The Analysis On Mechanism Of Transverse Crack In Thick Plate Welding

In recent years, more and more transverse crack was found in the manufacturing of thick plate welding structure in the application of offshore platforms. The majority viewpoint classified this phenomenon into cold crack. But according to method to control the cold crack, the transverse crack could not be eliminated completely. So that, the main influence factors of transverse crack defects should be thorough researched. Eliminating of Transverse crack is of significance for engineering practice. In this paper, the thorough researches of thick plate welding transverse crack formation mechanism were combined with case analysis.The diffusible hydrogen measurement was performed on the single welded layer sample with different residence time, the multi-pass weld with different interpass temperature and with different interpass surface treatment. The results showed that surface treatment has great influence on diffusion hydrogen escape. And then, imaging analysis and simulation methods was used on diffusible hydrogen in the five-pass analysis. The results indicated that the diffusible hydrogen in the cross-section of five-pass weld spill over at a position that is of 0.4-0.5 times the height of the full weld. The simulation result agrees well with the imaging analysis. Finally, the most commonly used Y-groove test and G-BOP test was simulated.The inhomogeneity of the microstructure and mechanical properties of multi-pass weld was studied. Firstly the difference in mechanical properties of the PF and the AF in the columnar grain was characterized by nanoindentation test. Secondly a small sized specimen was subjected to a tensile test with slow strain rate, and the finite element method was also performed. The results showed that that the PF at grain boundaries differs significantly with the AF. This aspect is not negligible for the crack analysis. Crack always formed at the PF that sustained large strain. The finite element analysis showed that the hydrogen concentration was redistributed due to the local stress concentration and microstructure inhomogeneity, and peak hydrogen concentration reached 21.6% higher than the initial level in the position where stress concentration was greater.The effect of the different inclusion on the formation of the crack was studied by failure analysis. The finite element method was also applied to analyze the distribution of stress, strain, hydrogen concentration of the sample having different inclusions. The results showed that in the case of hard metal matrix(i.e. high elastic modulus) containing soft inclusion, it is the edge of the inclusion that is more susceptible. The stress concentration led by inclusion can engender the redistribution of hydrogen, in which the hydrogen concentration level can be up to three times as the initial.On the basis of the above mentioned research of the transverse weld crack, The conclusion was proposed,transverse cracks caused by the peak concentration of the local diffusible hydrogen, by the inhomogeneity of the microstructure and mechanical properties of the weld, by inclusion and segregation in the weld metal and base metal and by the excessive strain in the multi-pass weld.