Research On Weldability,Microstructures And Mechanical Properties Of 9% Ni Steels Used For Giant LNG Cryogenic Storage Tank

Recently, China is playing clear energy strategy, LNG industry increased rapidly because of its low-cost storage and transportation. cryogenic 9Ni steel and its welding technology have a huge demand, But its assembling and welding technology are relay on foreign country, it has huge restricted to the development of our country's industry and energy's safe. So, master cryogenic 9Ni steel and its welding technology and push the national process of LNG equipment are urgent need to our country's energy development.9Ni steel as a material of inside storage worked at-162℃environment. So, in order to guarantee the safe of pressure vessel's welding, the research of the welding procedure qualification and microstructure has important significance.This article mainly research the domestically produced 9Ni steel's four kinds of welding process, including SMAW (shielded metal arc welding), GTAW (gas tungsten arc welding), SAW (submerged arc welding)and GMAW(gas metal arc welding), studies the microstructure and comprehensive mechanical properties of the weld joints which obtained via these kinds of welding procedure. and completes these four kinds of Welding Procedure Qualification.The structure property analysis mainly completed through the metallographic examination. It studies microstructure in the parent metal, emphatically, the welded joint and the welding heat-affected zone., and analyse the relation between the weld joint microstructure and welding procedure qualification.The comprehensive mechanical properties analysis mainly includes Cryogenic impact test, Vickers hardness test, the Tensile testing at ambient temperature and the Bending test. Studying the weld joint's comprehensive mechanical properties, establish the correct welding process evaluation.The test result indicated:the metallographic examination showed that the parent metal area performed the obvious Martensite structure, the welded seam area represented Austenite dendritic structure, the heat-affected zone manifested the tempered Martensite; Looking from cryogenic impact test fracture faces, it was tough tearing and following the massive strong nests. Vickers hardness test result showed that value satisfied with the standard EN288-3, completely, the request (<400); The cryogenic impact test demonstrated that there were close relation between low temperature toughness and the welding heat input value; the Tensile testing at ambient temperature and the Bending test indicated that shielded metal arc welding and gas metal arc welding should improve in later study work.