火力電廠水牆管破損機構探討

Investigation on Waterwall Tube Failure Mechanism of Fossil Fired Power Plant

台灣電力需求仍以火力發電為主，台電公司大林電廠二號機鍋爐屬於強制循環負壓式鍋爐已運轉達35年之久，累計運轉時數超過26萬小時。在2006年初，水牆管arch tube位置接連發生兩次破管事件，水牆管材質為ASME SA 210 grade A-1碳鋼。破損爐管取樣後，經由目視檢查、金相組織檢查、硬度分佈、電子顯微鏡分析等一系列分析工作，發現破管位置面火側管壁的波來鐵相已有輕微碳化物球化現象，雖然仍滿足ASTM A210 gradeA-1規範之拉伸性質和硬度要求，但已顯現材質老化的跡象；確認主要破損原因為內壁起始的沿晶腐蝕破裂，其形成機構為氫害現象（hydrogen damage）；而在爐管內壁氧化物層中亦偵測到銅金屬附著，此乃冷凝器銅管發生腐蝕作用，肇致銅遷移至水牆管表面，會促使碳鋼腐蝕速率增加。

The fossil-fired power plants are the backbone of electricity generation in Taiwan. The boilers of Taipower’s Ta-Lin Power Plant have been in service for over 260,000 hours since its first commissioning in 1970. ASME SA 210 grade A-1 carbon steel was selected as the construction material for waterwall tubes to undertake the severe circumstance. However, there were two successive failure cases occurred at the arch tube of waterwall in early 2006. The results of investigation works, inclusive of visual inspection, microstructural examination, electron microscopic analysis, hardness and tension tests, indicated the fire-side failure of arch tubes was caused by void formation on grain boundary firstly, and then disintegrated by cracks propagation along grain boundaries, i.e., the so-called hydrogen damage mechanism. A slight carbide spheroidization on the hot side of tubes also demonstrated an evidence of material aging effect. The damage rate was further accelerated by copper deposits on the inner surface of waterwall tubes.