石化工場高鉻鎳離心鑄造爐管高溫潛變破裂行為探討

Creep Behavior of a Ruptured High Cr-Ni Centrifugal Casting Furnace Tube in a Petrochemical Plant

本研究主要藉由一使用6年之高鉻鎳離心鑄造爐管破裂的破損分析，來探討此類材料之潛變破裂特性以及影響因素，同時從操作條件和鑄造品質來討論對爐管壽命的影響，破裂爐管材質為HP-40 Mod Nb Micro alloy，爐管外徑為125.6 mm，壁厚為9 mm，內容物組成主要為合成氣、H2、CO2和CO，內容物溫度為875℃，操作壓力為20 bar，煙道氣溫度平均為1050℃，分析結果顯示爐管存在晶界孔洞、串聯孔洞和裂紋，以及沿晶破裂型態等特徵，研判爐管破裂機制為潛變破裂，而過熱以及材料存在鑄造瑕疵是爐管潛變破裂的肇因；塊狀的共晶碳化物提供更短的路徑，會加速潛變裂紋的成長速率；此外利用LMP曲線計算不同壁厚位置潛變壽命損傷比的結果和實際爐管截面之潛變裂紋分佈特徵吻合。

Creep behavior and the pertinent causes of a high Cr-Ni centrifugal casting furnace tube ruptured after a six-year service were investigated by failure analyses. The effects of operating parameters and casting quality on the creep life of the tube were discussed. The material of the ruptured tube is HP-40 Mod Nb micro-alloy. The outside diameter of the tube is 125.6 mm and the wall thickness is 9 mm. The operating temperature and pressure were 875 ℃ and 20 bar, respectively. The average flue gas temperature was 1050 ℃. The content of the fluid include synthesis gas, H2, CO2 and CO. After thorough inspections, isolated voids, oriented voids, fissure, and intercrystalline fracture morphology were found. The rupture mechanism of the furnace tube was creep damage. Overheating and materials defects were the root cause of the creep rupture. Blocky eutectic carbides provided a shorter path for creeping and enhanced the crack growth. Calculated results of creep life by the LMP curve at different depth positions of the tube wall were consistent with the observed creep cracking morphologies at the ruptured surface.