熱交換鰭片破損分析

Fracture Analysis of Cast Heat Transfer Fins

鑄造熱交換鰭片為控制鋼捲封盒退火溫度部件；一組熱交換器由兩片鰭片經銲接後組成，其外部通COG氣體並點火燃燒進行升溫，內部則通入空氣並藉由熱交換以調節溫度。熱交換鰭片為GX-CrNiSi 22-9耐熱鑄鋼，此合金設計為抗高溫氧化需求，且使用溫度範圍為900 ~ 1100℃。於現場操作上，熱交換鰭片由室溫經不同昇溫速率加熱至900℃後進行持溫。然而，部件經使用後發現，熱交換鰭片發生彎曲變形及破損之現象。經微觀觀察及相鑑定發現，破損件表層及內部分別有氧化物及析出物生成。表層氧化物為鐵氧化物而非具有抗高溫氧化特性之鉻氧化物；內部則以硫化物、碳化物居多並含微量之富鐵鉻相析出物。分析散熱鰭片化學組成顯示，鉻及鎳元素含量小於合金設計規範，導致其抗高溫氧化溫度僅約870℃，顯示部件抗高溫氧化能力不足為導致破損發生之因素。

Batch annealing temperature of steel was controlled by heat transfer which was welded by two heat transfer fins. COG gas was ignited for raising temperature on external heat transfer fins. Ambient air, on the contrary, flew inside the part for temperature control. According to high temperature oxidation resistance requirement, alloying elements of cast heat transfer fins were based on GX-CrNiSi 22-9 design, and the appropriate using temperature was suggested to be in the range from 900 to 1100 ℃. In practice, annealing temperature was raised from ambient to 900 ℃ and soaked at that temperature for hours. However, deformation and fracture of heat transfer fins occurred during long-term soaking. After microstructure observation and phase identification of the fracture part, the product formed on the surface layer was found to be Fe oxide instead of high temperature resistant Cr oxide. Meanwhile, sulfide, carbide and a few Fe-Cr rich products could be detected nearby the surface layer. According to chemical composition analysis of heat transfer fins, Cr and Ni contents were less than alloying design requirement, resulting in the reduction of the scaling temperature to 870 ℃. The improper chemical composition of heat transfer fins would be the factor of fracture occurrence.