熱時效鑄造沃斯田鐵系不銹鋼於模擬核能電廠冷卻水環境中之應力腐蝕龜裂敏感性研究

Abstract

本研究探討CF8A鑄造沃斯田鐵系不銹鋼，在高溫水環境下的應力腐蝕龜裂敏感性。研究中針對δ-肥粒鐵的含量、熱時效的時間以及軋延的程度對CF8A不銹鋼應力腐蝕龜裂敏感性影響，進行了詳細且全面的研究，並提出了應力腐蝕龜裂的機制。研究的結果表示，熱時效對CF8A不銹鋼的顯微組織影響較小，而軋延則會導致滑移線和αʹ-麻田散鐵在沃斯田鐵基體中形成。熱時效會增加δ-肥粒鐵的硬度，但對沃斯田鐵硬度的影響則較小。軋延會使δ-肥粒鐵和沃斯田鐵的硬度提升，且隨著軋延程度的增加，δ-肥粒鐵和沃斯田鐵的硬度也會隨之提升。材料的δ-肥粒鐵含量、熱時效時間及軋延程度的增加，均會顯著提高材料的最大抗拉強度，並降低材料的延展性。此外，越長的熱時效時間及越大的軋延程度，均會導致CF8A不銹鋼有更高的應力腐蝕龜裂敏感性。值得注意的是，高δ-肥粒鐵含量試樣的應力腐蝕龜裂敏感性較容易受熱時效影響，而低δ-肥粒鐵含量試樣的應力腐蝕龜裂敏感性則較容易受軋延影響。對於經過熱時效和軋延的低δ-肥粒鐵含量試樣，裂紋傾向沿滑移線擴展，從而使穿晶應力腐蝕成為主要的應力腐蝕龜裂機制；而對於經過熱時效和軋延的高δ-肥粒鐵含量試樣，裂紋則傾向沿δ-肥粒鐵與沃斯田鐵之間的相介面擴展，表示相介面的破裂為主要的應力腐蝕龜裂機制。

This study evaluated the stress corrosion cracking (SCC) susceptibility of CF8A cast austenitic stainless steels (CASS) in a high-temperature water environment. The combined effects of δ-ferrite content, thermal aging duration, and rolling degree on the SCC susceptibility of CF8A stainless steels were thoroughly examined, and the corresponding SCC mechanisms were proposed. The results showed that thermal aging has minimal impact on the microstructure of CF8A stainless steels, while rolling promotes the formation of slip lines and αʹ-martensite within the austenite matrix. Prolonged thermal aging increases the hardness of δ-ferrite but has little effect on the hardness of austenite. Rolling hardens both δ-ferrite and austenite, with hardness increasing as the degree of rolling increases. Increases in δ-ferrite content, thermal aging time, and rolling degree all lead to a significant rise in ultimate tensile strength (UTS) and a reduction in elongation of CF8A stainless steels. Additionally, longer thermal aging times and greater rolling degrees contribute to higher SCC susceptibility. Notably, the SCC susceptibility of specimens with higher δ-ferrite content is more affected by thermal aging, while those with lower δ-ferrite content are more influenced by rolling. For thermally-aged + rolled specimens with lower δ-ferrite content, cracks are more likely to propagate along slip lines, making transgranular stress corrosion cracking (TGSCC) the dominant SCC mechanism. For thermally-aged + rolled specimens with higher δ-ferrite content, cracks tend to propagate along the δ-ferrite/austenite interface, indicating that interface cracking is the predominant SCC mechanism.