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Acta Metall Sin  2017, Vol. 53 Issue (8): 983-990    DOI: 10.11900/0412.1961.2016.00545
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Effects of Nitrogen Addition on Microstructure and Grain Boundary Microchemistry of Inconel Alloy 690
Bo CHEN(), Xianchao HAO, Yingche MA, Xiangdong CHA, Kui LIU
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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Abstract  

Inconel alloy 690 is an austenitic nickel-based corrosion resistant alloy with about 30%Cr, which is considered as the most ideal steam generator tubing materials in nuclear power plants because of its superior resistance to intergranular attack (IGA). However, the existence of impurities and the addition of minor alloying elements cause significant difference of carbide morphology, microstructure and chromium depletion of Inconel alloy 690. In this work, the microstructure and grain boundary chemistry of Inconel alloy 690 with four different nitrogen contents have been investigated by SEM and TEM. Stacking fault probability (SFP) and IGA with respect to the microstructure was tested and analyzed. The results indicated that thermal treatment at 715 ℃ following solution annealing (SA) at 1080 ℃ caused a wide range of intergranular carbide morphology with the associated chromium depletion in the vicinity of grain boundaries. With the increasing of nitrogen content, the characters of the carbides ranged from thin continuous bands along boundaries to coarse discrete particles. Stacking fault probability was increased with the increasing of nitrogen content, and the value reached the peak at 100×10-6 of nitrogen content, then it dropped. The corrosion tests showed that moderate nitrogen content alloy performed favorable intergranular attack correlated with the presence of semi-continuous grain boundary carbide and chromium depletion was mitigated. The consequent nitrides were appeared in high nitrogen alloy. So, about 100×10-6 contents of nitrogen in alloy 690 is suitable by synthesis considering of carbides, nitrides and chromium depletion.

Key words:  Inconel alloy 690      microstructure      carbide      N     
Received:  05 December 2016     
ZTFLH:  TG132.32  

Cite this article: 

Bo CHEN, Xianchao HAO, Yingche MA, Xiangdong CHA, Kui LIU. Effects of Nitrogen Addition on Microstructure and Grain Boundary Microchemistry of Inconel Alloy 690. Acta Metall Sin, 2017, 53(8): 983-990.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00545     OR     https://www.ams.org.cn/EN/Y2017/V53/I8/983

Alloy N C Cr Fe Ti Al Ni
690-38 0.0038 0.018 29.2 10.5 0.26 0.28 Bal.
690-100 0.0100 0.015 29.8 9.8 0.27 0.29 Bal.
690-220 0.0220 0.018 29.8 10.0 0.27 0.30 Bal.
690-330 0.0330 0.015 29.7 10.0 0.25 0.26 Bal.
Table 1  Chemical compositions of alloy 690(mass fraction / %)
Fig.1  SEM images of carbides precipitated on grain boundaries in alloy 690 with nitrogen contents of 38×10-6 (a), 100×10-6 (b), 220×10-6 (c) and 330×10-6 (d) after 1080 ℃, 10 min solution annealing and 715 ℃, 15 h thermal treatment
Fig.2  TEM images of carbides precipitated on grain boundaries in alloy 690 with nitrogen contents of 38×10-6 (a), 220×10-6 (b) and 330×10-6 (c) after 1080 ℃, 10 min solution annealing and 715 ℃, 15 h thermal treatment
Fig.3  TEM image of twin boundary carbides (a) and corresponding SAED patters of twin (b) and carbide (c) in alloy 690 with 38×10-6 nitrogen content after 1080 ℃, 10 min solution annealing and 715 ℃, 15 h thermal treatment
Fig.4  SEM images of TiN precipitates in alloy 690 with nitrogen contents of 38×10-6 (a), 100×10-6 (b), 220×10-6 (c) and 330×10-6 (d) after 1080 ℃, 10 min solution annealing and 715 ℃, 15 h thermal treatment (Arrows in Figs.4b~d show TiN precipitates)
Fig.5  TEM images showing stacking fault in alloy 690 with nitrogen contents of 38×10-6 (a), 220×10-6 (b) and 330×10-6 (c) after 1080 ℃, 10 min solution annealing and 715 ℃, 15 h thermal treatment
Fig.6  Chromium depletions of grain boundaries in alloy 690 with nitrogen contents of 38×10-6 (a) and 330×10-6 (b) after 1080 ℃, 10 min and different thermal treatment times at 715 ℃
N content / 10-6 Lowest of Cr content / % Width of Cr depletions zone / nm
38 24.6 500
100 26.4 400
220 27.2 300
330 28.6 200
Table 2  Grain boundaries chromium contents and width of Cr depletions in alloy 690 with different nitrogen contents after 1080 ℃, 10 min and 715 ℃, 15 h thermal treatment
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