of carbon precipitation and intergranular chromium depletion.