Gyu Baek An

Effect of welding sequence to minimize fillet welding distortion in a ship’s small component fabrication using joint rigidity method:

This study was performed to investigate the effect of welding sequence on fillet welding distortion. It proposes a new model of joint rigidity method, which can determine the welding sequence for minimum welding distortion, through a reliability validation done via experiments and welding distortion analysis using the equivalent load method. A new joint rigidity method considers the gap between base plate and stiffener and the fillet welding sequence at left/right bead. Fillet welding sequence at left/right bead could be considered by setting welding beads at left/right as a separate solid element after welding sequence was drawn by new joint rigidity method. Analysis and experiment for welding distortion for the test specimen with different stiffener distances were conducted. It was confirmed that the experimental and analysis results were well-matched. The validity of welding sequence using joint rigidity method was followed after the comparison of the welding distortion for the small components for ship building. Note that the welding sequence by which welding distortion became maximum and minimum was drawn using joint rigidity method for the small assembly components. Welding distortion was also analyzed using equivalent load method. Analysis results revealed that the maximum welding distortion less than 10 mm was generated. Welding distortion less than around 5 mm occurred in welding sequence which minimized welding distortion compared with welding sequence, generating maximum welding distortion.

Development of High Strength and High Performance Linepipe and Shipbuilding Steels

Advanced TMCP products, such as X80/X100 linepipe steels and YS 460MPa grade shipbuilding steel, have been developed by POSCO. In order to apply X80/X100 linepipe to strain-based design (SBD) concept, the main issue is to prevent strain aging after thermal coating. In case of YS 460MPa shipbuilding steel, challenging study is necessary to increase plate thickness up to 80 mm without deterioration of strength and toughness. It was found that solute carbon started to segregate at dislocation tangles in coarse ferrite during and after UOE pipe forming, resulting in strain aging. And it was shown that the optimum microstructure of X80 and X100 steels was the mixture of acicular ferrite, bainitic ferrite and fine ferrite. Those X80 and X100 steels revealed good strain hardening resistance as well as reasonable compressive strain capacity. The developed YS 460MPa grade steel plate for shipbuilding showed a good mechanical property at quarter and mid-thickness and also an excellent weldability. It was confirmed that the developed steel had a high CTOD value both HAZ and base metal to prevent an initiation of brittle crack and a good crack arrestablity to suppress long distance propagation of brittle crack.