Precipitation of Nanosized MX at Coherent Cu-Rich Phases in Super304H Austenitic Steel
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Super304H austenitic steel is based on 18/8 Cr-Ni stainless steel alloyed mainly with about 3 pct Cu and a small amount of Nb, which is widely used in ultra-super critical (USC) power plants due to its excellent hightemperature performance, especially the outstanding high-temperature creep resistance.[1,2] The outstanding creep strength at high temperature of Super304H steel arises from the precipitation of nanosized Cu-rich and MX (Nb(C, N)) phases in the austenitic matrix during service at high temperatures.[1] Among these two kinds of precipitates, the nanosized Cu-rich phase is regarded to be of the most significant precipitation strengthening effect in Super304H steel, and many studies have concentrated on the precipitation behaviors and strengthening effect of the nanosized Cu-rich phase in Super304H austenitic steel.[3–5] In contrast, the precipitation of nanosized MX phase in Super304H austenitic steels receives little attention until now. The precipitation of MX phase occurs when strong carbide/nitride former elements like Ti, Nb, V, and etc. are added in steels. One of the main aims of adding these elements is to stabilize the steels against intergranular corrosion during service at elevated temperature since most of carbon is tied up by MC. Additionally, MC carbides form in preference to M23C6, lessening sensitisation and improving mechanical properties.[6–9] The PING OU, Ph.D. Student, HUI XING, Assistant Professor, and JIAN SUN, Professor, Principal Researcher, are with the Shanghai Key Laboratory of Advanced High-temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200240, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted July 2, 2014. Article published online October 24, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
growth rate of MX is usually thought to be lower than other carbides, and the precipitation of fine MX carbides in the steels provides good creep resistance at elevated temperature.[10] The MX precipitates have a NaCl face-centered cubic structure. They have a typical shape of thin platelet and a Baker–Nutting (B–N) orientation relationship with the matrix of ferritic steels.[11] Meanwhile, the MX precipitates usually have a characteristic cuboidal shape and a cube-on-cube orientation relationship with the matrix of austenitic steels. In this work, the nanosized MX precipitates in Super304H austenitic steel after creep test under 250 MPa at 923 K (650 °C) for 447 hours were investigated by transmission electron microscope (TEM). The precipitation of the nanosized MX at Cu-rich phases and formation of the nanosized MX on dislocations have been observed in the Super304H steel. Based on these TEM results, the precipitation mechanism of the nanosized MX phase in Super304H steel was discussed additionally. The solution-treated Super304H austenitic steel supplied by a steel plant was used for creep test under 250 MPa at 923 K (650 °C) for 447 hours in this study. The samples of the Super304
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