Microstructure and Its Influence on the Mechanical Properties of Ni-28W-6Cr-Based Alloy-Welded Joints by GTAW

doi:10.1007/s40195-019-00881-8

Abstract

Abstract:

Microstructure and mechanical properties of Ni-28W-6Cr alloy-welded joints produced by gas tungsten arc welding were investigated in this work. Results showed that original fine-grain base metal near fusion line totally transformed into coarse heat affected zone after welding. Carbides with different shapes were found in the weld metal and base metal, which all were determined as M₆C carbides. In comparison with carbides in base metal, M₆C carbides in weld metal are rich in Si and Cr but deficient in W. Moreover, M₆C carbides are extremely scarce and hard to be detected in weld metal. Mechanical tests show that the hardness value of weld metal is only about 60% of base metal; the yield strength and tensile strength of welded joint are much lower than those of base metal due to the absence of carbides in weld metal.

Key words: Welding, Ni-W-Cr alloy, Carbides, Microstructure, Hardness, Tensile properties

Shuang-Jian Chen, Xiang-Xi Ye, D.K. L.Tsang, Li Jiang, Chao-Wen Li, Kun Yu, Zhi-Jun Li . Microstructure and Its Influence on the Mechanical Properties of Ni-28W-6Cr-Based Alloy-Welded Joints by GTAW[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(8): 1032-1040.

Figures/Tables 12

Table 1 Chemical composition of Ni-28W-6Cr alloy

Element	Si	Cr	W	Fe	C	Mn	P, S	Ni
Content (wt%)	0.31	6.85	27.91	0.601	0.04	0.34	0.004	Bal.

Table 2 Welding parameters

Current (A)	Welding speed (mm/min)	Pulse frequency (Hz)	Peak pulse duration (%)	Base/peak pulse value (%)	Gas flow rate (L/min)
150	100	2.5	50	50	15

Fig. 1 Dimensions of the tensile specimens

Fig. 2 OM images of microstructure of welded joint: a overall morphology of welded joint, b base metal and HAZ, c weld metal

Fig. 3 SEM images a and all-Euler map b of the welded joint measured by EBSD. Microstructures of BM c, HAZ d, WM e under high magnification

Fig. 4 XRD patterns of base metal and weld metal. The inset was zoomed in to present the characteristic peaks of the carbides

Fig. 5 SEM images and selected area electron diffraction (SAED) pattern of precipitates in different region of the welded joint; a, d carbides in BM; b, c, e, f carbides in WM

Table 3 Chemical compositions of precipitates in different region of the welded joint by TEM-EDS and the contents of carbon were not detected

Element (at.%)	Si	Ni	Cr	W	Ni:(W?+?Cr)	Ni:(W?+?Cr?+?Si)
Precipitates in BM	24.9	31.7	7.4	36.0	1:1.4	1:2.2
Precipitates in weld (particle)	22.9	32.2	18.4	26.5	1:1.4	1:2.1
Precipitates in weld (dentritic)	22.3	32.8	17.8	27.2	1:1.4	1:2.0

Fig. 6 Hardness and microstructure of the welded joint: a Vickers hardness values varied with the distance; microstructures of the BM b, WM c

Fig. 7 Tensile properties of welded joints under different temperatures: a yield and tensile strengths, b elongation

Fig. 8 Fracture morphologies of base metal specimens at a RT, b 550 °C, c 650 °C, d 750 °C, e 850 °C

Fig. 9 Fracture morphologies of as-welded specimens at a RT, b 550 °C, c 650 °C, d 750 °C, e 850 °C

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