Solder rework during manufacturing cannot be completely avoided. In order to ensure the quality of welding repair, there are the following requirements.
Welded joints have excess defects, which are often repaired by welding after the defects are removed. For the surface defects located in the welded joints, it is advisable to use methods such as a grinding wheel to remove the defects. If the grinding depth does not exceed the standard allowable value, the welding repair can be exempted. This is especially important for pressure vessels in use because the welding conditions (such as welding position, welding environment, preheating and slow cooling, etc.) and the operation level of the welders often have a certain gap with the manufacturing enterprises. There may be other hidden dangers.
Careful analysis of the causes of over-standard defects and research on preventive measures is the premise of doing a good job in welding repair. Generally speaking, welding rework consists of the following steps:
(1) The first is defect removal
Defects can be removed by cold (such as grinding wheel grinding) and heat (such as carbon arc gouging). The choice of removal method is related to the material strength grade and alloy content. For steels and Cr-Mo low-alloy steels with the lower limit value of standard tensile strength Rm ≥ 540MPa, attention should be paid to removing defects by hot working method to avoid the expansion of original defects or the initiation of new defects due to the existence of the surface hardened layer. . A combination of cold and hot processing can also be used, that is, carbon arc air gouging and then grinding off the hardened layer on the surface with a grinding wheel.
(2) Inspection of defect removal
After defect removal, non-destructive testing methods such as magnetic powder and penetration are generally used to confirm whether the defect removal is clean and thorough, which is the key to the quality of welding repair.
(3) Process qualification before welding repair
Since the process of repairing may be different from the process of product weldings, such as automatic welding and manual electrode arc welding repairing, it is necessary to carry out welding process qualification before repairing. If the original qualified process can cover the repair process according to the relevant provisions of NB/T 47014, it is deemed that the process evaluation has been carried out before the repair.
(4) Inspection after welding repair
In order to ensure the quality of rework, the same non-destructive testing method (internal RT or UT, surface MT or PT) and qualification level as that used for welding should be used to inspect the reworked weld to confirm that there are no out-of-standard defects.
The purpose of post-weld heat treatment is to eliminate excessive welding stress. Therefore, containers requiring post-weld heat treatment should generally be welded and repaired before post-weld heat treatment.
If the welding repair is required after the pressure test, when the repair depth is greater than 1/2 of the wall thickness, the pressure test should be performed again according to the relevant regulations and standards of our country (such as GB/T150).
Number of repairs
In the past, I was worried that if the same part of the weld was repaired many times, the grain growth would affect the operation safety of the container due to repeated heating. It poses a threat to the safe operation of the container. Therefore, it is unfounded to judge the welding seam as scrap when the number of repairs on the same part of the welding seam exceeds 2 times. Based on the above reasons, the restriction on the number of repairs on the same part of the weld is not mandatory in my country’s standards. Therefore, the expression “should not exceed 2 times” is adopted, and the following warning measures are prescribed for the phenomenon of more than 2 times, namely “before repair All should be approved by the general technical director of the manufacturing unit, and the number of repairs, the location and the repair situation should be recorded in the quality certificate of the container.” The purpose is to strictly enforce the welding process discipline.
Welding Rework of Stainless Steel Vessels
Corrosion that occurs along metal grain boundaries is called intergranular corrosion. Intergranular corrosion is one of the main corrosion forms of stainless steel. Intergranular corrosion often occurs in the welds and heat-affected zones of welded joints. Precipitation causes intergranular chromium depletion, which leads to intergranular corrosion. Intergranular corrosion is an extremely dangerous form of damage in stainless steel containers, which is characterized by the fact that the external dimensions and metallic luster are almost unchanged, but the mechanical properties of the material are greatly reduced. In order to prevent intergranular corrosion, it is necessary to select an appropriate method from the relevant standards of stainless steel intergranular corrosion susceptibility test methods and strictly screen materials (including welding materials) to ensure that they meet the requirements of intergranular corrosion resistance. During welding repair, due to improper material selection or operation, the original intergranular corrosion resistance may be damaged. Therefore, GB 150 and other requirements require that “for containers or pressure components with special corrosion resistance requirements, the repaired parts should still be guaranteed not to be lower than the original one. some corrosion resistance.” Therefore, the welding heat input as low as possible should be used during rework, and the interpass temperature should be strictly controlled during multi-pass welding, and the next pass should be welded after the previous pass is cooled to about 60 °C to reduce the stay of the welded joint within the sensitization temperature range. In addition, under the condition of ensuring strength, try to use ultra-low carbon austenitic stainless steel electrodes to reduce the possibility of carbon precipitation and intergranular chromium depletion.
The following are welding pictures from Longtaidi.
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