304不銹鋼鑄件之缺陷分析及對策

Abstract

本研究主要是針對304不銹鋼鑄件之各種不同缺陷，如：晶界碳化鉻(Cr23C6)析出、鑄造縮孔缺陷、焊接破裂等，進行分析，並提出解決方法與對策。此外，本研究亦針對304不銹鋼鑄件，建立適當之酸洗與鈍化處理條件來去除不銹鋼表面之鐵份或含鐵汙染物，以形成保護性鈍化膜來提升其抗蝕性。最後，本研究亦以次氯酸鈉水溶液作為腐蝕液，建立腐蝕檢測之標準，以作為鑄件品質管制之方法。 針對晶界碳化鉻Cr23C6析出問題，採用固溶處理(1080oC - 2hr)及水淬，將晶界上所析出之Cr23C6碳化物完全分解，以解決粒間腐蝕之問題。至於鑄件縮孔缺陷問題，研究上採用solidcast電腦軟體進行凝固模擬，以預測縮孔可能發生之位置，並進行最佳冒口設計，以解決縮孔問題。另外，本研究探討以酸洗與鈍化處理來嘗試彌補鑄件表面缺陷可能產生之腐蝕問題，實驗結果顯示，雖可完全通過ferroxyl測試，但仍舊無法通過鹽霧試驗之測試標準。最後，本研究建立了5% NaOCl-50 oC-24hr之加速腐蝕測試方法，測試結果顯示試片之腐蝕程度與原試片之品質有密切的關係，腐蝕結果易於判別，且測試時間可縮短至24hr以內，可作為鑄件品質管制之方法。

The purposes of this research are three fold：(1)To analyze the casting defects, including Cr23C6 carbides precipitation at grain boundaries, shrinkage porosities, welding cracks, etc, and to propose corrective measures; (2)To obtain appropriate pickling and passivation treatment conditions to remove surface defects and hence increase the corrosion resistance ability; (3)To establish a standard corrosion resistance test method by using sodium hypochlorite(NaOCl) as corrosive etchant to serve as quality control scheme. Regarding the precipitation of Cr23C6 carbides at austenite grain boundaries, solution treatment (1080oC/2hr) followed by water quench was performed. As a result, the Cr23C6 precipitants can be eliminated almost completely. In addition, solidification simulation by employing solidcast software was conducted to predict the locations of possible shrinkage porosities formation. And then, riser design was performed to solve the shrinkage defect. Furthermore, this study investigated the effects of pickling and passivation treatments on corrosion resistance of defective castings. The results show that the treated castings can pass the ferroxyl tests, but fail to pass the salt-spray tests. Finally, this study also developed a hasty corrosion resistance testing method, namely, 5% NaOCl-50 oC-24hr, which has demonstrated a very good correlation between casting quality and degree of corrosion, and can serve as an effective method for quality control.