Ladle furnace水冷式爐蓋冷卻水管破損原因分析

Failure Analysis of Cooling Pipe in Ladle Furnace Water Cooling Lid

本文探討Ladle furnace煉鋼精煉設備之水冷式爐蓋發生冷卻水管破裂之原因。研究方法包括現場調查、材料成分分析、破損件巨觀檢查、顯微組織觀察、腐蝕生成物分析、水質分析等，結果發現冷卻水管內表面沉積大量含有Cl-之硫酸鈣，並可在內壁找到許多垂直軸向且互相平行之微裂縫，但在外壁並未發現有明顯裂縫存在，顯示裂紋的傳播由管內至管外。經OM及SEM金相觀察結果，裂縫之延伸屬於穿晶破壞現象，且在外表層之金相並無碳化物球化析出現象，顯示冷卻水管無過熱現象。切取裂縫處剝開，可觀察到有明顯之海灘紋存在。另外在銲道破裂方面，經巨觀、微觀及硬度等檢測結果，推測可能是由銲補不良所留下之孔洞開始，沿硬度差異大之處向銲趾部延伸破裂。綜合以上之分析，推斷冷卻水管底材之破裂屬於腐蝕疲勞，腐蝕因子為冷卻水所含之氯，而高溫碳棒之反覆熱應力是造成其疲勞之主因，銲道之破裂可能因銲補不良所引起。

The purpose of this paper is to analyze the failure of a cooling pipe in ladle furnace water cooling lid. Through the actual field investigation, analyses of the material composition, macro and micro - fractographic study, analyses of both the corrosion product and the quality of cooling water, it was found a large amount of chloride - containing calcium sulfate deposited on the inner surface of the cooling pipe. Furthermore, there existed a large amount of radial microcracks in the inner tube. It revealed that the crystalline fracture was a transgranular type, which was also demonstrated that overheating never happened. Finally, some beach marks were observed on the fractured surface of the crack. About the crack of the weld, through macro- and microfractographic inspection and hardness measurement, we proposed that the crack was initiated by the porosity of incomplete fusion, and propagated toward the toe of the weld along the large hardness difference area. From the above analyses, we proposed the failure of the cooling pipe was corrosion fatigue. Corrosion was induced by the chlorides in cooling water, while fatigue was induced by the intermittent heating stress from high temperature carbon heater.