Introduction to 9Ni steel

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9Ni steel is a medium-alloy steel with a W(Ni) content of 9%, developed in 1944 by the Product Research Laboratory of International Nickel Company in the United States. It is a low-carbon quenched and tempered steel whose microstructure consists of martensite and bainite. This type of steel exhibits good toughness and high strength at extremely low temperatures. Compared to austenitic stainless steels and aluminum alloys, it has a low coefficient of thermal expansion, is cost-effective, and can be used at temperatures as low as -196°C. Since research conducted in 1960 demonstrated that it could be used safely without post-weld stress-relief heat treatment, 9%Ni steel has become one of the main materials used for manufacturing large LNG storage tanks. The main characteristics of steel are high nickel content, high purity, high strength, high low-temperature impact toughness, and good weldability. Through research and development, steel can achieve the following performance parameters: 9Ni steel: ReH≥585MPa, Rm = 680~820MPa, A≥18%, at -196°C, AKv(T)≥150J, AKv(T)≥80J (EU standard). 9Ni steel is primarily used in the manufacture of liquefied natural gas (LNG) storage tanks and transport vessels. It can operate at temperatures as low as -196°C, boasts high strength, excellent low-temperature toughness, and good weldability, making it the material with the best toughness for use in cryogenic environments. The liquefaction temperature of natural gas is -162°C. The volume of liquefied natural gas (referred to as LNG) is 1/600 of that of its gaseous state, which requires high-quality materials for LNG storage tanks. The materials commonly used for such tanks include 9Ni steel, 5Ni steel, austenitic stainless steel, and aluminum-magnesium alloy steel. Among these, 9Ni steel is widely used in LNG storage tank systems due to its high strength, ease of processing, and excellent welding properties. 　　In 1952, the world’s first 9Ni steel storage tank was put into use in the United States. In 1960, research proved that it could be used safely without post-weld stress-relief heat treatment, and since then 9Ni steel has become one of the main materials used for manufacturing large-scale low-temperature storage tanks. The largest capacity of the first LNG storage tank built in Japan in 1969 reached 20×104 m3. In 2004, China’s first large-scale LNG cryogenic storage tank project began construction in Dapeng, Guangdong; the LNG projects that were later established in Fujian, Dalian, Ningbo and other places also used 9Ni steel. 　　9Ni steel is a typical low-carbon martensitic steel that is resistant to low temperatures (–196°C; the design temperature for LNG storage tanks is –162°C). Its low-temperature mechanical properties depend mainly on its chemical composition, particularly the contents of Ni and C; in addition, the toughness of 9Ni steel is also influenced by the purity of the steel and its microstructure. In terms of chemical composition, it has a high Ni content, which gives it certain hardenability. A decrease in carbon content can **reduce** the precipitation of carbides in tempered martensite, thereby improving low-temperature toughness, whereas an excessive carbon content leads to a severe deterioration in weldability and cold brittleness. Therefore, during the alloying process, the carbon content must be strictly controlled to keep it within a low-carbon range; at the same time, the contents of alloying elements such as P, S, Mn, Mo, and Cr also need to be kept at low levels. 　　To date, 9Ni steel has been used in LNG equipment for nearly 60 years. With the rapid development of the world economy and trade, liquid natural gas has become a new focus in the global oil and gas industry. To promote the diversification of energy supply and improve the structure of energy consumption, Offshore Oil Engineering Co., Ltd. has placed increasing emphasis on the development of the LNG industry, investing substantial human and material resources; as a result, it has quickly established itself as a key player in this field. However, the construction of LNG storage tanks also requires the resolution of a range of technical challenges. Research and analysis conducted on the characteristics of the 9Ni low-temperature steel used in LNG cryogenic storage tanks, as well as the problems that tend to occur during welding such as cold cracks, hot cracks, reduced low-temperature toughness, and arc magnetic deflection, indicate that the following key control measures can be adopted to achieve optimal welding results when using 9Ni steel: (1) Select welding materials with low hydrogen and low carbon content, so that the linear expansion coefficients of the welding material and the base metal are similar at both room temperature and high temperatures, thereby avoiding thermal stresses caused by uneven heating and cooling. 　　(2) Before welding, clean the surface of the weld groove by using organic solvents or grinding ； When the ambient temperature is below 5°C, preheating of the base material is necessary before welding ； Welding materials must be stored in strict accordance with the requirements and used as needed; they must not be left exposed to the air for long periods of time. 　　(3) Select the appropriate welding materials; use welders with a high nickel content. Even due to heat loss during welding and the dilution of the weld metal by the base material, an adequately high austenite structure is maintained, thereby preventing the formation of brittle martensite bands at the fusion line. 　　(4) Strictly control the line energy, generally keeping it between 0.7 and 3.0 kJ/mm, and prefer welding with a lower heat input. 　　(5) During processing and transportation, the base material should be kept away from magnetic materials. If residual magnetism remains, the 9Ni steel must be demagnetized before welding to ensure that the residual magnetism level of the base material is within the specified limits.