9. Prevention of End Cracking in One-Side Automatic Welding : Numerical Analysis of Transient Phenomena in Welding

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Recently one-side automatic welding has widely prevailed in the assembly stage of Japanese shipyards. As one of the problems of welding procedure, as well as any other welding techniques, cracking which sometimes occurs near the end of weld, has become an important problem. As regards the prevention of end cracking in one-side automatic welding, a new preventing method is proposed. In the process of welding, hydraulic jack should be used to control the opening displacement of the plates at the end of weld and furthermore specimen should be used with no bead placed extended over 2000 mm from the specimen end. Experimentally it is confirmed that end cracking could be prevented almost perfectly where the new preventing method is applied. In order to analyze this preventing method, on the other hand, the authors have proposed new calculation technique in the seam welding. The most significant feature of this technique differing from ordinary welding thermal stress analysis is that the conditions of calculation are simulated to be those of more realistic welding procedures. Then the following assumptions are imposed in order to calculate stress and strain during welding. 1) Before welding, two plates are only connected with tack welds and welding groove is free edge. 2) Stresses are always assumed to be elastic except in metal beyond 700℃ in which stiffness is taken as zero, so molten pool near the arc is free from stress. 3) When the arc reaches to a tack weld, the tack weld is melted away and redistribution of the forces is taken place. 4) As the arc proceeds, the temperature of deposit metal decreases at a certain portion, then melted metal is solidified and the metal at the portion now becomes stiff, so the portion should be changed to supported edge from free grooved edge beforehand. This calculation method is carried out by using the finite element method and it proves to be very effective for a time-depending welding phenomena. The following results are obtained. 1) End cracking in one-side automatic welding is greatly influenced by the displacement and its rate to which weld metal is subjected. 2) Cracking can be completely prevented by use of dynamical method whereby the end is externally restrained by a hydraulic jack and in addition by the method improving the bead shape and increasing hot ductility of weld metal by using of multiple electrodes welding process.3) To analyze transient phenomena in one layered butt joint welding, a new numerical calculation technique by using the finite element method is proposed. This method proves to be very effective for such a time-depending welding phenomena. 4) It is confirmed that the possibility of occurrence of end cracking in 20Y specimen seems to be seldom, according to the calculated results by using this numerical method. 5) When weld length is about 6,000 mm, changes in groove opening caused by the welding heat source are almost the same as those for welding actually done in shops. Therefore the crack-preventing method which is established in the case of specimen of 6,000 mm weld length can be directly applied to the assembly unit and this preventing method is also effective in the assembly stage.
社団法人日本船舶海洋工学会の論文

9. Prevention of End Cracking in One-Side Automatic Welding : Numerical Analysis of Transient Phenomena in Welding

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