熱交換管高溫破裂分析

Analysis of High Temperature Failure of Heat Exchange Tube

本分析之高溫破裂熱交換管，係中鋼公司煉焦乾式淬火（CDQ）設備剛更新完成之管件，從暖機到發生破裂時間僅三天，造成現場運轉極大困擾。經由成分分析、機械性質試驗、硬度試驗、金相分析、腐蝕生成物分析、破斷面觀察及模擬試驗之結果證實：破管曾遭遇約900℃/3天之不正常熱履歷，使得內部彎曲部位生成厚達0.5 ~1mm之氧化物，金相組織從肥粒鐵加波來鐵轉變為硬脆之變韌鐵加惠德曼肥粒鐵。此時，材料內部之硫及磷有機會擴散至晶界，降低先前沃斯田鐵晶界之結合力，當蒸汽壓力大於晶界鍵結力，熱交換管即發生變形而導致沿晶破裂。熱交換管彎管部位局部處於高溫之原因，乃包商安裝不當所殘留之異物阻塞水蒸汽循環有關。

The operation time of high temperature failure of the heat exchange tube for coke dry quenching (CDQ) equipment in this analysis was only three days after CDQ revamping. This non-scheduled shut down led to an appreciable loss. The composition analysis, mechanical property testing, hardness testing, optical microstructure observation, corrosion product detecting, fracture analysis, and simulation were performed in this study. The results showed that a 0.5~1mm thickness of the oxide existed on the inner part of the failure tube. The original microstructure of ferrite and pearlite transformed to brittle bainite and Widmanstatten ferrite, indicating that the failure tube had the abnormal heat history for three days at 900℃. In this condition, the sulfur and phosphorus could diffuse to the grain boundary of the tube material and decrease the binding energy of pre-austenite grain boundary. When the vapor pressure of cooling water was larger than the grain boundary coalescence force, the heat exchange tube would induce intergranular cracking due to the occurrence of distortion.