Microstructural evolution and stress relaxation cracking mechanism for Super304H austenitic stainless steel weld metal

doi:10.1016/j.jmst.2021.06.010

Abstract

Abstract:

The pre-compressed CT technique was used to quantitatively investigate the formation of stress relaxation cracks under different tensile residual stresses and aging time in Super304H austenitic stainless steel weld metal. The statistical results revealed that intergranular cracks could occur within 2000 h under 650 °C when the residual stress was applied with greater than 18 KN pre-compression force. Detailed grain interior and boundary analyses showed that the growth of intragranular Cu-rich particles could induce a strong grain interior, and the intergranular Nb(C, N) carbides were one of the causes to crack under short-term aging time. For long-term aging time conditions, the intergranular M₂₃C₆ carbides were more susceptible to crack than intergranular Nb(C, N) carbides. Finally, the mechanism responsible for stress relaxation cracking formation was carefully illustrated for the weld metals after short-term aging and long-term aging, respectively.

Key words: Stress relaxation cracking, Super304H steel, Weld metal, Aging treatment, Residual stress, Precipitates

Xiaopeng Xiao, Dianzhong Li, Yiyi Li, Shanping Lu. Microstructural evolution and stress relaxation cracking mechanism for Super304H austenitic stainless steel weld metal[J]. J. Mater. Sci. Technol., 2022, 100: 82-90.

Figures/Tables 15

Table 1 Chemical compositions of the Super304H austenitic stainless steel base metal and the self-made YT-304H welding wire (wt%).

	Nb	C	Si	Mn	Cu	Ni	Cr	Mo	P	S	N
Base metal	0.45	0.10	0.27	0.81	3.16	9.08	17.68	0.36	0.003	0.0007	0.10
Welding wire	1.0	0.10	0.19	3.04	2.90	16.6	18.28	0.98	0.002	0.0005	0.20

Fig. 1. Weldment design and dimensions of the CT specimen.

Fig. 2. (a) The schematic diagram of residual stress generation. (b) The pre-compression force versus displacement curves of the measured CT specimen. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. (a) FEM model of the CT specimen, (b) true stress-true strain curve measured from a uniaxial tensile test of Super304H weld metal at room temperature, (c) predicted residual stress ${\sigma _{XX}}$ and (d) equivalent plastic strain at the notch root of the CT specimens with different pre-compression forces. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. The schematic diagram of CT specimen characterization.

Fig. 5. CT specimens with different test conditions and corresponding cracks observed. (Noting: hollow triangle indicated no crack observed in CT specimens, while solid triangles indicated that the CT specimens have already cracked.).

Fig. 6. Optical micrographs showing the cracks on the middle-thickness surfaces of the tested CT specimens: pre-compressed at 20 KN and then aging for (a) 10 h, (b) 425 h, (c) 2000 h; pre-compressed at 25 KN and then aging for (d) 10 h, (e) 250 h, (f) 500 h.

Fig. 7. (a) SEM image showing the as-welded weld metal microstructures, (b) EDS analysis result showing the existence of the Nb(C, N) carbide, (c1) BF STEM image at the ID region, (c2-c4) STEM-EDX spectrum images showing the presence of nanoscale Nb-rich particles.

Fig. 8. SEM images showing the precipitates inside grains of the pre-compressed CT specimens with different time: (a) 10 h (25 KN pre-compressed), and (b) 2000 h (19 KN pre-compressed). (c1) BF STEM image at the ID region of the damaged CT specimen (10 h (25 KN pre-compressed)), (c2-c4) corresponding STEM-EDX spectrum images showing the nanoscale Nb-rich and Cu-rich particles. (d1) BF STEM image at the ID region of the damaged CT specimen (2000 h (19 KN pre-compressed)), (d2-d4) corresponding STEM-EDX spectrum images showing the nanoscale Nb-rich and Cu-rich particles.

Fig. 9. SEM Characterization of the SRC crack tip in the damaged CT specimen (10 h (25 KN pre-compressed)). (a, b) BSE images showing the SRC crack tip with carbides, (c, d) Solved Kikuchi patterns and EDS analyses obtained as spot 1, (e1) SEM image showing the SRC crack tip with Nb(C, N), (e2-e4) EDS spectrum images showing the Nb-rich Nb(C, N) carbides.

Fig. 10. SEM Characterization of the SRC crack tip in the damaged CT specimen (2000 h (19 KN pre-compressed)). (a) SEM image showing the SRC crack tip with precipitates, (b1, c1) Solved Kikuchi patterns and EDS analyses obtained as spot 1, (b2, c2) Solved Kikuchi patterns and EDS analyses obtained from as spot 2, (d1) SEM image showing the SRC crack tip with precipitates, (d2-d4) EDS spectrum images showing the Nb-rich Nb(C, N) carbides and Cr-rich M23C6 carbides.

Fig. 11. SEM analysis showing the different fracture surface morphologies at the notch root of the damaged CT specimen (10 h (25 KN pre-compressed)). (a) Schematic diagram of the fracture surface, (b-d) corresponds to the fracture morphology at the position of b, c, d in figure (a), respectively. (e1) SEM images showing the precipitates in the intergranular fracture surface, (e2-e4) EDS spectrum images showing the presence of Nb-rich Nb(C, N) carbides.

Fig. 12. SEM analysis showing the different fracture surface morphologies at the notch root of the damaged CT specimen (2000 h (19 KN pre-compressed)). (a) Schematic diagram of the fracture surface, (b-d) corresponds to the fracture morphology at the position of b, c, d in figure (a), respectively. (e1) SEM images showing the precipitates in the intergranular fracture surface, (e2-e4) EDS spectrum images showing the presence of Nb-rich Nb(C, N) carbides and Cr-rich M23C6 carbides.

Fig. 13. Illustration of SRC formation in the Super304H weld metal during short-term aging.

Fig. 14. Illustration of SRC formation in the Super304H weld metal during long-term aging.

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