| 9 Nickel steel was developed by American in the early 20th century. Which cryogenic steel and the content of nickel was 9%, due to its high strength and good toughness at -162℃.9 Nickel was widely used to build LNG tanks. However, when welding 9 Nickel steel plate, there was low temperature toughness brittle zone in the welding joint coarse grain near fusion line, prone to cracking and damage. So to ensure the safety of LNG tank, avoiding to calamity accident happen, this paper will study micro-structure and performance on welding joint with 9 Nickel steel by mechanical tests such as the joint tensile、bending、impact test and hardness experiments, microscopic test instruments such as Optical Microscope (OM), Scanning Electron Microscope (SEM), Transmission Electron Microscopy(TEM). Electron Back scattered Diffraction. In this paper, experimental design content mainly includes the welding thermal simulation and the actual welding parts.Adopting welding thermal simulation technique, respectively to simulate the single and double channel welding thermal cycle, set up different peak temperature and cooling time t8/5,cryogenic impact test and micro-structure analysis on the welding thermal simulated specimens, The experimental results show that:(1) Simulation of single channel welding thermal cycle, micro-structure of welding thermal simulation sample on peak temperature of 1350℃ and 950℃ were mainly lath martensite, poor impact toughness, fracture mechanisms belongs to brittle fracture; when peak temperature was 700℃, the retained austenite appear in the micro-structure, impact toughness become better, fracture mechanisms belongs to the ductile fracture. (2) Simulation of single channel welding thermal cycle, when peak temperature was consistent, cooling time t8/5 changes, martensite lath micro-structure.. account of a large angle grain boundary and grain size can be change to some extent. (3) Simulation of double channel welding thermal cycle, when cooling time was 15s, micro-structure of welding thermal simulation sample on the second peak temperature of 1350℃ were mainly lath martensite, when the second peak temperature become 700℃, the retained austenite appear in the micro-structure, when the peak temperature changes, account of a large angle grain boundary and grain size did not changed obviously, Impact toughness was general, fracture mechanisms belongs to brittle fracture. On the basis of welding thermal simulation, testing and analyzing cryogenic performance of 9 Nickel steel. Then 9 nickel steel was carried respectively out by manual arc welding and submerged arc welding. Analyzing welding process performance of 9 Nickel and the influence of welding joint’s micro-structure and performance through the joint tensile, bending and impact test, micro-structure observation and fracture analysis. The experimental results show that:(1) LNG storage tank with 9 nickel steel at-196℃ has better strength better elongation and better toughness. (2) 9 nickel steel was carried respectively out by two welding processes, have access to better welding joint. With the observation and analysis the micro-structure of welding metal and heat affected zone, with the two welding processes, welding metal micro-structure consists of austenite and precipitated phase, Crystal morphology of welding metal were dendritic crystal and cellular dendrites, and has a certain direction welding heat affected zone micro-structure was composed of lath martensite. (3) with the two welding processes, welding joint strength was larger, bending performance was better, low temperature impact toughness was better, and no embrittlement appearance, the chemical composition matching between welding metal and LNG Storage tank with 9 nickel steel was better. (4) Compared with manual arc welding, with the submerged arc welding, welding metal cellular crystal was small, uniform distribution, segregation scattered, in welding heat affected zone, lath martensite become thin, lath beam increased, low temperature impact toughness become better. |
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