**1. Introduction**

With the global environmental regulations to prevent climate change, regulations on emissions of ships are intensifying. In the shipbuilding industry, the use of Liquefied Natural Gas (LNG) for marine fuel is continuously studied as an alternative to existing fossil fuels [1–5]. By using LNG as fuel, CO2 emission is reduced by about 20%, nitrogen oxides (NO*x*) by 80%, sulfur oxides (SO*x*) by 90% and particulate matter (PM) by 99%, in comparison to using Heavy Fuel Oil (HFO), which is the existing marine fuel. It enables Tier III compliance, which is the latest regulation from International Maritime Organization (IMO). In other words, in the case of ships using LNG as fuel, it is possible to operate eco-friendly vessels without the need for additional equipment, to reduce harmful exhaust gas when operating.

A total of four materials (9% Nickel, STS 304L, Al 5083-0 and Invar) are available for the low-temperature/cryogenic energy storage/transport vessels approved by International Code of the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) [6]. Moreover, mechanical properties are shown in Table 1.


**Table 1.** IGC Code list of cryogenic fuel tank material.

A553-1 is used as a material for cryogenic tanks, such as LNG, and has recently been used as an LNG fuel tank material because of its relatively higher yield strength/tensile strength than other materials. There are several factors that determine the thickness of LNG fuel tanks, among which the minimum yield/tensile strength is an important factor. A553-1 has a weak point in that the thickness of the tank becomes thicker due to weakened strength at the weld after welding [7]. When laser welding is applied, the thickness of the tank can be reduced by increasing the strength of this part. The base material used in this study is A553-1, and the chemical composition is shown in Table 2.

**Table 2.** The chemical composition of ASTM A553 Type 1 (9% nickel steel).


Laser is the abbreviation of "Light Amplification by Stimulated Emission of Radiation" and refers to the welding method applying the output of the laser beam. Laser welding has been studied for application to various materials due to its low welding deformation, easy automation and its deep, narrow heat-affected part [8–22]. The purpose of this study was to analyze the penetration characteristics of A553-1, according to the welding speed and power among the laser-welding parameters, and to improve the price competitiveness, by securing the weldability with good mechanical strength in the future.
