Weldability and Mechanical Behavior of the Thin Plate Cryogenic High Manganese Steel

Abstract

Recently, Due to the gas resources and shale gas resources development such as shipbuilding and shipping industry demand for LNG market is steadily increasing. In addition, Green-ship demand has increased because IMO strongly regulated ship exhaust gas regulations. Hence high-efficiency, electric propulsion, Green ship-related research has expanded. Because LNG propulsion ship is rated as the most realistic alternative, demand has been gradually increased. In the future, demand of the LNG cargo and LNG Tank is expected to increase significantly. The material used in LNG cargo and fuel tank must keep excellent ductile characteristics under the cryogenic environment, down to –163℃, in order to avoid the sudden catastrophic brittle fracture during the operation condition. STS, Ni steel and Al alloy used in LNG tank has a excellent cryogenic properties. Currently, High Manganese steel is considered to be alternative material. The conventional alloy steel are expensive because of high Ni content. On the other hand, austenitic High Manganese steel have the advantage of low cost and high yield strength because of high Mn content. In this study, the world's first 3t 18% High Manganese for adoption of LNG tank were carried out by FCAW. High Manganese with 3mm thickness is used for this study as base metal. The optimum welding parameters were voltage, ampere, shielding gas, flow rate, C.T.W.D and welding speed. The detailed parameters are as follows. Voltage : 25V, Ampere : 170~180A, Shielding gas : Ar and CO₂mixture gas, Flow rate : 20/min, C.T.W.D : 17~18mm, Welding speed : 34.3cm/min. The weldability was evaluated in terms of mechanical and metallurgical characteristic. The average yield and tensile strength of the FCA welded joints was 385.6 MPa and 598 MPa, which is approximately 91% and 63.2% of base metal. The fracture occurred in welded joints after tensile test. The Vickers hardness were found about 269HV for base metal, and 153HV for welded joints. The fracture occurred in welded joints. Welded joints quality was evaluated according to ISO 5817(Undercut, Excess weld metal, Excess penetration, Incorrect weld toe, Overlap, Linear misalignment). The result for estimate welded joints could be obtained quality level for stringent(B).