Xu Liming, He Yinsheng, Kang Yeonkwan, Jung Jine-Sung, Shin Keesam
School of Materials Science and Engineering, Changwon National University, Changwon 51140, Korea.
KEPCO Research Institute, Korea Electric Power Corporation, Daejeon 34056, Korea.
Materials (Basel). 2022 Jul 5;15(13):4704. doi: 10.3390/ma15134704.
HR3C (25Cr-20Ni-Nb-N) is a key material used in heat exchangers in supercritical power plants. Its creep properties and microstructural evolution has been extensively studied at or below 650 °C. The precipitation evolution in HR3C steel after creep rupture at elevated temperatures of 700 °C and 750 °C with a stress range of 70~180 MPa is characterized in this paper. The threshold strength at 700 °C and 750 °C were determined by extrapolation method to be σ105700= 57.1 MPa and σ105750=37.5 MPa, respectively. A corresponding microstructure investigation indicated that the main precipitates precipitated during creep exposure are Z-phase (NbCrN), MC, and σ phase. The dense Z-phase precipitated dispersively in the austenite matrix along dislocation lines, and remained stable (both size and fraction) during long-term creep exposure. MC preferentially precipitated at grain boundaries, and coarsening was observed in all creep specimens with some continuous precipitation of granular MC in the matrix. The brittle σ phase formed during a relatively long-term creep, whose size and fraction increased significantly at high temperature. Moreover, the σ phases, grown and connected to form a large "island" at triple junctions of grain boundaries, appear to serve as nucleation sites for high stress concentration and creep cavities, weakening the grain boundary strength and increasing the sensitivity to intergranular fracture.
HR3C（25Cr - 20Ni - Nb - N）是超临界发电厂热交换器中使用的关键材料。其蠕变性能和微观结构演变已在650℃及以下温度下得到广泛研究。本文对HR3C钢在700℃和750℃的高温下，应力范围为70~180MPa的蠕变断裂后的析出相演变进行了表征。通过外推法确定700℃和750℃时的门槛强度分别为σ105700 = 57.1MPa和σ105750 = 37.5MPa。相应的微观结构研究表明，蠕变暴露期间析出的主要析出相为Z相（NbCrN）、MC和σ相。致密的Z相沿位错线在奥氏体基体中弥散析出，并且在长期蠕变暴露期间保持稳定（尺寸和体积分数均不变）。MC优先在晶界析出，并且在所有蠕变试样中均观察到粗化现象，基体中还有一些颗粒状MC的连续析出。脆性σ相在相对较长时间的蠕变过程中形成，其尺寸和体积分数在高温下显著增加。此外，σ相生长并连接，在晶界的三叉晶界处形成一个大的“岛”，似乎作为高应力集中和蠕变空洞的形核位置，削弱了晶界强度并增加了对沿晶断裂的敏感性。
