RESEARCH ON THE CARBIDE PRECIPITATION AND CHROMIUM DEPLETION IN THE GRAIN BOUNDARY OF ALLOY 690 CONTAINING DIFFERENT CONTENTS OF NITROGEN
Yingche MA(),Shuo LI,Xianchao HAO,Xiangdong ZHA,Ming GAO,Kui LIU
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
Yingche MA,Shuo LI,Xianchao HAO,Xiangdong ZHA,Ming GAO,Kui LIU. RESEARCH ON THE CARBIDE PRECIPITATION AND CHROMIUM DEPLETION IN THE GRAIN BOUNDARY OF ALLOY 690 CONTAINING DIFFERENT CONTENTS OF NITROGEN. Acta Metall Sin, 2016, 52(8): 980-986.
Nickel-based alloy Inconel 690 (hereinafter called alloy 690) is currently replacing alloy 600 as steam generator tubes in pressurized water nuclear reactors, owing to its excellent resistance to intergranular stress corrosion cracking (IGSCC) and good mechanical properties. The carbide precipitation is a major microstructural characteristic during heat treatment of stainless steels and nickel-based alloys. The carbide precipitation and chromium depletion in the grain boundary of alloy 690 were investigated. The grain size and carbide of alloy 690 with 0.001% and 0.03% (mass fraction) nitrogen contents were observed and analyzed. The extent of chromium depletion in the vicinity of grain boundaries was quantitatively determined as a function of thermal treatment time. The solution treatment of the samples was at 1080 ℃ for 10 min, and then the samples were thermally treated at 715 ℃ for 1~25 h. The results show that the nitrogen addition decreases the intergranular carbide density and the average carbide length but increases its distance. The level of chromium in the depleted regions in alloy 690 with 0.03%N is higher than that with 0.001%N. This is attributed to the beneficial role of nitrogen addition against grain growth and sensitization.
Table 1 Chemical compositions of the alloy 690(mass fraction / %)
Fig.1 SEM images of carbides in hot-rolled 10N (a) and 300N (b) alloys where intergranular carbides are indicated with arrows
Fig.2 OM images of 10N (a) and 300N (b) alloys solution treated at 1080 ℃ for 10 min (SA)
Fig.3 TEM image of carbides in the 10N alloy after SA and thermally treated at 715 ℃ (TT) for 1 h (a), electron diffraction pattern (b) and corresponding index result (c) of γ matrix and M23C6
Fig.4 TEM images of carbides (M23C6) precipitated at grain boundaries of 10N alloy after SA and TT for 1 h (a), 5 h (b), 15 h (c) and 25 h (d)
Alloy
Treatment
Average density μm-1
Average length μm
Average distance between 10 particles / μm
10N
SA+TT, 1 h
7.9
0.082
0.043
SA+TT, 5 h
7.6
0.102
0.023
SA+TT, 15 h
3.8
0.132
0.132
SA+TT, 25 h
2.7
0.174
0.196
300N
SA+TT, 1 h
6.8
0.068
0.079
SA+TT, 5 h
6.1
0.090
0.074
SA+TT, 15 h
3.0
0.108
0.223
SA+TT, 25 h
2.4
0.152
0.265
Table 2 Quantitative analysis of precipitate characteristics
Fig.5 TEM images of carbides (M23C6) precipitated at grain boundaries of 300N alloy after SA and TT for 1 h (a), 5 h (b), 15 h (c) and 25 h (d)
Fig.6 Cr concentration curves of 10N and 300N alloys after SA and TT for 1 h (a), 5 h (b), 15 h (c) and 25 h (d)
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