A new high strength 9% Ni steel with tensile strength 30% higher than conventional ASTM A553 Type I has been developed for cryogenic applications such as liquid natural gas (LNG) transportation and storage. Because of the high toughness requirements at cryogenic service temperatures, mechanical properties of the heat affected zone and deposited weld metal (WM) are key issues for successfully using this newly developed steel. The current investigation evaluated the weldability of high strength 9% Ni steel plates using the gas metal arc welding (GMAW) process with Ni-based alloy FM680 welding wire. The study concluded that average heat affected zone (HAZ) toughness of high strength 9% Ni steel exceeds 50J at room and −196 °C; and weld metal toughness values of FM680 welds are excellent at both room and cryogenic temperatures. Tensile testing of welded joints with FM680 welding wire shows promise for meeting the 900 MPa minimum tensile strength requirement for joining high strength 9% Ni steel.
With the expansion of natural gas production and transportation globally, steel producers have begun to offer new innovative materials to supply this market. To date, the steels offered focus on lowering costs through reducing alloy content as compared to traditional ASTM A553 Type I, also known as 9% Ni steel. As an alternative approach, a new steel has been developed that meets the toughness requirements of ASTM A553 Type I but with an increase of the minimum tensile strength from 690 MPa to 900 MPa (Fraley et al., 2018). By increasing the plate steel strength, a lower volume of steel and weld metal can be used providing additional benefits.
The strength of this newly developed high strength 9% Ni steel is 30% higher than that of conventional 9% Ni steel, that is commonly used for cryogenic applications such as LNG transportation and storage vessels. LNG tanks are fabricated using fusion welding processes. Therefore, the weldability of the new steel is a key issue for designers and fabricators. Because of the 900 MPa tensile strength of the new steel, it is a challenge to find a suitable welding consumable that meets both the requirements for mechanical strength and the stringent toughness at both room and cryogenic temperatures.
