熱處理對英高鎳600顯微結構及敏化行為的影響

Effects of Thermal Treatment on the Microstructural Evolution and Sensitization of Alloy 600

顯微結構為決定英高鎳600在加氫脫氧的PWR核能電廠一次側水質及純水環境中產生晶間應力腐蝕破裂最重要的影響因素之一，因為英高鎳合金之破裂敏感度可經熱處理而大大的變化。溫度及碳含量為決定固溶及退火處理英高鎳600顯微結構的兩個重要因素，而析出溫度及時間為決定熱力析出英高鎳680鉻耗乏、碳化物分佈形態及晶界偏析的重要參數。SEM及EDX分析工具被用以確認經各種熱處理後英高鎳600的碳化物分佈、晶粒大小及晶界耐蝕性等和晶間應力腐蝕破裂相關的結構特徵，而EPR電化學測試用以決定鉻耗乏、偏析及孔蝕的電化學行為。本研究發現不只晶界碳化物可機械性地鈍化裂縫尖端，且晶界本身的耐蝕性亦為決定英高鎳600 產生PWSCC的重要因素。英高鎳600和不銹鋼敏化行為不同，不尋常敏化行為及不均匀的硫偏析在本研究被發現，其和PWSCC的關係仍待深入探討。

The microstructure of alloy 600 has been recognized as one of the most important variables in determining IGSCC of alloy 600 in high temperature hydrogenated and deaerated PWR primary water and/or pure water since the susceptibity to IGSCC of alloy 600 can be modified significantly by thermal treatment. Temperature and carbon content are two important factors influencing various microstructures of alloy 600 in solution annealing and mill annealing states while as aging temperature and time are two main parameters deciding the morphologies of carbides, Cr depletion and grain boundary segregation in thermally treated alloy 600. SEM and EDX were applied to identify the microstructure characteristics, such as g.b.d. corrosion ristance, carbides distribution and grain size etc., of alloy 600 after various heat treatments. EPR tests were used to determine the degree of sensitization, impurities segregation and pitting during the tests. It has been found that not only intergranular carbides can mechanically enhance the IGSCC resistance, but also the corrosion resistance of g.b.d. itself may electrochemically influence IGSCC susceptibility of alloy in PWR environments. Contrary to that of stainless steel, unusual sensitization behavior and nonuniform sulfur segregation were discovered in mill annealing and short time thermally treated alloy 600.