轉爐廢氣處理系統上護罩膜管破裂原因分析

Analysis of the Fracture Mechanism of Membrane Tubes of Upper-hood in BOF OG System

轉爐廢氣處理系統（OG設備）上護罩於工廠製作完成後，即進行全面英高鎳合金（Inconel625）覆面銲，並上線使用。使用不到1年，陸續發生膜管橫向龜裂漏水，雖經緊急銲補搶修，仍發生多次漏水現象，影響生產甚鉅。為了瞭解本批護罩膜管龜裂原因，遂分別進行外觀觀察、成分分析、附著物及生成物分析以及破裂分析。主要結論如下：膜管破裂型態分管內爐管用鋼破裂及管外覆面銲合金破裂兩種，其中膜管內壁有明顯局部腐蝕現象，破裂為平行裂紋，少見分支，及破斷面觀察到明顯之海灘紋及疲勞紋理，可以確認破裂原因為腐蝕疲勞。造成腐蝕疲勞之原因，推測為管內鬆散腐蝕生成物沈積，導致沈積腐蝕及不利熱傳導，加上煉鋼吹煉週期之反覆熱應力，促進熱腐蝕疲勞發生。膜管外側覆面銲發現兩種不同合金，發生破裂的覆面銲合金為高溫耐蝕性較差之鐵基309不銹鋼。裂紋沿著覆面銲鑄造組織之樹枝狀晶介面傳播，屬於沿晶破裂之方式，並可以發現晶間分離之液化現象，此為沃斯田鐵組織常見之熱裂現象。造成熱裂之主要原因為膜管銲接時表面油污未清除乾淨，導致晶界面的雜質S濃度太高而誘發熱裂。

The membrane tubes of upper-hood in BOF OG (Oxygen converter gas recovery) system were overlay welded using Inconel 625 alloy after manufracture in workshop. However, the transverse cracks occurred in membrane tubes within one year. This leakage of membrane tubes would disturb the melting process seriously and induce the abnormal shutdown. Therefore, outer observation, composition analysis, corrosion product analysis, and failure analysis were conducted to analyze the crack mechanism. The main conclusions were below: the failure modes were divided into inner cracks of membrane tubes and outer cracks of overlay welding alloys. Obviously localized corrosion existed on the inner surface of membrane tubes. The crack was straight with no branch. The beach marks and striations were observed on the fracture surface. This fracture mechanism was corrosion fatigue. Loose corrosion products deposition and cyclic thermal stress were the cause to bring about corrosion fatigue. On the other hand, there were two different alloys existed on the outer side of membrane tube. One is 309 stainless steel and the other is Inconel 625 alloy. Crack occurred in the 309 stainless steel. Cracks would propagate along the dendrite arm interfaces of overlay welding cast microstructure. In addition to the intergranular propagation, the separation of grain boundary would suggest the liquefaction. This fracture mode belonged to hot cracking. Surface greasy dirt during welding was the major responsibility for sulfur segregation on grain boundary to induce hot cracking.