動力場渦輪機葉片破損原因分析

Failure Analysis of the Steam Turbine Blades of A Power Plant

動力場發電機因軸承振動大停機檢查，發現動葉片因不明原因發生進口端及出口端根部鳩尾斷裂，故進行相關葉片破損分析。期望能找出葉片斷裂原因，避免斷裂所造成之維修及發電損失一再發生。分別進行設備操作條件調查、外觀觀察、材料化學成分分析、腐蝕生成物取樣及SEM/XRD分析、材質表面及截面金相觀察、硬度試驗、機械性質試驗、破斷面巨觀及微觀SEM觀察及EDS分析等，來推斷汽輪機轉子葉片可能之斷裂原因。結果發現：發電機第14級第38號葉片斷裂鳩尾前腳根部裂紋起始位置受到鍋爐水含S污染及蒸汽凝結，而有局部腐蝕現象，腐蝕誘發裂紋起始，破斷面呈現沿晶破裂（IGC）及穿晶破裂（TGC）混合型態，且可以發現疲勞紋理及略成平行的二次裂紋，此種破裂方式為腐蝕疲勞。至於，第117號葉片破斷面全區均呈現穿晶破裂型態，腐蝕扮演之角色較輕微，且可以發現疲勞紋理及略成平行的二次裂紋，故裂紋之傳播屬於疲勞破壞。

Emergent shutdown of the steam turbine of a power plant occurred due to the high vibration of the bearing. Failure was revealed in the root parts of dovetails of the fourteenth stage. The scope of this paper was to analyze the main cause of blade cracking and to prevent the loss of generated electric power. The survey of operation conditions, observation of blade appearance, analysis of chemical composition, corrosion products analysis, optical microstructure observation of surface and cross-section, hardness test, mechanical property test, fracture surface observation and energy dispersive spectrum analysis were performed to infer the possible failure reason of the blades in this study. The results showed that the localized corrosion occurred in the crack initiation site of the front leg root part of dovetail of the 38th blade of the fourteenth stage owing to the sulfur contamination and steam condensation of boiler water. The mixed mode of the intergranular and transgranular cracking was found in this fracture surface. We could also find the fatigue striations and parallel secondary cracks in this blade. This fracture mechanism was defined as corrosion fatigue. On the other hand, the transgranular cracking was shown in all the fracture surface of the 117th blade. The extent of corrosion was minor and the fatigue striations and parallel secondary cracks were also found in this fracture surface. This fracture mechanism belonged to fatigue, too.