Remaining Life Prediction of Ethylene Cracking Tubes Under Carburizing and Creep Joint Action

Remaining life prediction model of ethylene cracking tubes (ECT) suffered joint damage of carburizing and creep have been set up in this study. Materials employed in this work were cut from Cr35Ni45-type radiation section of ECT. Layered structure, including oxide layer, carburized layer and carburizing transition layer, have been found from inside to outside of the tubes in scanning electron microscopy (SEM) images. However, no abrupt transition occurs between carburized layer and the base material. Both carburizing related layers caused the damage of the tubes, together with oxide layer. In order to facilitate the life prediction accuracy, carburizing transition layer was considered as a part of the damage layer. The remaining life of ECT was investigated by review of the microstructure and stress-rupture tests. Stress-rupture tests have been finished to obtained the rupture life of the tubes at 1000°C, 1040°C, 1080°C and 1125°C under six loading stresses (10MPa, 15MPa, 17MPa, 20MPa, 25MPa and 30MPa, respectively). In the results of stress-rupture tests, the combination of testing temperature at 1040°C and loading stress with 15MPa got the highest rupture life of 830h. Finally, a modified Larson-Miller remaining life prediction model of ECT, considering the comprehensive effect of carburizing and creep damage, has been established.