服役态Super304H过热器钢管显微组织及力学性能

采用光学显微镜、扫描电镜和透射电镜等对某超超临界机组服役40 000 h后Super304H过热器钢管不同区域的显微组织和力学性能进行了分析。结果表明,服役40 000 h后Super304H钢管外壁出现异常长大的奥氏体晶粒,TEM测试表明外壁M₂₃C₆颗粒的粗化明显,且大量M₂₃C₆颗粒沿晶界连续分布,而MX相和富Cu相长大不明显;异常长大的晶粒导致服役态Super304H钢管外壁粗晶区的室温抗拉强度和冲击性能较供货态下降了21.9%和50%,粗晶区冲击试样断口沿晶断裂特征明显,其脆性显著增加,对Super304H钢管的服役安全性构成威胁,应加强监督消除因外壁奥氏体晶粒异常长大而带来的爆管等安全隐患。

Microstructure and mechanical properties of Super304H superheater steel tube in service

Microstructure and mechanical properties of different areas of the Super304H superheater steel tube of an ultra-supercritical unit after 40 000 h service were analyzed by means of optical microscope, scanning electron microscope and transmission electron microscope. The results show that the Super304H steel tube has abnormally grown austenite grains in the outer wall after 40 000 h service. TEM test shows that the coarsening of M₂₃C₆ particles in the outer wall is obvious, and continuous M₂₃C₆ particles precipitate on grain boundary, while the growth of MX phase and Cu rich phase is not obvious. The abnormally grown grains cause the room temperature tensile strength and impact property of coarse-grained zone of outer wall of the Super304H steel tube in service to decrease by 21.9% and 50%, respectively, compared with the supply state. The fracture characteristics of the impact specimen in the coarse-grained zone are obvious, and its brittleness is significantly increased, which poses a threat to the service safety of the Super304H steel tube. Supervision should be strengthened to eliminate the potential safety hazards such as tube explosion caused by the abnormal growth of austenite grains on the outer wall.