Study on Delayed Cracking of a Dissimilar-Metal Weld
Date Issued
2009
Date
2009
Author(s)
Kuo, Hung-Ta
Abstract
In the present dissertation, simulation is carried out to study the likelihood of occurrence of delayed cracking in a load-carrying weld. In particular, a big weld joining together two separate heavy decks to construct a huge annular steel structure in a construction site is studied in detail. A two-dimensional axial symmetric model is built for finite element simulation. The multi-pass welding joining two base metals made of different materials, the iterated and coupled calculation between structural deformation and thermal-elasto-plastic process, the material phase transformations, the residual phase percentage after phase transformation, the structural constraint, and the residual hydrogen concentration are all taken into consideration in the simulation. A hydrogen concentration model resulted from a real shielded metal arc welding process is also considered in the third weld bead for analyzing the diffusible hydrogen distribution pattern. Fourteen nodes in three specific directions around the third weld bead are chosen for evaluating the likelihood of delayed cracking. From the simulation, variation histories of the residual stress, the residual hydrogen concentration, and the temperature gradient during cooling are obtained. They are used for evaluating the likelihood of occurrence of delayed cracks based on a so-called “Alan Glover-Brian Graville” model. The result indicates that, among the selected fourteen nodes, six will crack after welding processes are completed. Four of them are owing to that their residual stresses are higher than the ultimate strength; and the other two is attributed to the combined effect of residual stress and diffusible hydrogen concentration. For the latter case, cracks occurs during the cooling process even the residual stresses are not substantially high. The evaluation result is very similar to cracks found in a construction site in northern Taiwan.
Subjects
weld delayed cracking
residual hydrogen content
multi-passes weld
welding simulation
Type
thesis
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