Multi-scale study of ductility-dip cracking in nickel-based alloy dissimilar metal weld

doi:10.1016/j.jmst.2018.10.023

Abstract

Abstract:

A ductility-dip-cracking (DDC)-concentrated zone (DCZ) in a width of about 3 mm was observed adjacent to the AISI 316 L/52 Mw fusion boundary (FB) in 52 Mw. The morphology, microstructure, mechanical and thermal properties and corrosion behavior in simulated primary water of DDC/DCZ were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), 3D X-ray tomography (XRT), 3D atom probe (3DAP), slow strain rate tensile (SSRT) testing and thermal dilatometry. The results indicate that DDCs are random-shaped and disc-like cavities with corrugated structure of inner surface and are parallel in groups along straight high-angle boundaries of columnar grains, ranging from micrometers to millimeters in size. Large-size M₂₃C₆ carbides dominate on the grain boundaries rather than MC (M=Nb, Ti), and thus the bonding effect of carbides is absent for the straight grain boundaries. The impurity segregation of O is confirmed for the inner surfaces of DDC. The oxide film formed on the inner surface of DDC (about 50 nm) is approximately twice as thick as that on the matrix (about 25 nm) in simulated primary water. The yield strength, tensile strength and elongation to fracture of 52 Mw-DCZ (400 MPa, 450 MPa and 20 %, respectively) are lower than those of 52 Mw-MZ (460 MPa, 550 MPa and 28 %, respectively). The intrinsic high-restraint weld structure, the additional stress/strain caused by the thermal expansion difference between AISI 316 L and 52 Mw as well as the detrimental carbide precipitation and the resulting grain boundary structure all add up to cause the occurrence of DCZ in the dissimilar metal weld.

Key words: Dissimilar metal weld, Nickel-base alloy, Ductility-dip cracking (DDC), Slow strain rate tensile (SSRT) testing, Thermal expansion coefficient

Yifeng Li, Jianqiu Wang, En-Hou Han, Wenbo Wu, Hannu H?nninen. Multi-scale study of ductility-dip cracking in nickel-based alloy dissimilar metal weld[J]. J. Mater. Sci. Technol., 2019, 35(4): 545-559.

Figures/Tables 24

Fig. 1. Schematic diagram of the dissimilar metal weld joint.

Table 1 Chemical compositions (wt %) of the welds in the dissimilar metal weld joint.

Material	Ni	Cr	Fe	Mn	Nb	Si	Ti	C	Cu	Mo
52 Mb	59.25	29.70	8.88	0.82	0.81	0.12	0.22	0.008	0.025	0.01
52 Mw	59.19	29.52	9.03	0.78	0.86	0.11	0.20	0.008	0.024	0.03
316 L	12.42	17.60	64.79	1.62	0.04	0.65	-	0.028	0.08	2.69

Table 2 Welding parameters of the welds in the dissimilar metal weld joint.

Welds	Welding process	Filler metal	Diameter (mm)	Amperage (A)	Voltage (V)	Welding speed (mm/min)
52 Mb	GTAW	52 M	Φ 1.2	210-265	9-10	190-210
52 Mw	GTAW	52 M	Φ 0.9	170-190	8-11	60-70

Fig. 2. Sampling scheme for DDC statistics and DDC-concentrated zone (DCZ) in DMW slab.

Fig. 3. Schematic preparation processes for 3DAP tip. (a) The region of interest protected by Pt deposition; (b) the sample attached to the micro tip post; (c) the sample after cutting off; (d) the final 3DAP tip.

Fig. 4. Diagram of specimen size and sampling position for SSRT testing. (a) Specimen size; (b) 52 Mw-DCZ; (c) 52 Mw-MZ.

Table 3 Statistics of DDCs in 52 M weld in an axial-radial section of DMW.

Sample No.	Total crack amount	Crack length Max. /Avg. (μm)	Crack amount in DDC-concentrated zone	Percentage
1#	2	32/29	2	100%
2#	22	315/83	19	86.3%
3#	54	247/91	49	90.7%
4#	22	240/92	17	77.3%
5#	11	2060/335	7	63.6%

Fig. 5. Microscopic morphology of DDCs in DMW. (a) BSE image of a typical DDC-concentrated zone; (b) SE image of the square-marked grain boundary with a representative DDC; (c) close-up of the marked DDC; (d) inner surface morphology of the marked DDC.

Fig. 6. EBSD results of a typical DDC-concentrated area in DMW. (a) Image quality (IQ) map; (b) inverse pole figure (IPF) map; (c) grain boundary character distribution (GBCD); (d) Kernel average misorientation (KAM) distribution; (e) KAM distribution around crack A; (f) KAM distribution around crack B.

Fig. 7. EBSD results of recrystallization upon DDC in 52 Mw. (a) IQ; (b) IPF; (c) GBCD; (d) KAM distribution.

Fig. 8. (a) XRT sampling position; (b) 3D X-ray tomography of DDC; c) DDC cluster I as imaged; (d) Front view of DDC cluster I; (e) Side view of DDC cluster I; (f) DDC cluster II as imaged; (g) Front view of DDC cluster II; (h) Side view of DDC cluster II.

Fig. 9. Equivalent disc model and the statistics of measured volume and equivalent radius of measured DDCs within the XRT sample.

Fig. 10. TEM characterization of the open DDC. (a) STEM-BF image of DDC cross-section; (b) STEM-BF image of Cr23C6 at DDC; (c) HRTEM diffraction pattern of Cr23C6; (d) EDS mapping of DDC (corresponding to Fig. 10(a)).

Fig. 11. TEM characterization for locked grain boundary at DDC tip. (a) STEM-BF image of DDC tip; (b) STEM-BF image of grain boundary at DDC tip; (c) Nano-size carbides rich in Nb and Ti on the grain boundary; (d) EDS mapping of DDC tip (corresponding to Fig. 11(b)).

Fig. 12. TEM characterization for cracked grain boundary at DDC tip. (a) STEM-BF image of DDC tip; (b) STEM-BF image of grain boundary at DDC tip; (c) Cr23C6 on the grain boundary; (d) EDS mapping of DDC tip (corresponding to Fig. 12(b)).

Fig. 13. 3DAP composition distribution around DDC. a) The distribution of Ni, Cr, Fe and CrO adjacent to DDC; b) Atomic concentration profiles in the marked analysis cylinder (10 nm in diameter).

Fig. 14. Secondary electron (SE) image of DDC after exposure test in simulated primary water at 325 °C for 720 h. (a) Accelerating voltage = 20 kV; (b) high magnification of square area in Fig. 14(a) (Accelerating voltage = 5 kV).

Fig. 15. TEM characterization of DDC after exposure test in simulated primary water at 325 °C for 720 h. (a) STEM-BF image of DDC after exposure test; (b) EDS maps of Fig. 15(a); (c) STEM-BF close-up of Area I in Fig. 15(a); (d) oxygen concentration gradient of matrix surface and DDC inner wall; (e) HRTEM diffraction pattern of needle-like oxides.

Fig. 16. SSRT stress-strain curves for 52 Mw specimens in simulated primary water at 325 °C with strain rate of 4 × 10-7 s-1.

Fig. 17. SEM fracture morphologies of SSRT specimens. (a) Top view of 52 Mw-DCZ fracture; (b) DDC 1 found in 52 Mw-DCZ fracture; (c) DDC 2 found in 52 Mw-DCZ fracture; (d) side view of 52 Mw-DCZ fracture on surface (same as in Fig. 17(a)); (e) Cracking zone (square marked) in (d); f) DDC as crack initiation site; (g) top view of 52 Mw-MZ fracture; (h) ductile dimpled fracture; i) dimples of different sizes on step surface.

Fig. 18. Linear thermal expansion rates and mean coefficients of linear thermal expansion of 52 M and AISI 316 L (ambient temperature 20 °C).

Fig. 19. Schematic diagram of the welding process of DMW. (a) Overlay 52 M buttering on free-end surface of RPV nozzle (low stress restraint for 52 Mb); (b) Butt welding of the narrow-gap preparation of 52 Mb and AISI 316 L by 52 M (high stress restraint for 52 Mw).

Fig. 20. Schematic diagram of the welding process of 52 Mw. (a) Greater thermal expansion of AISI 316 L than that of 52 Mw caused by welding heat; (b) reversed residual stress after cooling down of previous beads and AISI 316 L; (c) thermal expansion difference is mitigated as the beads are further away from AISI 316 L thermal expansion affected zone.

Fig. 21. Schematic diagram of grain boundary stress concentration as a function of boundary geometry and precipitation behavior. (a) Straight grain boundary; (b) tortuous grain boundary.

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