Effect Of The Creep-carburization Interaction On Creep Stress Of Ethylene Cracking Furnace Tubes

Ethylene cracking furnace tube,as the core equipment of the ethylene production unit,is facing an increasingly harsh service environment with the continuous large-scale and high parameters development of the ethylene unit.Therefore,the failure of the furnace tube has been upgraded from single carburization and single creep failure to the coupling failure affected by variety of factors.Thus,the failure of ethylene cracking furnace has seriously threatened the safety of the production and economic benefits of the enterprise.Compared with single failure of carburizing or creep damage,the impact of creep-carburizing coupling effect on failure of the tube is even more serious.Previous studies are mainly based on the study of microstructures evolution and mechanical properties of single damage,while the studies of the coupling effect of creep and carburization are less.CSEI inspection results of the petrochemical equipment especially the inspection of the ethylene cracking furnace tube were consulted.In this paper,the experiments and analysis of the microstructure and mechanical behavior for single creep or carburization and creep-carburizing coupling damage mechanism were carried out.More importantly,the effect of carburization on the creep behavior was studied systematically..Creep and carburizing tests was carried out to quantify the damage affected by the creep-carburizing coupling effect.OM,XRD,SEM,EDS,micro-hardness testing were applied to characterize the microstructure and mechanical properties before and after creep,carburization and creep-carburizing coupling testing.Key parameters of the creep constitutive equation for Cr35Ni45 Nb material affected by the creep-carburizing coupling effect was obtained,which may further optimize the life prediction of the cracking furnace tube.Results of the microstructure and mechanical properties of the tubes showed that the microstructure changes by carburizing.Three layers including carburized layer,transition layer and non-carburized layer were formed during carburization testing.The micro-hardness testing results demonstrated that similar phenomenon was found compared to the microstructure.The effect of carburization on the microstructure and mechanical properties of the material shows a positive correlation with the distance from the surface of the specimen.Carburization influencing the constitutive equation has been studied by creep rupture testing for carburized specimens,which also influences the creep stress.With the help of the finite element analysis software – ANSYS,10h-carburzed and 30h-carburized creep specimens were analyzed by considering the creep analysis of the stress.von-Mises stress and steady creep rate were obtained to establish the stress-strain model.By using this model,carburizing stresses were calculated.It is found that the proportion of the carburizing stress in the total stress of the specimen increase with the rise of the carburizing level.