Differences in welded structures
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What is the difference between welding structures DU3 and DU4 as specified in HG/T20583-2011? Some people say that DU4 requires root cleaning in order to achieve full penetration during welding. I work in the field of blind welding; could you please explain: 1. What does root cleaning mean? Is it necessary to drill inside in order to perform root cleaning? 2. What is the relationship between root cleaning and full penetration? Is full penetration not possible without root cleaning?
1. Simply put, the difference between the two is that one involves single-sided welding while the other involves double-sided welding. DU4 refers to double-sided welding; the process of using appropriate tools to clean the root of the completed weld on the opposite side before welding that side is called “root cleaning”. The purpose of root cleaning is, first, to remove defects at the root of the weld such as pores and pits, and second, to create an appropriate welding groove (since the original groove has been damaged by the backside weld). Root removal tools generally use carbon arc gouging, and pneumatic chisels are also used. 2. It should be said that there is no inevitable connection between the two. As mentioned above, full penetration refers to the condition in joints with beveled welds where the weld metal penetrates the entire thickness of the base material. Without double-sided welding, TIG welding is used for the root pass and MIG welding for the cover pass; this also results in full penetration. In 20583, all DU-type structures are fully penetrative structures.
Thank you for your answer. It means that DU3 is single-sided welding. If we are talking only about DU3 and DU4, then is single-sided welding with TIG welding as the root pass equivalent to DU3? It seems that the bottom part of DU4 has an additional section compared to DU3; I’m not sure what this extra portion means
Some people say that both DU3 and DU4 involve single-sided welding with double-sided formation, with DU3 smoothing out the protrusions.
Root cleaning involves, after welding on the V-face, removing from the back side any areas that were not fully welded or did not fuse properly (usually by using air blasting), grinding those areas clean, and then welding on the back side again; this is equivalent to welding on both sides. If it cannot be done internally, use U3; a conscientious welder will usually use TIG welding for the root pass
I’ve learned that, generally, root cleaning is required for medium and thick plates, while thin plates can be welded through without such cleaning.
Backside root cleaning: Some weld metal and base material are removed from the side of the weld joint’s root in order to ensure complete penetration when welding continues from that side.