Research On Material And Technology Of Repair Welding Of 18Ni Maraging Strength Steel

18Ni maraging steel is a kind of ultra-high strength steel strengthened by ageing precipitation hardening on Fe-Ni martensite matrix which is carbon-free or extra-low carbon. It has excellent formability before ageing treatmentand almost non-deforming during aging, after aging the steel has high strength and high toughness.18Ni maraging steel is often used in important components of ship, aircraft, offshore-oil-drilling platform, etc. The components bear high stress and work in corrosive environment, weld area may crack and cause a lot of serious problems. How to repair weld cracks and breakage when emergency happens will be very meaningful. This thesis introduced the weldability and welding methods of 18Ni maraging steel, produced a kind of repair electrode, studied the electrode's welding parameters and hot treatment after welding, analysed the effect of alloy content on the properties of deposited metal, tested the mechanical properties of weld joints.Domestic and foreign similar Welding material of maraging steel can be divided into electrodes, solid welding wire with metal powder and flux-cored wire, etc. which are mainly used for overlaying metal mold surface. Maraging steel has good weldibility. Its welding methods including TIG welding, MIG welding, electron beam welding, resistance welding, etc, which has high welding quality compared with SMAW. Studies on the 18Ni maraging steel electrode are few, however, SMAW is more simple, flexible and adaptable for a variety of spatial location and irregular weld, which is an ideal repair welding method. So it has some potential and application prospect.18Ni maraging steel repair electrode developed in the thesis has excellent performance when welding, which has small welding spatter, low stomatal sensitivity. Hardness of surfacing layer after welding is low, about 30³5HRC. After aging, there is a great improvement of hardness, the maximum hardness can reach more than 54HRC. After butt welding tests, the composition, hardness, tensile strength, crack resistance of deposited metal can reach domestic and foreign similar welding material. The test results show that using the AT # spot electrode is feasible for repair welding at the right moment.After solution treatment, the deposited metal structure of the developed electrode is well-distributed, the hardness values are generally lower than the welded state, after aging, the hardness has a significant improvement and the hardness of the various parts is alike.The appropriate solution treatment process is 820℃×1h, and the best aging treatment is 500℃×3h. Considered the actual welding operation, the solution treatments is very difficult, In this paper taking into in, so mining.Welded directly to the limitation of using heat treatment.By adjusting the content of Ti, Al, Mo in the electrode, we can control the alloy elements content in deposited metal, thus obtain the best mechanical properties. Through the study we found that: With the increase of Ti content, the hardness of surfacing layer is also increase. When the content of Ti in electrode was at 1.0%, the hardness of surfacing layer reached a maximum. The tensile strength of deposited metal increases with increasing Ti content, elongation and impact energy at room temperature increases first and then decreases. As the content of Al increases, the tensile strength of the deposited metal is also increases, elongation and impact energy at room temperature decreased significantly.By microscopic analysis, the thesis proved the strengthen phases in deposited metal are mainly intermetallic compounds such as Ni₃Ti, Ni₃Al, Ni₃(Ti, Al), etc.