負荷頻率與溶氫量對沸水式反應器壓力槽之低合金鋼疲勞裂縫成長速率的影響

Effects of Loading Frequency and Dissolved Hydrogen on the Fatigue Crack Growth Rate of Low-Alloy Steel in BWR

低合金鋼在模擬沸水式反應器的高溫水環境下，疲勞裂縫成長速率與負荷頻率和水中的溶氫量有關。在相同水質與預裂縫長度的條件下，裂縫延伸速率（da/dt）會隨著頻率的升高而增加。此外，低合金鋼在高溫純水中的電化學電位與水中的溶氧與溶氫量有十分密切的關係。實驗發現：在水中溶入少量的氫氣，亦即降低電化學電位，可以有效地抑制裂縫的成長速率。本實驗所求得之裂縫成長速率的數據大致上都可以被國際上通用的裂縫成長速率預測模型所涵蓋。因此，同時結合較低的負荷頻率與少量的溶氫可以確實地降低低合金鋼的裂縫成長速率，進而延長材料的使用壽命與增加電廠的操作安全。最後，比較低合金鋼在經過去氧化層處理前後的破裂面金相可發現環境促進破裂的延伸幾乎均屬於河流狀的劈裂與穿晶破裂的型式。

Fatigue crack growth rate of low-alloy steels exposed to a simulated boiling water reactor (BWR) environment was found to be dependent upon the loading frequency and the amount of dissolved hydrogen. The time-dependent crack propagation rate (da/dt) increased as the loading frequency increased under the same water quality and pre-crack length. In addition, the electrochemical corrosion potential (ECP) of low-alloy steel in high temperature pure water, is closely related to the amounts of dissolved oxygen and hydrogen. It was observed that dissolving a little amount of hydrogen, i.e. to lower the ECP, could effectively suppress the crack growth rate (CGR). The CGR data from this study were comparable with those obtained from some well-known CGR prediction models for BWR environments. Combining lower loading frequencies with the addition of a small amount of dissolved hydrogen can result a significant reduction in CGR, an extended lifetime of the material and an increase in operating safety. Finally, a comparison of the fractographs of the low-alloy steel before and after the oxide removal shows that the fracture appearance of EAC (environmentally assisted cracking) reveals mainly cleavage and transgranular cracking (TGC).