核電廠X-750合金組件應力腐蝕破裂研究

Study of Stress Corrosion Cracking of Alloy X-750 Components in Nuclear Power Reactor

合金X-750是一種析出硬化型的鎳基合金，含有鎳、鐵、鉻、鈮及少量鈦、鋁等元素。由於合金X-750廣泛使用於爐心組件上，故其損壞極可能造成安全上的問題，同時在更換時亦造成大量的成本損失。目前各國研究的方向皆集中在（1）熱處理與元素成分的控制，（2）元件之幾何、負荷應力及殘留應力控制，（3）尋找新的替代材料等三方面。本研究確認及分析在不同熱處理條件下，合金X-750的重要微結構特徵，並利用慢速拉伸試驗測試材料在不同化學環境下之破裂敏感度。研究顯示，經由材料表面的破裂情形，可知X-750確實有沿晶應力腐蝕的情形；經過704℃/20hr時效處理的X-750合金有最多的Cr23C6析出，沿晶應力腐蝕抗性最高；經過885℃/24hr時效處理後，材料的沿晶應力腐蝕抗性最差。水環境中的離子雜質會影響材料的沿晶應力腐蝕程度，其中以氯離子比硫酸根離子容易使材料產生沿晶應力腐蝕。根據實驗結果及相關資料，合金X-750經過704℃/20hr單階時效處理後，其沿晶應力腐蝕抗性最強。

Alloy X-750 is a precipitation-hardenable, nickel-based alloy, which contains nickel, iron, chromium, niobium, and little amount of titanium, aluminum. Since alloy X-750 was extensively applied for reactor core components, its failure may cause safety problems, and also cost loss due to replacement of components. At the present day, the research scopes of the related countries include (1) control of heat treatment and elements of alloy; (2) design of the component geometry, load and residual stresses; (2) the alternate materials. This study analyzes and characterizes the important microstructure features of X-750 for various heat treatments and evaluates stress corrosion cracking (SCC) susceptibility of alloy X-750 by the slow strain rate tensile test in different chemical environments. It was shown that the alloy X-750 suffered intergranular stress corrosion cracking (IGSCC) problem through the observation on the surface cracking of material. Alloy X-750 has the richest Cr23C6 precipitate which shows the highest resistance to IGSCC after 704 /20hr aging treatment, while it has the lowest resistance to IGSCC after 885 /24hr aging treatment. Impurity content in water chemistry can influence the resistance to IGSCC, in which the chlorine ion is more corrosive than the sulfuric ion. According to the experiment results and the related literature review, alloy X-750 shows the highest resistance to IGSCC after 704 /20hr aging treatment.