IT-15-P-2661 Evaluating the detection of cracks in underwater wet welds by x-ray microtomography

Despite efforts to improve the mechanical properties of wet welds, welding in direct contact with water still presents critical problems. High cooling rates and the presence of hydrogen derived from water dissociation leads to the formation of defects, such as pores and cracks in the weld metal (WM) and in the heat affected zone (HAZ) which adversely affect mechanical properties. The occurrence of hydrogen assisted cold cracking is considered as one of the most important factors for the usual low ductility in the WM [1, 2].
During cooling, weld beads contract both in transverse and longitudinal directions. It is well established that longitudinal contractions are responsible for higher residual stress after welding. As a consequence, the low WM toughness associated with hydrogen embrittlement can lead to nucleation of cracks with a predominant orientation transverse to the weld axis.
In previous works, both Optical Microscopy (OM) [3] and x-ray microCT [4,5], linked to Image Analysis (IA) have been used to characterize crack size, density, shape and orientation. Cracks are challenging to image and measure from microCT due to their strong anisotropy and the small dimension of the tip, which can go beyond the resolution of microCT.
In this work, a weld sample with varying cross-section (Fig. 1) was imaged by microCT before and after being submitted to a tensile test up to failure. The variation in strain due to varying thickness led to changes in crack tip opening and crack length. Fig. 2 shows a typical reconstructed layer of the weld with cracks. The presence of noise and limited contrast hinder the detection of cracks. After noise filtering and segmentation the cracks were rendered, as shown in Fig. 3, and measured, allowing the estimation of detection limits for this kind of defect by microCT.

REFERENCES

1. A. Q. BRACARENSE et al., “Comparative study of commercial electrodes for underwater wet welding”, (In International Congress of the International Institute of Welding, São Paulo, 2008).
2. V. R. SANTOS et al., “Recent Evaluation and Development of Electrodes for Wet Welding of Structural Ship Steels” (In 29th International Conference on Ocean, Offshore and Arctic Engineering, Shangai, 2010).
3. M. H. P. MAURICIO et al., “Quantitative Hydrogen Cracking Evaluation by Digital Optical Microscopy” (In IMC17, Rio de Janeiro, 2010).
4 . S. PACIORNIK ET al. “Characterization of Pores and Cracks in Underwater Welds by ct and Digital Optical Microscopy”. Proc. 1st International Conference on 3D Materials Science, 2012. p. 177-182.
5. PADILLA, E. et al . Image analysis of cracks in the weld metal of a wet welded steel joint by three dimensional (3D) X-ray microtomography. Mat Charac, 83, 139-144, 2013.