Microstructure and high temperature fracture toughness of NG-TIG welded Inconel 617B superalloy

doi:10.1016/j.jmst.2019.07.021

Abstract

Abstract:

In the present study, the microstructure, fracture toughness, and fracture behavior of Inconel 617B narrow gap tungsten inert gas (NG-TIG) welded joint were investigated systematically at the designed service temperature of 700 ℃. Fracture toughness (J_0.2) of base metal (BM) and heat affected zone (HAZ) was higher than that of weld metal (WM). In HAZ and BM, strain mainly localised at grain boundaries with large misorientation and there were lots of coincidence site lattice (CSL) ∑3 boundaries related to twins inside grains, which led to the much higher fracture toughness of BM and HAZ than WM. The high numbers of twins as well as the less serious strain localization at grain boundaries resulted in the most outstanding fracture toughness of BM.

Key words: Nickel alloys, Welding, Fracture behavior, Microstructure, Misorientation

Xiaogang Li, Kejian Li, Shanlin Li, Yao Wu, Zhipeng Cai, Jiluan Pan. Microstructure and high temperature fracture toughness of NG-TIG welded Inconel 617B superalloy[J]. J. Mater. Sci. Technol., 2020, 39: 173-182.

Figures/Tables 21

Table 1 Chemical compositions of Inconel 617B and filler metal (wt%).

Element	Cr	Co	Mo	Al	Ti	Fe	Cu	Nb
Inconel 617B	22.37	12.02	9.01	1.02	0.46	0.34	0.006	0.04
Filler metal	22.5	11.2	8.9	1.27	0.39	—	0.1	—
Element	Mn	B	C	Si	S	P	N	Ni
Inconel 617B	0.01	0.0044	0.051	0.04	0.007	0.013	0.004	Balance
Filler metal	0.4	—	0.07	0.3	0.002	0.003	—	55

Fig. 1. Sampling location and dimension of compact tension sample: (a) sampling location, (b) dimension (in mm).

Fig. 2. OM image of the cross-section of Inconel 617B NG-TIG welded joint: (a) welded joint, (b) Inconel 617B-WM, (c) Inconel 617B-HAZ, (d) Inconel 617B-BM.

Fig. 3. Microstructure of Inconel 617B-WM observed by SEM: (a) low-magnification, (b) detailed microstructure.

Fig. 4. Microstructure of Inconel 617B-HAZ observed by SEM: (a) low-magnification, (b) eutectic microstructure, (c) M23C6 carbides.

Fig. 5. Microstructure of Inconel 617B-BM observed by SEM: (a) low-magnification, (b) micro-scale M23C6 and small-scale M23C6 decorating the grain boundary.

Fig. 6. J-R curves of WN, HAZ, and BM at 700 ℃: (a) Inconel 617B-WM, (b) Inconel 617B-HAZ, (c) Inconel 617B-BM.

Table 2 The fracture toughness parameters of WM, HAZ, and BM at 700℃.

Position	α	β	γ	Correlation
WM	0.8807	345.3251	0.7084	0.9990
HAZ	0.8860	657.4997	0.8870	0.9996
BM	0.8940	694.8938	0.8232	0.9902

Table 3 Equations for J-R curves of WM, HAZ, and BM at 700℃.

Position	Equations
WM	J=0.8807+345.3251×(Δa)0.7084
HAZ	J=0.8860+657.4997×(Δa)0.8870
BM	J=0.8940+694.8938×(Δa)0.8232

Table 4 A comparison of J0.2 and Jstr for WM, HAZ, and WM.

Position	J_0.2 (kJ m²)	SZW (μm)	J_str (kJ m²)
WM	138.09	323.62	157.86
HAZ	225.21	328.64	246.49
BM	293.36	334.66	289.66

Fig. 7. Force versus CMOD curves of WM, HAZ, and BM at 700 ℃ in J-R tests.

Fig. 8. Low-magnification SEM fractograph of broken Inconel 617B-BM.

Fig. 9. Fracture appearances of Inconel 617B-WM: (a) boundary between the stretch zone and propagation zone, (b) stretch zone, (c) propagation zone.

Fig. 10. Fracture appearances of Inconel 617B-HAZ: (a) boundary between the stretch zone and propagation zone, (b) stretch zone, (c) propagation zone.

Fig. 11. Fracture appearances of Inconel 617B-BM: (a) boundary between the stretch zone and propagation zone, (b) stretch zone, (c) propagation zone.

Fig. 12. Crack profiles of WM, HAZ, and BM: (a) Inconel 617B-WM, (b) Inconel 617B-HAZ, (c) Inconel 617B-BM.

Fig. 13. EBSD analyses of an area on the mid thickness plane of broken Inconel 617B-WM sample close to fracture surface: (a) image quality (IQ)+IPF map, (b) IQ + KAM map, (c) IQ + misorientation distribution, (d) IQ + CSL ∑3 boundaries.

Fig. 14. EBSD analyses of an area on the mid thickness plane of broken Inconel 617B-HAZ sample close to fracture surface: (a) IQ + IPF map, (b) IQ + KAM map, (c) IQ + misorientation distribution, (d) IQ + CSL ∑3 boundaries.

Fig. 15. EBSD analyses of an area on the mid thickness plane of broken Inconel 617B-BM sample close to fracture surface: (a) IQ + IPF map, (b) IQ + KAM map, (c) IQ + misorientation distribution, (d) IQ + CSL ∑3 boundaries.

Fig. 16. The quantitative analysis of misorientation angle (0°-64°).

Fig. 17. The quantitative analysis of CSL boundaries.

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