Precipitation Kinetics of M 23 C 6 Carbides in the Super304H Austenitic Heat-Resistant Steel
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JMEPEG DOI: 10.1007/s11665-017-2982-2
Precipitation Kinetics of M23C6 Carbides in the Super304H Austenitic Heat-Resistant Steel Qingwen Zhou, Shaobo Ping, Xiaobo Meng, Ruikun Wang, and Yan Gao (Submitted October 12, 2016; in revised form May 12, 2017) The precipitation kinetics of M23C6 carbides in Super304H and TP304H steels were investigated using the selective-etching method, SEM backscattered electron images and Image-Pro-Plus 6.0 software. Precipitation–temperature–time (PTT) diagrams of M23C6 carbides in the as-received Super304H (fine grains), coarsened Super304H (coarse grains) and TP304H (coarse grains) steels all show the typical C-shaped character with nose temperature range from 800 to 850 °C. Compared with the TP304H steel, the same trend is found of the PTT curve of M23C6 carbides for both kinds of Super304H steels, but their start lines move to the right and finish lines to the left. The preferential formation of Nb(C,N) phase at grain boundaries in the Super304H steels inhibited the nucleation of M23C6 carbides in the early stage of precipitation, causing the right shift of the start line of PTT curve. The main reason for the left shift of the finish line of the two Super304H steels was the quicker growing and coarsening rate of M23C6 in the later precipitation stage due to their higher C content than in TP304H. For the difference in PPT curves between the two grain sizes of the Super304H steel, the lower diffusion rate of atoms in the coarse-grained Super304H steel may explain its righter finish line than the fine-grained counterpart, while the reason for its lefter start line is due to the higher solute segregation along coarse-grained boundaries. Keywords
M23C6 carbide, precipitation kinetics, PTT curve, Super304H austenitic stainless steel
1. Introduction The ultra-supercritical critical (USC) units have been installed quickly and widely all over the world due to their advantages of high efficiency and low CO2 emission (Ref 1-3). A novel austenitic heat-resistant steel named Super304H, with higher carbon content and additional alloying elements of Cu, Nb, N, B based on the traditional TP304H steel, has been developed for the superheater and reheater tubes in USC boilers. This steel exhibits fine-grained microstructure, high creep rupture strength, high anti-steam corrosion resistance and low cost (Ref 4-6). However, leakage accidents of Super304H superheater and reheater tubes in USC boilers occurred frequently at the stage of water pressure testing in China, which were found to be caused by intergranular corrosion (IGC)-induced stress corrosion cracking (Ref 7). It is well known that precipitation of chromium-rich M23C6 phase is the main reason for the intergranular corrosion sensitivity (IGCS) of 18-8 austenitic stainless steel (Ref 8). Similar failure cases caused by IGC in austenitic stainless steels were also reported in other works (Ref 9, 10). The high carbon content of Super304H will facilitate the formation of chromium-rich
Qingwen Zhou, Shaobo Ping, Ruikun Wang, and Yan Gao, School of Mat
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