Damage analysis of in-service ethylene cracking furnace tubes using small punch test

The outer wall of the ethylene cracking furnace tube is subjected to thermal radiation, while the inside of the tube has a high carbon potential and low oxygen pressure. Small punch test (SPT) technique can be used to sample the outer surface of the furnace tube. However, since the complex conditions of the inner and outer surfaces of the tube, the representativeness of the outer surface sampling needs to be studied. Therefore, in order to investigate the damage situation of the furnace tube in the radiation section of the in-service ethylene cracking furnace made of 25Cr35NiNb alloy along the thickness direction of the tube, microstructure and SPT were used to compare the in-service 7-year-old furnace tube with a new furnace tube, and the feasibility of using SPT instead of the uniaxial test was explored. The results showed that the microstructure, chemical elements, and hardness of the in-service furnace tube varied at different positions along the thickness direction compared with the new furnace tube. The SPT at room temperature revealed that the performance degradation at the inner and outer surfaces was more serious than that at the middle. The small punch creep test (SPCT) found that the outer surface had a shorter fracture time than the inner surface at the same temperature and load, indicating that the outer surface was more severely damaged than the inner surface. The SPT showed consistency with the uniaxial test in mechanical properties and fracture morphology. The results of this study provide strong technical support for future assessment of damage and remaining life prediction of inservice ethylene cracking furnace tubes based on the SPT technique.