Correlation Between Microstructure and Properties of New Heat-Resistant Alloy SP2215

doi:10.11900/0412.1961.2022.00539

Acta Metall Sin

2023, Vol. 59

Issue (6): 797-811 DOI: 10.11900/0412.1961.2022.00539

Research paper

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Correlation Between Microstructure and Properties of New Heat-Resistant Alloy SP2215

LIANG Kai, YAO Zhihao(

), XIE Xishan, YAO Kaijun, DONG Jianxin

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

Cite this article:

LIANG Kai, YAO Zhihao, XIE Xishan, YAO Kaijun, DONG Jianxin. Correlation Between Microstructure and Properties of New Heat-Resistant Alloy SP2215. Acta Metall Sin, 2023, 59(6): 797-811.

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Abstract

With the improvement of the steam parameters of thermal power units, the requirements put forward for the stress rupture strength and structural stability of heat-resistant materials for boiler superheater/reheater pipes become higher. SP2215, as a new heat-resistant alloy, is an excellent candidate for 620-650°C ultra-supercritical boiler superheater/reheater. In this study, the correlation between microstructure evolution and properties of the SP2215 heat-resistant alloy aging at different temperatures and time was studied via a series of creep and impact tests. The results show that the SP2215 alloy has excellent microstructure stability in high-temperature condition. Moreover, various nanoscale precipitations such as Cu-rich, MX, NbCrN, and M₂₃C₆ phases occur during aging. In the early period of aging, with the increase in aging temperature and aging time, the precipitations increase rapidly, improving the strength of the material; however, the impact toughness of the SP2215 alloy decreases considerably, with substantial intergranular fracture caused by the continuous precipitation and growth of M₂₃C₆ at the boundary, as shown with the quantitative calculation using the JMA model. In the late period of aging, the precipitations gradually stabilize, and the grain size remains in the range of 4.5-5 grade. As a result, the 1 × 10⁵ h stress rupture strength of the SP2215 alloy at 650 and 700^oC still remain more than 120 and 70 MPa, respectively. Hence, the alloy can be used as a domestic replacement for foreign HR3C, Super304H, and other similar heat-resistant alloys.

Key words: SP2215 heat-resistant alloy long term aging precipitation stress rupture strength property

Received: 24 October 2022

ZTFLH:

TG132.3

Fund: National Natural Science Foundation of China(51771017);National Natural Science Foundation of China(52271087)

Corresponding Authors: YAO Zhihao, professor, Tel:(010)62332884, E-mail: zhihaoyao@ustb.edu.cn

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https://www.ams.org.cn/EN/10.11900/0412.1961.2022.00539 OR https://www.ams.org.cn/EN/Y2023/V59/I6/797

Fig.1 Varieties of grain size of the SP2215 heat-resistant alloy aged at different temperatures and time (Insets show the microstructures of the alloy under different aging conditions, t—aging time)

Fig.2 OM image of SP2215 heat-resistant alloy aged at 650^oC for 500 h

Fig.3 TEM images of Cu-rich precipitates in the SP2215 heat-resistant alloy aged at 650^oC (a-d) and 700^oC (e-h) for 500 h (a, e), 2000 h (b, f), 6000 h (c, g), and 10000 h (d, h)

Fig.4 EDS analyses of Cu-rich precipitates (a) and the matrix (b)

Table 1 Average sizes of Cu-rich particles at different aging temperatures and time

Fig.5 SEM images of precipitates at and near the boundaries in the SP2215 heat-resistant alloy aged at 650^oC (a-c) and 700^oC (d-f) for 500 h (a, d), 4000 h (b, e), and 10000 h (c, f) (Insets show the higher magnification SEM images of precipitates at and near the boundaries)

Fig.6 EDS mappings of the precipitates at and near the boundaries in the SP2215 heat-resistant alloy after aging at 650^oC for 6000 h

Fig.7 Volume fractions of M₂₃C₆ as a function of aging time at different aging temperatures

Fig.8 Fitting curves of lnln[1 / (1 - f^{)] and lnt of SP2215 heat-resistant alloy at different aging temperatures (f—conversion rate of M₂₃C₆ phase)}

Fig.9 JMA kinetic curves of M₂₃C₆ of the SP2215 alloy aged at 650 and 700^oC

Fig.10 SEM images of M₂₃C₆ phases precipitated at the twin boundary (a), inside the grain (b, c) and at the grain boundary (d), and the EDS mappings (e) (Arrows point to the M₂₃C₆ phase at the specific locations)

Fig.11 TEM images of MX precipitates after aging at 650^oC for 500 h (a), and NbCrN precipitates after aging at 650^oC (b) and 700^oC (c) for 6000 h

Table 2 Varieties of main compositions in MX precipitates of SP2215 alloy during the aging at 650^oC

Table 3 Varieties of main compositions in MX precipitates of SP2215 alloy during the aging at 700^oC

Fig.12 EDS mappings of NbCrN precipitates in the SP2215 heat-resistant alloy after aging at 650^oC for 2000 h (a) and 6000 h (b)

Fig.13 TEM image of the ellipsoid NbCrN phase (a) and the selected area electrical diffraction (SAED) pattern (b) of circle in Fig.13a

Fig.14 Linear extrapolation curves of SP2215 heat-resistant alloy at different temperatures

Fig.15 C value calculated by fitting (t_r—rupture time, T—temperature)

Fig.16 Larson-Miller curves of SP2215 heat-resistant alloy (P—Larson-Miller parameter, σ—stress)

Fig.17 Variety of tensile strength (R_m) and yield strength (R_p0.2) at high temperature

Fig.18 Hardness curves of the SP2215 heat-resistant alloy after aging at 650 and 700^oC

Table 4 Comparison of impact energy of SP2215, HR3C^[30], and Super304H^[6] heat-resistant alloys after long-term aging at 650 and 700^oC for 3000 h

Fig.19 Impact energy curve of the SP2215 heat-resistant alloy after aging at 700^oC

Fig.20 Impact macrofractures of the SP2215 heat-resistant alloy aged at 650^oC (a, b) and 700^oC (c, d) for 500 h (a, c) and 6000 h (b, d) (Arrows point to the sources of cracks)

Fig.21 Impact micro fractures of the SP2215 heat-resistant alloy aged at 650^oC (a-c) and 700^oC (d-f) for 500 h (a, d), 4000 h (b, e), and 10000 h (c, f)

Fig.22 Effect of M₂₃C₆ content of on impact energy (a_k—impact energy, V—volume fraction)

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