Microstructure And Properties Of Deposited Metal For 690 Ni - Based Alloy Electrode

Nuclear power is a sustainable supply of clean energy, which has become an important part of energy in our contry. When nuclear power developing rapidly, the welding materials of 690 Ni-based alloy with good corrosion resistance and stress corrosion cracking resistance applied in cuclear power equipment. Research has shown that the welding materials of 690 Ni-based alloy is sensitive to hot cracking, especially the Ductility Dip Cracking(DDC). Currently, though the welding materials of nickel base alloy have been reserch for long time in China, can’t realize nationalization. So the welding materials of 690 Ni-based alloy need to further research.The experimental materials is four groups of weld metals, which is three groups of different composition 690 nickel-based alloy electrode and Inconel152 welded by TPS-5000 welding machine with the method of one-side welding on 9Ni steel. Adopting optical microscope, scanning electron microscope and THERMO-CALC, the microstructure of the four groups of weld metals are analyzed. By hard test, tension test, impact test, bending test and intergranular corrosion test, the mechanical of the four groups of weld metals are analyzed. Finally, the strain-to-fracture(STF) test and high-temperature tension test were studied with the four groups of weld metals, and the mechanism of ductility dip cracking(DDC) was analyzed. The results show that microstructure is columnar crystals austenitic matrix and the weld metals were dominant with dendrites and interdendritic phases. The most of dendrites and interdendritic phases were (Nb,Ti)C or NbC. The number of precipitates increased with the increase of Nb and Ti contents in the weld metals. The addition of Nb can inhibit the growth of grain, make grain fine, increase grain boundary area, twist the grain boundary, inhibit the propagation of crack and improve hardness, plasticity, toughness and bending properties of weld metals. When the content of Nb is 1.92%, large precipitates transformed from NbC/(Nb,Ti)C into Laves phases and their sizes increased, reduced the plasticity, toughness and bending properties of weld metals.The result of testing for nickel based alloy ductility dip crack sensitivity show that the M23C6 precipitates will not present at grain boundary when the test temperatures are 700℃ and 1000℃. The fractures of STF test and tensile test are dimple fractures. When the test temperature is 900℃, M23C6(M=Cr) becomes larger and dispersed at grain boundary. The fractures of STF test and tensile test are intergranular fractures. M23C6 aggravates DDC. The incoherent M23C6/γ interface is prone to crack initially, which exacerbates DDC in nickel based alloy at intermediate temperatures. The addition of Nb in the weld metal could prevent the grain growth, refine grain and increase the area of grain boundary; Adding Nb can fix C and form MC(M=Nb,Ti) precipitates, which were present to "pin" grain boundary motion and make boundary more tortuous, reduce number of M23C6 and sensibility of DDC.