EFFECT OF Nb CONTENT ON MICROSTRUCTURE, WELDING DEFECTS AND MECHANICAL PROPERTIES OF NiCrFe-7 WELD METAL

doi:10.11900/0412.1961.2014.00288

Acta Metall Sin

2015, Vol. 51

Issue (2): 230-238 DOI: 10.11900/0412.1961.2014.00288

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EFFECT OF Nb CONTENT ON MICROSTRUCTURE, WELDING DEFECTS AND MECHANICAL PROPERTIES OF NiCrFe-7 WELD METAL

MO Wenlin^1,², ZHANG Xu¹, LU Shanping^1,²(

), LI Dianzhong¹, LI Yiyi¹

1 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016
2 Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016

Cite this article:

MO Wenlin, ZHANG Xu, LU Shanping, LI Dianzhong, LI Yiyi. EFFECT OF Nb CONTENT ON MICROSTRUCTURE, WELDING DEFECTS AND MECHANICAL PROPERTIES OF NiCrFe-7 WELD METAL. Acta Metall Sin, 2015, 51(2): 230-238.

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Abstract

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。

Key words: NiCrFe-7 weld metal ductility-dip-cracking Nb M₂₃C₆ MX mechanical property

Received: 29 May 2014

ZTFLH:

TG422.3

Fund: Supported by National Natural Science Foundation of China (No.51474203) and Key Research Program of Chinese Academy of Sciences (No.KGZD-EW-XXX-2)

About author: null

莫文林, 男, 1986年生, 博士生

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	Wenlin MO
	Xu ZHANG
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	Dianzhong LI
	Yiyi LI

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https://www.ams.org.cn/EN/10.11900/0412.1961.2014.00288 OR https://www.ams.org.cn/EN/Y2015/V51/I2/230

Fig.1 Vertical cross-section phase diagrams of NiCrFe-7 alloy at low (a) and high (b) magnification

Fig.2 Effect of Nb content on mass fraction of NbC (a) and M₂₃C₆(b) for NiCrFe-7 alloys

Table 1 Composition design of filler wires

Fig.3 Schematic diagram of weld joint (unit: mm, the rectangle area indicates the sampling position)

Fig.4 Morphologies of 0Nb weld metal at low (a) and high (b) magnification

Fig.5 SEM images (a~d) and EDS analysis (e, f) of 0Nb (a, c, e) and 1.1Nb (b, d, f) weld metal (Figs.5e and f correspond to the EDS analysis of precipitates in the rectangle areas in Figs.5a and b, respectively)

Fig.6 Statistics of the intragranular precipitate number in NiCrFe-7 weld metals with different contents of Nb

Fig.7 SEM images (a~e) and TEM image (f) of M₂₃C₆ on the grain boundaries in 0Nb (a), 0.4Nb (b), 0.7Nb (c), 0.85Nb (d), 1.1Nb (e, f) weld metals (The insert in Fig.7f shows the corresponding SAED pattern in the circle area)

Fig.8 Ductility-dip-cracking (DDC) morphology in 0Nb weld metal

Fig.9 Statistics of the total crack length in the weld metals with different contents of Nb

Fig.10 Tensile test curves for the weld metals with different Nb contents (σ—stress, ε—strain)

Fig.11 Fractographs of 0Nb (a, a1, a2, a3), 0.4Nb (b, b1, b2, b3), 0.7Nb (c, c1, c2, c3), 0.85Nb (d, d1, d2, d3), 1.1Nb (e, e1, e2, e3) weld metal (Figs.a1~a3, b1~b3, c1~c3, d1~d3 and e1~e3 correspond to the magnified images of areas in Figs.11a, b, c, d and e, respectively)

Fig.12 Morphologies of bending surfaces for 0Nb (a), 0.4Nb (b), 0.7Nb (c), 0.85Nb (d) and 1.1Nb (e) weld metals

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