Search Results (10)

The International Maritime Organization (IMO) is tightening regulations on air pollutants. Consequently, more LNG-powered ships are being used to adhere to the sulfur oxide regulations. Among the tank materials for storing LNG, 9% nickel steel is widely used for cryogenic tanks and containers due to its high cryogenic impact toughness and high yield strength. Hence, numerous studies have sought to predict 9% nickel steel welding distortion. Previously, a methodology to derive the optimal parameters constituting the Goldak welding heat source for arc welding was developed. This was achieved by integrating heat transfer finite element analysis and optimization algorithms. However, this process is time-consuming, and the resulting shape of the weld differs by ~15% from its actual size. Therefore, this study proposes a simplified model to reduce the analysis time required for the arc welding process. Moreover, a new objective function and temperature constraints are presented to derive a more sophisticated heat source model for arc welding. As a result, the analysis time was reduced by ~70% compared to that previously reported, and the error rates of the weld geometry and HAZ size were within 10% and 15% of the actual weld, respectively. The findings of this study provide a strategy to rapidly predict welding distortion in the field, which can inform the revision of welding guidelines and overall welded structure designs. Full article

Various regulations are being devised and implemented to prevent the environmental pollution that is threatening mankind. The International Maritime Organization has strengthened regulations on sulfur, a notorious pollutant, to prevent sea pollution. In addition, the production of LNG fueled ships is increasing. Among various metals, 9% nickel steel is widely used in the shipbuilding industry because it is advantageous in terms of material strength and cryogenic impact toughness. Various studies are being carried out to predict and prevent its distortion, caused by welding, in the design. To predict welding distortion during flux core arc welding, this study found a way to refine the parameters constituting the Goldak welding heat source. The optimal heat source parameters were derived by using BOP experiments, cross-sectional analysis, finite element analysis and global optimization algorithm. When re-analyzed and verified based on the values, an error of up to 6.3% was found between simulation results and experimental values. The process was improved by clarifying the objective function and reducing the range of candidate welding efficiencies during global optimization and the process efficiency was also improved by reducing analysis time with a simplified model. Therefore, it is thought that this study can contribute to the productivity improvement of LNG storage containers, helping engineers apply it immediately in the industrial field. Full article

The International Maritime Organization has adopted the reduction of carbon dioxide emissions from ships as an important priority, and is continuously strengthening its regulations on marine air pollution. By 2035, it is expected that LNG-powered ships will account for more than 50% of the available ships. Accordingly, the demand for equipment related to LNG-fueled ships is expected to grow as well, requiring the development of a lot of equipment. However, the characteristics of LNG-powered ships mean that they require a high level of reliability and long history of operating reliably. Even when a product is developed, numerous demonstrations and quality assurance measures are needed to reach the technological level ship owners and customers require. Therefore, an optimization procedure to determine the welding quality for 9% Ni steel is necessary. In this study, the heat input criteria that induce brittle fracture characteristics were analyzed to optimize the flux core arc welding process for 9% Ni steel used in the manufacture of LNG storage tanks. We developed an optimization algorithm (Welding Current, Arc Voltage, Welding Speed) that can select a group of fracture conditions by examining the tendency of the tissue to brittle fracture due to excessive heat input among potential quality issues of cryogenic steel. Capable of selecting the range in which quality deterioration occurs, determining quality of a weld and avoiding the range in which toughness degradation occurs, through which a process to derive high quality 9% Ni welds is proposed. Full article

Various international organizations and governments of many countries are making efforts to prevent environmental pollution, with the IMO (International Maritime Organization) reinforcing related regulations. With these regulations, equipment related to LNG-fueled ships, which have the greatest carbon dioxide reduction effect among eco-friendly ships, are expected to increase. Although the IGC code designates the materials that can be used for LNG containers, such as 304L stainless steel and 9% nickel steel, these materials have a tendency to deteriorate the tissue around the heat-affected zone due to excessive heat input. In this study, we analyzed the effect of brittle fracture in the weld zone and heat-affected zone after fiber laser welding and found that welding quality improved with control of the heat input. SVM discriminant analysis was applied to classify the groups in which brittle fracture and ductile fracture occurred. The shape of the penetration section, hardness in the welding zone and heat-affected zone, and fracture surface were selected as factors for discrimination; these values were determined under various welding conditions. With these data, we derived a regression model and multi-objective optimization algorithm to predict mechanical properties after welding, as well as the conditions necessary to prevent brittle fracture. Finally, the prediction models were verified, as the results of welding under the derived conditions were classified as ductile fracture group. Full article

The demand for LNG-powered ships and related equipment is rapidly increasing among major domestic and foreign carriers due to the strengthened IMO regulations on the sulfur content of ship fuel oil. LNG operation in a cryogenic environment requires a storage tank and fuel supply system that uses steel with excellent brittleness and fatigue strength. A ship using LNG is very sensitive to explosion and fire. For this reason, 9% Ni is often used, because ships require high quality products with special materials and structural technologies that ensure operability at cryogenic temperatures. However, research to derive uniform welding quality is urgent because the deterioration of weld quality in the 9% Ni steel welding process is caused by high process difficulty and differences in welding quality depending on a welder’s skill set. This study proposes a method to guarantee a uniform quality of 9% Ni steel in a fiber laser welding process by categorizing weld joint hardness according to the dilution ratio of a base material and establishing a standard for quantitative evaluation. Full article

As a result of strengthened sulfur content standards for ship fuel oil in IMO regulations, major domestic and foreign carriers have a high and growing demand for liquefied natural gas (LNG) powered ships and related equipment. For LNG operation in a cryogenic environment, a storage tank and fuel supply system that uses steel with excellent brittleness and fatigue strength is required. Ships that use LNG have a high vulnerability to explosion and fire. For this reason, 9% Ni is typically used, since a ship requires high quality products with special materials and structural technologies that guarantee operability at cryogenic temperatures. However, there is an urgent need for research to derive a uniform welding quality, since high process difficulty and differences in welding quality related to a welder’s skills can cause a deterioration of the weld quality in the 9% Ni steel welding process. For 9% Ni steel, the higher the dilution ratio of the base metal, the lower the strength. In order to secure the required strength, excessive dilution of the base metal should be avoided, and the relationship between dilution ratio and strength should be investigated. According to previous research, if it exceeds 25% it may be lower than the API standard of 363 MPa for hardening welds. Therefore, in this study, the flux cored arc welding process is performed by establishing criteria that can be evaluated based on the SVM method in order to determine the structure of the weld to be cured according to the dilution rate of the base metal. We would like to propose a multipurpose optimization algorithm to ensure uniform quality of 9% Ni steel. Full article

The environment of the global shipbuilding market is changing rapidly. Recently, the International Maritime Organization (IMO) has tightened regulations on sulfur oxide content standards for marine fuels and tightened sulfur oxide emission standards for the entire coastal region of China to consider the environment globally and use LNG as a fuel. There is a tendency for the number of vessels to operate to increase significantly. To use cryogenic LNG fuel, various pieces of equipment, such as storage tanks or valves, are required, and equipment using steel, which has excellent impact toughness in cryogenic environments, is required. Four steel types are specified in the IGG Code, and 9% Ni steel is mostly used for LNG fuel equipment. However, to secure safety at cryogenic temperatures, a systematic study investigating the causes of quality deterioration occurring in the 9% Ni steel welding process is required and a discrimination function capable of quality evaluation is urgent. Therefore, this study proposes a plan where the uniform quality of 9% Nickel steel is secured by reviewing the tendency of the solidification crack susceptibility among the quality problems of cryogenic steel to establish the criteria for quality deterioration and to develop a system capable of quality discrimination and defect avoidance. Full article

To prevent the contamination of the marine environment caused by ship exhaust gas, the demand for LNG (liquefied natural gas) fueled ships is increasing worldwide. A tank to store LNG at cryogenic temperatures is indispensable to such LNG-fueled ships. Since the materials used for LNG fuel propulsion tanks must have excellent mechanical properties such as impact toughness at cryogenic temperatures, the International Maritime Organization limits the IGC Code only to four types. Most of the tank materials for LNG-fueled ships ordered recently are adopting ASTM A553-1 material, but a systematic study to analyze the problem of quality degradation that may occur when welding A553-1 steel is required to secure the safety of cryogenic tanks. Therefore, in this study, among various quality problems, the tendency of weld solidification crack vulnerability is identified, and a decision system and optimization procedure are developed. In addition, a method of securing the welding quality of A553-1 steel was proposed by setting quality deterioration standards. Full article

The International Maritime Organization (IMO) is tightening regulations to reduce greenhouse gas emissions from ship operations. As a result, the number of vessels using liquefied natural gas (LNG) as fuel has increased rapidly. At this time, ASTM A553-1 (9% nickel steel) is being used as a tank material for storing LNG as fuel because of its higher strength than other cryogenic materials. Currently, shipyards are manufacturing LNG fuel tanks using the flux cored arc welding (FCAW) method using 9% nickel steel material. However, fabrication through FCAW has two drawbacks. The first is that the welding filler is 20 times higher cost than the base metal, and the second is that the total production cost increases because the thickness of the tank increases due to the strength drop near the heat affected zone (HAZ) after welding. The laser welding of A553-1, which does not require additional welding fillers and has no yield and tensile strength reduction in the HAZ, can overcome the drawbacks of FCAW and ensure price competitiveness. Through the study of Part I (penetration shape by bead on plate), the penetration characteristics of laser welding were studied and the optimized welding conditions of 15 mm thickness of A553-1 were obtained. With optimized conditions, butt laser welding tests of A553-1 material were conducted in this study, and mechanical properties, which are tensile/yield strength, hardness, bending strength, and impact property on the cryogenic temperature of the weld zone after laser beam welding, are confirmed by comparing those after FCAW. In the case of tensile/yield strength, hardness, and bending strength at weldment, the values of laser beam welding (LBW) are higher than those of FCAW, and the value of the impact test after FCAW is higher than that of LBW, but both values are satisfied in ASTM. Through these conclusions, it is confirmed that there are no mechanical property problems in replacing the existing FCAW with LBW. Full article

The International Maritime Organization (IMO) is tightening regulations, in order to reduce greenhouse gas emissions from ship operations. As a result, the number of vessels using Liquefied Natural Gas (LNG) as fuel has increased rapidly. At this time, ASTM A553-1 (9% nickel steel) is being used as a tank material for storing LNG as fuel, because it has higher strength than other cryogenic materials. Currently, shipyards are manufacturing LNG fuel tanks by using the Flux Cored Arc Welding (FCAW) method, using 9% nickel steel material. However, fabrication through FCAW welding has two drawbacks. The first is to use a welding electrode that is 20 times higher in cost than the base metal, and the second is that the total production cost increases because the thickness of the tank increases due to the strength drop near the Heat Affected Zone (HAZ) after welding. Laser welding, which does not require additional welding rods and has no strength reduction in the HAZ, can overcome the drawbacks of FCAW welding and ensure price competitiveness. In this study, it is confirmed the characteristics of the penetration shape of Bead on Plate (BOP) after various laser welding conditions as a basic study to apply laser welding to A553-1 welding. For this, penetration characteristics of A553-1, according to laser welding speed and power, which is a main factor of laser welding, are confirmed. Full article