Formation Mechanism And Control Method Of Inclusions In 9Ni Weld Joint

With the acceleration of modernization in China,the reform of new energy is imminent.Because of the large reserves and the main products after combustion,natural gas has attracted much attention in the field of natural gas development and utilization.Because the natural gas distribution in place with much land and few people,or buried in the seabed,how to transport and store natural gas is a difficult problem.The transportation of natural gas after liquefaction will greatly improve the transport efficiency.However,the temperature of liquid natural gas is-161.5?,and very few low-temperature-steels can maintain excellent property at such low temperature.9Ni steel has good mechanical properties at low temperature,and the ductile brittle transition temperature is close to-200?,which can fully meet the transportation and storage problem of liquefied natural gas.At present,our country has mastered related technologies for 9Ni steel manufacturing,but its welding materials are still not available,and we need to rely on large quantities of imports.Therefore,in this paper,the widely used submerged arc welding and manual welding materials for 9Ni steel are studied.The inclusions and precipitates that affect the properties of deposited metal are analyzed,so as to achieve the localization of 9Ni steel welding materials.Thermodynamic calculation of inclusions and precipitates that may be generated during metallurgical reaction of deposited metal welding pool is carried out.It is believed that the pool reaction will generate oxide and sulfide inclusions.The precipitates are multiple MX?M=Nb,Ti;X=C,N?.Because the deposited metal is nickel based,it is easy to form eutectic products with low melting point with sulfur;Segregation of solute elements titanium and niobium will generate hard and brittle Laves phase;High temperature carbides and matrix?may produce molybdenum rich low temperature carbides under the action of thermal cycling,Such as M₆C.Oxide inclusions and precipitates with high melting point can refine grain and improve microstructure,The low melting point eutectic product will cause the crystallization crack.In this thesis,two ways of submerged arc welding and manual welding are used to weld9Ni steel.The test results show that:The microstructure of deposited metal is austenite under the two welding methods.The crystallization mode is dendrite and its directivity is obvious.Microstructural display that inclusions and precipitates are mostly distributed between dendrite arms and grain boundaries.The precipitation inside the grain has little effect on the properties of deposited metal.In submerged arc welding,with the increase of heat input,the metallurgical reaction of molten pool is more thorough.The weld time is prolonged at high temperature,the welding speed is slower,the cooling rate is slower,the grain growth is coarsening,and the dendrite widens.Heat input at 15kJ/cm,The tensile strength of 724MPa,yield strength 474MPa,elongation and contraction rate of around 45%,the side bend is good,and the low temperature impact is the highest,which is 68J.In manual welding,the effect of decarburization and desulphurization is weaker than that of submerged arc welding.The higher sulfur content leads to the production of low melting point sulfide,and the granular precipitation is beneficial to the deposited metal.With the increase of heat input,the precipitates distribute in chains or lamellae between dendrite arms,and the dendrite widens seriously,which can damage the properties of deposited metal.Impact fracture scanning showed that the high dimple oxide in the dimple adsorbed carbide compound inclusions.