Weld rework during the manufacturing process cannot be completely avoided. In order to ensure the quality of welding rework, there are several requirements as follows.

　　Rework conditions

　　There are oversized defects in the welded joint, and the defects are often repaired by welding after removal. For surface defects located in the welded joint, it is appropriate to use grinding wheel grinding and other methods to remove defects, such as grinding depth does not exceed the standard allowable value, it can be exempt from welding rework.

　　This is particularly important for pressure vessels in use, because the use of site welding conditions (such as welding position, welding environment and preheating and slow cooling, etc.) and the level of operation of the welder, and often there is a gap between the manufacturing enterprise, welding rework process may cause other hidden problems.

　　Rework requirements

　　Carefully analyze the causes of over-standard defects, research prevention and control measures, is to improve the premise of welding rework. Generally speaking, welding rework consists of the following processes.

　　(1) First of all, the defect removal

　　Defects can be removed using cold (such as grinding wheel grinding), hot (such as carbon arc gas planing) two processing methods. The choice of removal method is related to the material strength level and alloy content.

　　The standard tensile strength of the lower limit of Rm ≥ 540MPa steel and Cr-Mo low-alloy steel, the use of hot processing methods to remove defects should be noted to avoid the presence of surface hardening layer induced by the expansion of the original defects or the emergence of new defects.

　　Can also use a combination of cold and hot processing method, that is, after the carbon arc air planing and then use the grinding wheel to repair the hardened layer of the surface off.

　　(2)Inspection of defect removal

　　Defect removal is generally used after the magnetic particle, penetration and other non-destructive testing methods to confirm that the defect removal is clean and thorough, which is the key to the quality of welding rework.

　　(3) welding rework before the process evaluation

　　As the process of rework and product welding process may be different, such as welding is automatic welding and manual electrode arc welding rework, so it is necessary to rework before the welding process assessment. If the original process has been assessed according to the relevant provisions of NB/T 47014 can cover the rework process, it is deemed to have been assessed before the rework process.

　　(4) welding rework after the inspection

　　In order to ensure the quality of rework, the same non-destructive testing methods should be used with the application of welding (internal RT or UT, surface MT or PT) with the passing level, the reworked weld inspection to confirm that there is no over-standard defects.

　　Rework timing

　　The purpose of post-weld heat treatment is to eliminate excessive weld stress, therefore, require post-weld heat treatment of the vessel should generally be post-weld heat treatment before the welding rework.

　　After the pressure test requires welding rework, when the rework depth is greater than 1/2 wall thickness, according to China's relevant regulations, standards (such as GB / T150), should be re-pressure test.

　　Number of rework

　　Previously, there was concern that the same part of the weld reworked several times, will be repeatedly heated to cause grain growth affect the safety of the operation of the vessel, after several tests to prove that the tendency of grain growth due to repeated rework is not obvious, will not pose a threat to the safe operation of the vessel.

　　Therefore, when the same part of the weld is reworked more than 2 times when the weld is sentenced to scrap is not based on.

　　Based on the above reasons, our standards on the same part of the weld rework limit is non-mandatory, so the use of "should not exceed 2 times" this expression, the phenomenon of more than 2 times the following warning measures, that is, "rework should be approved by the technical director of the manufacturing unit before, the number of rework , parts and reworking should be recorded in the quality certificate of the container ", the purpose is to serious welding process discipline.

　　Stainless steel vessel welding rework

　　Along the metal grain boundary corrosion called intergranular corrosion. Intergranular corrosion is one of the main forms of corrosion of stainless steel, its welded joints and heat-affected zone often produce intergranular corrosion phenomenon, the reason is that stainless steel in 500 ℃ ~ 700 ℃ after a period of time, due to the precipitation of chromium carbide caused by intergranular poor chromium, which leads to intergranular corrosion.

　　Intergranular corrosion is a dangerous form of damage in stainless steel containers, which is characterized by the shape and size and metal luster almost unchanged, but the mechanical properties of the material decreased significantly. To prevent intergranular corrosion, the need to choose the appropriate method from the stainless steel intergranular corrosion susceptibility test method in the relevant standards, the material (including welding material) to be strictly screened to ensure that the requirements of intergranular corrosion resistance.

　　Welding rework due to improper selection or operation, may destroy the original ability to resist intergranular corrosion, therefore, GB 150 and other requirements "have special corrosion requirements of the container or pressurized components, rework parts still need to ensure no less than the original corrosion resistance".

　　Therefore, the reworking of the lowest possible welding heat input, multi-layer welding should be strictly controlled interlayer temperature, to be cooled to about 60 ℃ before the next weld, in order to reduce the residence time of the welded joint in the range of sensitization temperature;

　　In addition, to ensure the strength of the conditions as far as possible to use ultra-low carbon austenitic stainless steel welding rod, to reduce the possibility of carbon precipitation and intergranular chromium depletion.