Study On The Welding Material And The Microstructure And Properties Of The Welded Joint In 9Ni Steel

The technology of welding of 9Ni steel was still mastered by minority developed countries and the manufacturing technology and market was controlled by minority Transnational Corporation for welding materials of kitting nickel, which will lead to depend on foreign country in design and manufacture of LNG tanks with period long, cost high, application and extension of Domestic 9Ni steel disadvantageous. So it is very important for domestic 9Ni steel to develop its kitting welding materials and technology.In the paper submerged arc welding wire of 9Ni steel was studied. Deposited metal gainned possessed high strength (yield strength Rp0.2≥450MPa),tensile strength Rm≥720MPa) and excellent low-temperatrue toughness, satisfying deposeited metal of 9Ni steel to requirement of strength and toughness. Applied optical microstructure and SEM and so on to study microstructure of deposited metal and fractographs, the results showed that microstructure was consisted ofγtype solid solution and precipitation phase, fracture of deposited metal had two area: fibrous zone and shear lip. Type of fracture belonged to fracture of toughness.The three method of welding—tungsten inert gas arc welding (TIG) and shielded metal arc welding (SMAW) submerged arc welding (SAW) and foreign welding materials was applied to 9Ni steel and microstructure and porperties of joint was studied to analyse domestic of weldability of 9Ni steel. The result showed that weldability was excellent to 9Ni steel, three usability of weldingcan adapt to requirement of welding. Applying TIG to 9Ni steel crosswise tentile strength of welding joint was 745Mpa and situation of crack was base metal, which strength of weld bead above base metal. Center of weld bead, fusion-line and 3mm outside of fusion-line was measured in low temperature test, value of center of weld bead was minimum and average value was only 62J, but this value can satisfy the requriement of British system BS7777 to Akv/J(-196℃)of LNG tank weld joint >35J. Compared to TIG, low temperature impact toughness of weld bead in SMAW decreased to 50J with remained minimum. Three heat input was applied in SAW. With the increase of the heat input, tensile strength of welding joint incersed then decresed, while low temperature impact incresed. Under the one heat input, impact toughness of base metal was maximum, HAZ above meat bead. The results can be used as theoretic basis for making welding procedure of 9Ni steel.