CRYOGENIC STEEL PRODUCTION TECHNOLOGY X7Ni9: CURRENT STATE AND PROSPECTS

Abstract

With the global liquefied natural gas (LNG) market rapidly expanding, the development of reliable structural materials for cryogenic equipment is becoming increasingly important. X7Ni9 steel (1.5663 according to EN 10028-4), alloyed with 8.5–10% Ni, is one of the most sought-after materials for the construction of LNG storage and transportation tanks at temperatures down to –196 °C. This paper presents a comprehensive analysis of the current state of steel production technology, encompassing the metallurgical aspects of smelting, hot rolling, heat treatment, and welding. Particular attention is paid to the development of a unique combination of high strength properties and exceptional impact toughness at cryogenic temperatures, achieved through specialized heat treatment regimes. This paper examines the traditional quenching and tempering (QT) process, as well as the advanced double quenching with intercritical heating (QLT) process, which allows for an increase in impact toughness to 238 J at –196 °C by stabilizing residual austenite. A separate section is devoted to welding technologies, including modern laser and hybrid laser-arc welding methods, which minimize the heat-affected zone and ensure mechanical properties of welded joints comparable to those of the base metal. The paper also highlights domestic experience in developing a full production cycle for X7Ni9 steel in Russia as part of import substitution efforts: in 2022, Severstal successfully certified the production of cryogenic thick plates that meet European standards and outperform imported equivalents in several respects. The results of the study confirm that modern technological solutions can achieve a high level of material performance. Prospects for further development are associated with the use of QLT processing and innovative welding processes to expand the operating temperature range of steel, including for storing liquid hydrogen at –253 °C.