某超临界汽轮机主汽阀杆断裂原因分析

目的 某热电厂超临界机组汽轮机发生主蒸汽阀阀杆(标称材质为22Cr12NiMoWV(C422)钢)断裂事故,对发电安全造成不良影响.为此,分析该阀杆断裂失效的原因,提出改进措施,以避免类似事故的发生.方法 从外观检查、成分测定、金相检验以及断口观察等方面入手,结合硬度和冲击性能测试,探讨阀杆断裂的主要原因.结果 该断裂阀杆材质符合22Cr12NiMoWV钢的成分标准,但其内部存在网状分布的组织偏析,导致其力学性能达不到标准要求.此外,该断裂阀杆还存在零件设计、加工及热处理工艺不合理等问题.结论 该阀杆变径处采用直角过渡,十字贯穿孔加工刀痕明显,易造成应力集中,诱发裂纹,裂纹选择性地沿力学性能薄弱的组织偏析区失稳扩展,最终导致阀杆断裂失效.据此,从质量控制、工艺规范化等方面提出改进建议.

Failure Analysis on Main Steam Valve Stem of An Ultra-supercritical Turbine

Fracture of the nominal 22Cr12NiMoWV (C422) steel main steam valve stem of the ultra-supercritical turbine took place, which brought about an adverse influence on power generation security of the thermal power station. Based on the failure analysis of the valve stem, improvement approaches should be put forward in order to avoid the occurrence of similar accidents. Appearance inspection, composition and metallography tests and fracture surface observation, combining with hardness and impact toughness tests, were employed to investigate the fracture causes of the main steam valve stem. The results show that Composition of the valve stem meets the requirement of the 22Cr12NiMoWV steel. However, there are net-like distributed segregation microstructures in the steel, leading to that the mechanical properties of the steel cannot meet the standard requirement. Moreover, other disadvantages such as unreasonable mechanical design and manufacturing and unreasonable heat treatment of the valve stem were also found. Both right-angle transition at the location of reducing diameter and apparent machining tool mark through the cross-holes result in stress concentration in the valve stem, even induce microcracks over there. The microcracks preferentially propagate through the segregation microstructures of low mechanical properties, leading to fracture failure of the valve stem. Accordingly, reasonable proposals were put forward from quality control, process normalization and so on.