Abstract:
Shape change and dimension change in association with the plastic flow of residual stresses are the perennial problems as far as welding is concerned. Apart from shape and dimensional changes the distortion poses problem of difficulties on further assembly and maintaining the aesthetics. Correcting distortion involves heavy cost and time and in many cases the correction becomes impossible. These difficulties prompt to develop methods to control distortion during various stages of fabrication. They involve theoretical prediction and proper sequencing to balance the heat input to control distortion rather than correcting it after occurring. It is to be noted that the prediction of distortion is not an easy task since it involves analysis of transient thermal stresses and computation of incompatible plastic strains.Hence in many, the practical application of proper sequence of weld would relieve us from the worry of distortion..
The objective was to control distortion during fabrication of a vessel which has many internals. Each internal has a number of stiffeners which were to be welded on the internal. Considering the amount of welding and high heat input, it is found out that the distortion would likely to be more. Further, to add to this problem, the stiffeners were to be welded only on one side of the all the internals and also the maximum thickness of internals were 8 mm. After a detailed analysis on heat input, size of the internal and one side welding, it was decided to devise proper weld sequence of each internal and also proper sequences for the internal to the main vessel welding. Accordingly weld sequences were devised for more than 20 internals. Due to heavy welding on one side, one of the internals got deformed slightly above the required levels. Since heating on the component was restricted, this deformation was corrected mechanically. After fabricating all the internal assemblies they were welded to the main shell with suitable sequences so the mismatching problem was avoided.
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