復熱器管腐蝕破損原因分析

Failure Cause Analysis of Air Recuperator Tubes

某廠加熱爐的復熱器於2019年1月歲修時進行檢修，發現復熱器管熱風管及冷風管皆有斷裂，冷風管在端板與管口交接處腐蝕破損嚴重，為了解該復熱器之破損原因以改善其使用壽命，乃進行該復熱器之破損原因分析。
研究分析方法包括外觀觀察、spark成分分析、截面微硬度量測、草酸電解試驗、金相光學顯微鏡觀察、破斷面掃描式電子顯微鏡／能量散佈光譜儀觀察分析、腐蝕生成物分析等。
經分析結果研判，復熱器熱風管斷裂之主要原因為材質中C含量高或Ti添加量不足，導致在高溫下發生敏化，以及由於局部滲碳導致晶界碳化物明顯析出，在外界之S腐蝕因子侵入晶界造成沿晶腐蝕，最後導致斷裂；而冷風管斷裂主要原因研判為原來材質之組織異常所造成；端板交接處之冷風管腐蝕破損主要原因為硫酸露點腐蝕所引起。為延長設備使用壽命及避免類似狀況再發生，提出改善建議。

The exhaust gas recuperator of the heating furnace of a factory was maintained annually in January 2019. It was found that both the hot air tube and the cold air tube of the recuperator were fractured and the cold air tubes were seriously damaged at the intersection of the end plate and the pipe joint. In order to understand the failure causes of the recuperator and to improve its service life , some analysis study was conducted.
For speculating the failure cause, investigations and studies were performed. They were appearance examination, material composition analysis , hardness test, electrolytic etching in oxalic acid solution, metallography analysis, surface SEM/EDS analysis and corrosion products analysis etc..
According to the analysis results, it was speculated that the main failure cause for the breakage of the hot air tube was that the content of C in the material was higher and the amount of Ti was insufficient, this would result in material sensitization at high temperature, which combination with the precipitation of grain boundary carbides due to local carburization and the intrusion of S element from outside. They altogether caused intergranular corrosion of the hot air tube and leaded to fracture eventually. The main failure cause of the fractured cold air tube was caused by the abnormality of the original material; the main cause of corrosion of the cold air tubes at the junction of the end plate was caused by sulfuric acid dew point corrosion. Some suggestions for improvement and extending the service life of the equipment and to avoid similar situations were proposed.