Life Prediction Of Welded Joint For Ethylene Cracking Furnace Under Multiple Conditinos

Ethylene Cracking Furnace Tube (CFT) usually used in high-temperature and complicated environment. Creep, carburizing and temperature change during decoking process had a strong influence both on the life of the welded joints and the furnace. The paper focused on the creep and damage life of CFT without cracks under multi-factor conditions, and performed numerical analysis of the impact under various constraint conditions on the crack-tip stress field and crack tip high-temperature fracture mechanics parameters under creep conditions, finally analyzed the constraint effects on the life prediction of CFT with defects.Firstly, investigation on creeping property of the CFT HP alloys showed that the disappear of skeleton-like carbide in the microstructure and compositional variation of carbide weakened CFT’s high-temperature properties, meanwhile, the infiltration of carbon atoms resulted in the formation of carbide at grain boundary, so that it increased in number, unwanted carbon aggregated at grain boundary, which broke the grain boundary succession, resulted in the microstructure deterioration of CFT. The lowest creep rate of HP base alloys and welded metal was in line with strain, the creep property of weld metal was better than base metal.Secondly, finite element analysis was employed for life prediction of CFT’s creep damage under multiple conditions. The results showed that if the creep was taken into account, when the CFT reaches its design life, is far from failure. Therefore, it was improper to design tube life according to the creep strength. The simulation on the creep damage under multi-coupling conditions showed that the outer wall of CFT failed firstly after 30000h service, the damage caused by multiple stresses coupling effect was identified for the said early failure of CFT.Thirdly, the numerical analysis on the geometric constraint caused by crack length and constraint resulting from mismatch of creep property was performed under different creep conditions. The results showed that Q can serve as a measure of geometric constraint. When steady state creep was properly approached, Q could be approximately considered had no relationship with crack-tip distance r. when the crack size was given, the constraint drops with the load increase. The creep constraint resulted from the mismatch of creep could be expressed by M. The softer welded seam was, the greater M, a greater constraint and vice versa. Mismatch constraint parameters M was not sensitive to creep index n, but width of weld seam c had great impact on M. When crack was located in base metal and paralleled to weld seam, whether creep soft weld seam or hard one, the crack-tip stress tend to crack-tip stress of single base metal sample; when the crack was perpendicular to weld seam, the crack-tip stress was controlled by two parts, for the cracks in base metal, is by creep property of itself; for the crack-tip opening stress in weld seam, controlled by seam’s creep property.Finally, for the welded joints of CFT, the joints were creep hard weld seam, the stress redistribution during creep will increase the stress in welded seam, and quicken crack growth rate. If mismatch of materials was not considered, the creep crack growth rate will be underestimated in practical, furthermore, less conservative result will be reached for the corresponding high temperature components in the subsequent life prediction, therefore brought unsafe evaluation.