고출력 화이버 레이저를 이용한 LNG선용 스테인리스강의 용접성에 관한 연구

Abstract

These days, world wide interest about global warming and environmental pollution and exhausting fossil fuel which have been main energy source in all around the world. So many country have tried to find out the solution by investing new & renewable and clean energy. Therefore LNG have been widely used as a substitution of fossil fuel and clean energy that emits less pollutant like SOx, NOx. Therefore LNG consumption has been quickly raised and LNG carriers have been getting larger for decades. Our shipbuilding industry has been the most powerful in the world. On the other hand, it's also in the middle of stiff competition between developed nations with original technique and quickly progressing chinese shipbuilding companies. In this situation, increasing productivity is one of the most important factor to lead the industry.

Meanwhile, although the laser welding technique has been already used for various industry such as automobile, semiconductor, electronics and so on, not very widely applied in shipbuilding except for cruise ships. Laser is high density heat source, so it can make high speed welding with minimum heat input possible. Especially the high power fiber laser is recently commercialized and has high beam quality and the smallest system size compare with conventional laser due to it's unique oscillating mechanism. Because of these advantages it's thought that the most suitable heat source for LNG cargo tank welding precess which has to be conducted inside of ships. So we suggest fiber laser welding technique as a substitutional heat source of exiting plasma and arc welding. In this study fiber laser was used for welding of stainless steel for LNG carrier to applicate laser welding technique for shipbuilding industry. Used material was STS304L and STS316L which have low carbon less than 0.03% and their thickness was 1.2mm and 8mm respectively.

First of all, we carried out bead on plate welding changing defocused distance and shield gas-related factor such as kind of shield gas, nozzle angle, distance of nozzle tip from the laser incident point and flow rate. Through this experiment, penetration characteristic was analyzed to understand characteristic of fiber laser because there's a few research about it's basic quality. And we could find out it's distinctive nature in welding process. Optimal welding condition decided from the results was shield gas : Ar(25ℓ/min), nozzle angle : 40°, nozzle distance : 20mm and defocused distance(fd) : -4mm for STS316L, 0mm for STS304L.

Second, butt and lap welding was conducted changing laser power, welding speed, Then penetration characteristic was analyzed and optimal parameters for each materials and joint configuration was decided based on experimental results.

Third, mechanical properties of weld such as hardness, tensile strength and toughness were measured and compared to that of base metal. As a result same or better mechanical properties were obtained in weld.

Last, weldability, microstructure and mechanical properties of stainless steel welded by fiber laser were compared to those by plasma. As a result, although the laser welding has several times faster speed similar properties and smaller weld and heat affected zone were obtained. Consequently. this study proves the superiority of fiber laser welding for LNG carrier.

Increasing welding speed and controling defects technique are important factor related to productivity. Therefore, the achievement in this study can be utilized to prove the superiority of laser as a substitutional heat source of exiting plasma and arc.