The Development Of NV5Ni And NV9Ni Steels For LNG Storage Under Cryogenic Environment

New technologies and equipment are emerging following the development of petrochemical industry.Gas liquefaction,dissociation,storage,transportation and application have been quite common around the world.Therefore,the advance of cryogenic technologies and equipment is driving the development of cryogenic pressure vessel steels.The main features of such steels include higher level of nickel content,higher purity and tensile strength,higer ductibility under cryogenic impact test an well as decent weldability.Using NV9Ni and NV5Ni as technical indicators,a total of 5 steel grades of 9Ni,9NiCu,7.5Ni,and 7.5NiCu series steels for storing LNG containers and 5Ni steels for storing LEG containers have been designed and developed,and have reached corresponding technical specifications.9Ni steel and 5Ni steel are traditional cryogenic container steels;9NiCu is newly developed as an ultrahigh-strength 9Ni steel with a yield strength of more than 710 MPa,-196℃ Charpv impact energy of more than 120 J,and 7.5Ni and 7.5NiCu of 9Ni steel.Alloy reduction products.Optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD)and electron backscatter diffraction(EBSD)were used to observe the fine structure of cryogenic container steel.Various types of steels were systematically heat-treated,and their mechanical properties and microstructures were observed and tested.The mechanism for maintaining low-temperature toughness in cryogenic environments was thoroughly studied,and storage of LEG and LNG containers steel chemical composition,heat treatment process and its relationship with the organization and performance is revealed.Combining laboratory research and pilot plant trials,a series of industrial production solutions for LEG and LNG containers and shipboard steels have been proposed.The developed steel grades can be applied to the construction of large-scale LNG and LEG storage tanks,and the implementation of energy structure adjustment and sustainable development in China.The key strategies are consistent and have very good development prospects.