1. Department of Materials Engineering - PUC-Rio; Department of Mechanical Engineering – UFMG; Department of Industrial Mechanics – IFMG; Materials Research Institute - Aalen University Luciana Ferreira 1, Sidnei Paciornik 1, Timo Bernthaler 2, Valter dos Santos 1, Mauricio Monteiro 1, Marcos H. P. Mauricio 1, Alexandre Bracarense 3, Ezequiel Pessoa 4 Characterization of Pores and Cracks in Underwater Welds by µCT and Digital Optical Microscopy Introduction Underwater wet welding suffers from harsh conditions that still make it hard to obtain welded joint with full structural quality similar to that obtained under atmospheric conditions and with the same classification according to AWS standard D3.6M:2010. Defects such as pores and cracks are detrimental to the weld mechanical properties. The presence of water during welding creates high cooling speeds and hydrogen absorption by the weld pool. In turn, the presence of hydrogen leads to the formation of cracks and pores in the weld metal. In the present work, x-ray microtomography (μTC) was evaluate as a tool to image and quantify pores and cracks in underwater weld samples. The visibility of these defects under different experimental conditions was evaluated and image processing procedures were established to segment and measures 3D parameters. ResultsConclusions Main Objective To evaluate the use of x-ray microtomography technique as a tool to visualize and to quantify pores and cracks in underwater wet weld samples. Welding Procedure  Mechanized gravity system, simulating the drag technique used by divers-welders, along with an electronic constant current type power source.  Simulation of the wet welding at 0.5 m depth: a tank with a capacity of 200 liters, filled with fresh water.  Simulation of the wet welding at 20 m depth: a hyperbaric welding chamber, capable of operating up to 20 bar.  Multipass welds were performed on a 19 mm thick A36 plate using 3.25 mm rutillic electrode in the flat position.  The V preparation : 3 mm root opening and 45º angle. Samples Two types of samples were cut from the original welded plates:  Blocks (5S1): 15x15x19 mm³ blocks containing both base metal (A36) and weld metal. The weld were performed with a mechanized gravity system in a hyperbaric chamber. They were mainly used to reveal pores.  Slabs (1N1): 2 mm thick slabs containing just weld metal machined from a larger block. The welds were performed with a mechanized gravity system in a tank. They were mainly used to reveal cracks, as porosity is essentially nil at this depth. Contact e-mail: sidnei@puc-rio.br Acknowledgments The authors are grateful to Markus Schwenger and Thomas Samtleben, from Aalen University, Germany, for their support in tomography and image file organization. The support of Allen Gu and S. H. Lau, from XRadia, USA, with high resolution μCT, is also acknowledged. Conditions5S11N1 Electrode typeRutillic Depth200.5 Diffusible hydrogen measured in samples welded at 0.5 m (ml/100g) 85.488.7 Samples Microtomography Conditions5S11N1 TomographGE VTome|X Source Settings (kV/μA)240/120120/60 Pixel Size (μm)305 Acquisition Time (s)0.750.5 Views/Angular Step (#/º)600/0.6800/0.45 Samples Sample 5S1 - Pores Sample 1N1 - Cracks  In this μCT evaluation of underwater wet welded samples it was shown that it is possible to visualize pores and cracks, their spatial distribution and orientation.  The results are consistent with the known behavior of these defects, but substantially extend the traditional 2D analysis performed in cross- sections.  When the goal is to foresee mechanical properties and the qualification of underwater welds under strict requirements, the 3D visualization and quantification of the defects is a very powerful tool.  The main limitations of μCT are the power of the tomograph, necessary to traverse large and representative samples of a dense material such as steel, and the compromise between sample size and resolution, to allow visualization of small defects.  Initial results (not shown) with an XRadia MicroXCT tomograph, with special x-ray optics, reached 2.17 μm resolution in 2 mm thick slabs, allowing an even finer visualization of cracks. A single reconstructed slice revealing pores Pore concentration in the weld metal (left) and pore alignment (right) Image slice, with cracks, before and after application of Kuwahara filter 3D model showing cracks