Abstract
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.
Highlights
Since January 2020, the International Maritime Organization (IMO) has been applying a stricter standard for the sulfur content of ship fuel oil, announcing a plan to restrict the sulfur content of ship fuel oil from the current level of 3.5% to 0.5% in 2020
This study focused the specific welding method and a material, FCAW and 9% nickel steel
An analysis based on various perspectives is needed to clearly identify the specific conditions that can have a similar bead geometry compared to the intermittent variables, and it is necessary to identify the phenomenon in which the structure of a weldment is hardened by matching the characteristics generated from the correlation between the partially divided bead geometry within a weldment to the dilution ratio of a weldment
Summary
Since January 2020, the International Maritime Organization (IMO) has been applying a stricter standard for the sulfur content of ship fuel oil, announcing a plan to restrict the sulfur content of ship fuel oil from the current level of 3.5% to 0.5% in 2020. The correlation between diverse variables and mechanical characteristics applied to the welding process of cryogenic steels such as STS series or Ni alloy series has been reviewed and the process problems and quality deterioration that occurred when those were used in the LNG-related equipment were reviewed. Similar size areas and heat-affected zones are derived intermittently even for different welding process variables, so the applicability of analysis and consideration limited to bead geometry in the actual field has been reduced In this way, an analysis based on various perspectives is needed to clearly identify the specific conditions that can have a similar bead geometry compared to the intermittent variables, and it is necessary to identify the phenomenon in which the structure of a weldment is hardened by matching the characteristics generated from the correlation between the partially divided bead geometry within a weldment to the dilution ratio of a weldment. We have proposed a multipurpose algorithm that can be systematically avoided
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