采用相图计算、焊材设计、焊材制备、现场焊接和焊缝金属解剖的方法, 研究Nb含量对NiCrFe-7焊缝金属组织、缺陷和力学性能的影响. 结果表明, 随焊缝金属中Nb含量增加, 焊缝金属晶内MX (M=Nb, Ti, X=C, N)析出相增加, 晶界M₂₃C₆(M=Cr, Fe)的初始析出温度降低, 晶界M₂₃C₆析出相减少, 且M₂₃C₆由多列连续分布转变为单列离散分布; 焊缝金属中的高温失延裂纹(ductility-dip-cracking, DDC)的数量和尺寸减少; 焊缝金属的强度、塑性和弯曲性能显著提高。

Ni-based filler metal is one of the most important filler metals in building the key components of nuclear power plants, however, ductility-dip-cracking (DDC) and inclusion defects form easily in the weldment and need to be repaired afterward. The precipitation of M₂₃C₆ (M=Cr, Fe) at grain boundaries will promote the nucleation and propagation of DDC. Adding Ti can form Ti(C, N) and reduce M₂₃C₆ precipitate at grain boundaries, which reduces DDC in the weld metal. However, the increase of Ti content in filler metal will cause the inclusion defects. Nb replacing part of Ti in Ni-based filler metal is proposed in this work. The reduction of Ti in filler metal is to reduce the sensitivity of inclusion defects in the weld metal. Nb can form MX (M=Nb, Ti, X=C, N) precipitates to reduce the M₂₃C₆ and DDC in weld metal. The effect of Nb on the size, number, and location of MX and M₂₃C₆ in Ni-based weldment has been investigated systematically in this work. Phase diagram calculations show that Nb is an element forming high temperature MX precipitate, and its affinity with oxygen is poor and not easy to form oxide. According to the phase diagram calculations, five different filler metals are designed and made with 0, 0.4%, 0.7%, 0.85%, 1.1%Nb content. The results show that the intragranular precipitates are distributed along sub grain boundaries. The intragranular precipitate for the Nb-free weld metal is Ti(C, N), whereas the intragranular precipitate in the Nb-bearing weld metals is MX. For the increased Nb in weld metals, more MX is produced, and more C is fixed within the grain. As the Nb content increased in weld metals, the initial precipitation temperature of M₂₃C₆ decreases, the intergranular M₂₃C₆ precipitate decreases and M₂₃C₆ turns discreted at grain boundaries. As Nb content increases in weld metals, the total crack length of DDC decreases. When the Nb content is over 0.85%, little DDC is found in the weld metals. The addition of Nb can improve the tensile strength, plasticity and bending property of the weld metals。