Effect of Nb and Mo on the Microstructure, Mechanical Properties and Ductility-Dip Cracking of Ni-Cr-Fe Weld Metals

doi:10.1007/s40195-016-0469-z

Abstract

Abstract:

A series of Ni-Cr-Fe welding wires with different Nb and Mo contents were designed to investigate the effect of Nb and Mo on the microstructure, mechanical properties and the ductility-dip cracking susceptibility of the weld metals by optical microscopy (OM), scanning electron microscopy, X-ray diffraction as well as the tensile and impact tests. Results showed that large Laves phases formed and distributed along the interdendritic regions with high Nb or Mo addition. The Cr-carbide (M₂₃C₆) was suppressed to precipitate at the grain boundaries with high Nb addition. Tensile testing indicates that the ultimate strength of weld metals increases with Nb or Mo addition. However, the voids formed easily around the large Laves phases in the interdendritic area during tensile testing for the weld metal with high Mo content. It is found that the tensile fractographs of high Mo weld metals show a typical feature of interdendritic fracture. The high Nb or Mo addition, which leads to the formation of large Laves phases, exposes a great weakening effect on the impact toughness of weld metals. In addition, the ductility-dip cracking was not found by OM in the selected cross sections of weld metals with different Nb additions. High Nb addition can eliminate the ductility-dip cracking from the Ni-Cr-Fe weld metals effectively.

Key words: Nb, Mo, Laves phase, Microstructure, Mechanical properties

Xu Zhang,Dian-Zhong Li,Yi-Yi Li,Shan-Ping Lu. Effect of Nb and Mo on the Microstructure, Mechanical Properties and Ductility-Dip Cracking of Ni-Cr-Fe Weld Metals[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(10): 928-939.

Figures/Tables 20

Fig. 1 Schematic diagrams of the weld joint and test specimen: a sampling location, b impact specimen, c tensile specimen

Fig. 2 Welding plates under strong restraint conditions

Fig. 3 Optical micrographs of six weld metals: a WM-A(0Mo), b WM-B(2Mo), c WM-C(4Mo), d WM-D(6Mo), e WM-E(1.8Nb), f WM-F(4Nb)

Fig. 4 XRD pattern of the precipitates electrochemically extracted from the Nb-bearing weld metals: a WM-E(1.8Nb), b WM-A(2.5Nb), c WM-F(4Nb)

Fig. 5 Precipitates in the weld metals: a WM-E(1.8Nb), b WM-A(2.5Nb), c WM-F(4Nb). The compositions of a1,b1, c1 are presented in Table 3

Fig. 6 XRD patterns of the precipitates electrochemically extracted from the weld metals: a WM-A(0Mo), b WM-B(2Mo), c WM-C(4Mo), d WM-D(6Mo)

Fig. 7 Interdendritic precipitates in the Mo-bearing weld metals: a WM-A(0Mo), b WM-B(2Mo), c WM-C(4Mo), d WM-D(6Mo). The compositions of a1,b1,b2, c1, c2 and d1 are presented in Table 4

Fig. 8 Average ultimate strength of the Nb-bearing weld metals at room temperature and 350 °C

Fig. 9 SEM fractographs of the weld metals under tension at room temperature: a WM-E(1.8Nb), b WM-A(2.5Nb), c WM-F(4Nb)

Fig. 10 Micrographs of longitudinal section of the tensile rupture for the weld metals under tension at room temperature: a WM-E(1.8Nb), b WM-A(2.5Nb), c WM-F(4Nb)

Fig. 11 Average impact energy of Nb-bearing weld metals

Fig. 12 Average ultimate strength of Mo-bearing weld metals

Fig. 13 SEM fractographs of the weld metals under tension at room temperature: a WM-A(0Mo), b WM-B(2Mo), c WM-C(4Mo), d WM-D(6Mo)

Fig. 14 Micrographs of longitudinal section of the tensile rupture for the weld metals under tension at room temperature: a WM-A(0Mo), b WM-B(2Mo), c WM-C(4Mo), d WM-D(6Mo)

Fig. 15 Average impact energy of each as-welded deposited metal

Fig. 16 SEM micrographs of M23C6 distributing along the grain boundary a1, b1, c1, d1 and optical micrographs of longitudinal section of tensile rupture a2, b2, c2, d2: a1, a2 weld metal with 0 wt% Nb from the published article [23, 30], b1, b2 WM-E(1.8Nb), c1, c2 WM-A(2.5Nb), d1, d2 WM-F(4Nb)

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