Liquified hydrogen is considered a new energy resource to replace conventional fossil fuels due to environmental regulations by the IMO. When building tank for the storage and transportation of liquified hydrogen, materials need to withstand temperatures of -253°C, which is even lower than that of LNG (-163°C). Austenitic stainless steel mainly used to build liquified hydrogen tank. When building the tanks, both the base material and welding zone need to have excellent strength in cryogenic condition, however, manual arc welding has several issues due to prolonged exposure of the base material to high temperatures. Laser welding, which has some benefits like short period of exposure time and decrease of thermal affected zone, is used many industries. In this study, laser bead on plate welding was conducted to determine the laser butt welding conditions for STS 304 and STS 316L steels. After the BOP test, cross-section observations were conducted to measure and compare four bead parameters. These tendency result of laser BOP test can be used as conditions laser butt welding of STS 304 and STS 316L steel.

Demand for research on the use of hydrogen, an eco-friendly fuel, is rapidly increasing in accordance with global environmental problems and IMO environmental regulations in the shipbuilding and marine industry. In the case of hydrogen, similar to liquefied natural gas, it has a characteristic that its volume decreases hundreds of times during phase transformation from gas to liquid, so it must be stored in a tank in the form of liquefied hydrogen for transport efficiency. The material of the liquid hydrogen tank is selected in consideration of mechanical properties and hydrogen embrittlement at cryogenic temperatures. In this study, welding research was conducted on STS316L material, which was most commonly used in the space industry. In this study, flux cored arc welding was performed under 4 welding conditions to derive the optimal welding conditions for STS316L material, and then mechanical properties of the welded part were compared and analyzed.

In this study, additive manufacturing of a functionally graded material (FGM) as an alternative to joining dissimilar metals is investigated using directed energy deposition (DED). FGM consists of five different layers, which are mixtures of austenitic stainless steel (type 316 L) and low-alloy steel (LAS, ferritic steel) at ratios of 100:0 (A layer), 75:25 (B layer), 50:50 (C layer), 25:75 (D layer), and 0:100 (E layer), respectively, in each deposition layer. The FGM samples are successfully fabricated without cracks or delamination using the DED method, and specimens are characterized using optical and scanning electron microscopy to monitor their microstructures. In layers C and D of the sample, the tensile strength is determined to be very high owing to the formation of ferrite and martensite structures. However, the elongation is high in layers A and B, which contain a large fraction of austenite.

In the powder bed fusion (PBF) process, a 3D shape is formed by the continuous stacking of very fine powder layers using computer-aided design (CAD) modeling data, following which laser irradiation can be used to fuse the layers forming the desired product. In this method, the main process parameters for manufacturing the desired 3D products are laser power, laser speed, powder form, powder size, laminated thickness, and laser diameter. Stainless steel (STS) 316L exhibits excellent strength at high temperatures, and is also corrosion resistant. Due to this, it is widely used in various additive manufacturing processes, and in the production of corrosion-resistant components with complicated shapes. In this study, rectangular specimens have been manufactured using STS 316L powder via the PBF process. Further, the effect of heat treatment at 800 °C on the microstructure and hardness has been investigated.

The penetration depth, bead height, width, and internal porosity were analyzed to select the perfect penetration conditions for the STS316L tube material with an outer diameter of 38.1mm and a thickness of 3.4 mm. The welding conditions to secure a penetration depth of 3.4mm or more were selected. In addition, a welding range in which underfill does not occur was selected. The range of the selected conditions is the condition of a welding speed of 0.75 to 1.25m/min with an output of 2.0kW. The selected welding conditions were applied to STS316L tube orbital welding, and as a result of cross-sectional inspection after welding, a welded part of less than 4% of complete penetration and porosity was secured. The strength of the weld was measured to be more than 800kgf, and the hardness of the weld was found to decrease compared to the base material. The decrease in the hardness of the weld is judged by the annealing effect of the heat treated base material.

As the part of efforts to respond to the environmental pollution, the demand for clean energy is increasing. Natural gas is one of the most realistic alternatives, and interest in the storage and transportation containers to utilize the natural gas is growing. The production area of ​​natural gas is limited, and in Korea, it is imported in the form of liquefied natural gas. LNG is a cryogenic state with a vaporization point of -163°C, so the ordinary metal cannot be used due to its brittleness. The international maritime organization (IMO) defines the metals that can handle LNG in the IGC Code, and the research is ongoing. This study was a preliminary study to check the weldability of related metals and confirmed the shape of bead on plate (BOP) during the laser welding for each material. In part I, a study was conducted on the high manganese steel, and in part II, a study was conducted on the materials of STS304L and STS316L among the stainless steel. Based on this study, it can be used as an optimal welding condition for the butt and fillet welding.

In this study, STS316L powders prepared by gas atomization are used to manufacture bulk structures with dimensions of 10 × 10 × 10 mm3 using selective laser melting (SLM). The microstructures and hardness of the fabricated 316L stainless steel has been investigated with the laser beam overlap varied from 10% to 70%. The microstructures of the fabricated STS316L samples show a decrease in the balling and satellite of powders introducing defect in the bulk samples and the porosity caused by the gap between the molten metal pools disappearing as the overlap ratio increases, whereas a low overlap ratio results in significant balling and a large amount of isolated powders due to the increased gap between the melt pools. Furthermore, the highest value in Vickers hardness is obtained for the sample fabricated by 30% overlapped laser beams. These results show that the overlap ratio of laser beams in the SLM process should be considered as an important process parameter.

As wrought stainless steel, sintered stainless steel (STS) has excellent high-temperature anti-corrosion even at high temperature of 800ºC and exhibit corrosion resistance in air. The oxidation behavior and oxidation mechanism of the sintered 316L stainless was reported at the high temperature in our previous study. In this study, the effects of additives on high-temperature corrosion resistances were investigated above 800ºC at the various oxides (SiO2, Al2O3, MgO and Y2O3) added STS respectively as an oxidation inhibitor. The morphology of the oxide layers were observed by SEM and the oxides phase and composition were confirmed by XRD and EDX. As a result, the weight of STS 316L sintered body increased sharply at 1000oC and the relative density of specimen decreased as metallic oxide addition increased. Compared with STS 316L sintered parts, weight change ratio corresponding to different oxidation time at 900oC and 1000oC, decreased gradually with the addition of metallic oxide. The best corrosion resistance properties of STS could be improved in case of using Y2O3. The oxidation rate was diminished dramatically by suppression the peeling on oxide layers at Y2O3 added sintered stainless steel.

In this study, nitrogen ions were implanted into STS 316L austenitic stainless steel by plasma immersion ion implantation (PIII) to improve the corrosion resistance. The implantation of nitrogen ions was performed with bias voltages of −5, −10, −15, and −20 kV. The implantation time was 240 min and the implantation temperature was kept at room temperature. With nitrogen implantation, the corrosion resistance of 316 L improved in comparison with that of the bare steel. The effects of nitrogen ion implantation on the electrochemical corrosion behavior of the specimen were investigated by the potentiodynamic polarization test, which was conducted in a 0.5 M H2SO4 solution at 70 oC. The phase evolution and texture caused by the nitrogen ion implantation were analyzed by an X-ray diffractometer. It was demonstrated that the samples implanted at lower bias voltages, i.e., 5 kV and 10 kV, showed an expanded austenite phase, γN, and strong (111) texture morphology. Those samples exhibited a better corrosion resistance.

In this study, porous stainless steel (STS316L) sintered body was fabricated by powder metallurgy method and its properties such as porosity, compressive yield strength, hardness, and permeability were evaluated. 67.5Fe-17Cr-13Ni-2.5Mo (wt%) powder was produced by a water atomization. The atomized powder was classified into size with under 45 μm and over 180 μm, and then they were compacted with various pressures and sintered at 1210oC for 1 h in a vacuum atmosphere. The porosities of sintered bodies could be obtained in range of 20~53% by controlling the compaction pressure. Compressive yield strength and hardness were achieved up to 268 MPa and 94 Shore D, respectively. Air permeability was obtained up to 79 l/min·cm2. As a result, mechanical properties and air permeability of the optimized porous body having a porosity of 25~40% were very superior to that of Al alloy.

In this study, analysis on the oxidation behavior was conducted by a series of high-temperature oxidation tests at both 800oC, 900oC and 1000 in the air with sintered STS 316L. The weight gain of each oxidized specimen was measured, the oxidized surface morphologies and composition of oxidation layer were analyzed with Scanning Electron Microscope-Energy Dispersive x-ray Spectroscopy (SEM-EDS), finally, the phase change and composition of the oxidized specimen were shown by X-Ray Diffraction (XRD). As a result, the weight gain increased sharply at 1000oC when oxidation test was conducted for 210 hours. Also, a plentiful of pores were observed in the surface oxidation layers at 900oC for 210 hours. In addition, the following conclusions on oxidation behavior of sintered STS 316L can be obtained: Cr2O3 can be formed on pores by influxing oxygen through open-pores, (Fe0.6Cr0.4)2O3 can be generated on the inner oxidation layer, and Fe2O3 was on the outer oxidation layer. Also, NiFe2O4 could be precipitated if the oxidation time was kept longer.

In this study, STS 316L powders with 3 wt.% Cu and 1 wt.% Sn known as corrosion-resistance reinforce- ment elements, are prepared to make different kinds of specimens, in which, 3 wt.% Cu and 1 wt.% Sn are added in different forms by mixing, alloying and fully alloying. After sintering in the same condition, the corrosion resistance, wear resistance and their mechanical properties of specimens are tested respectively. According to the comparison, STS 316L specimen sintered at 1270o C showed the most excellent mechanical property: HRB 78 (hardness), 1130.7 MPa (RCS), 26.6% (Fraction Wear), It was similar with the specimen made of STS316L and fully alloyed Cu and Sn pow- ders, meanwhile, the latter one appears the best corrosion resistance, 75hrs-salt immersion test results. In addition, the specimens with Cu and Sn powders additive showed relatively worse wear resistance in compared with STS316L spec- imen.

The purpose of the present study is to investigate the influence of thermal debinding and sintering conditions on the sintering behavior and mechanical properties of PIMed 316L stainless steel. The water atomized powders were mixed with multi-component wax-base binder system, injection molded into flat tensile specimens. Binder was removed by solvent immersion method followed by thermal debinding, which was carried out in air and hydrogen atmospheres. Sintering was done in hydrogen for 1 hour at temperatures ranging from 1000℃ to 1350℃ The weight loss, residual carbon and oxygen contents were monitored at each stage of debinding and sintering processes. Tensile properties of the sintered specimen varied depending on the densification and the characteristics of the grain boundaries, which includes the pore morphology and residual oxides at the boundaries. The sinter density, tensile strength (UTS), and elongation to fracture of the optimized specimen were 95%, 540 MPa, and 53%, respectively.