The Residual Life Predication Of Ethylene Cracking Furnace Tube

The ethylene cracking furnace is the core equipment of ethylene production and directly related to the operation of the ethylene unit.The lifetime of furnace tubes are the key to normal operation of cracking furnace.This paper focuses on the study of HP40Nb heat resistant steel,also known as 25Cr35NiNb alloy,which is a kind of material commonly used in ethylene cracking furnace tube.By means of chemical composition analysis,ametallographic examination and scanning electron microscopy(SEM)together with energy dispersive spectroscopy(EDS),applied and new furnace tubes provided by a chemical plant were alalyzed to indentificate the failure mechanisms,the author has an in-depth study of the organization transformation and composition change of the material.Thus resulting in skeleton-shaped eutectic carbide(r+M7C3+a small amount of M23C6)of the original organization of 25Cr35NiNb alloy under high temperature conditions,which afterlong time's application gradually becomes M23C6 in strips and patches,and have a certain amount of secondary carbide precipitation.The disappearance of skeletal eutectic carbide in alloy may result in decline in creep strength.Through an experiment on stretching of the new and old furnace tubes at normal atmospheric temperature,we found that the yield strength and tensile strength of the applied furnace tube has dropped 50%and 60%respectively when compared to the new one,showing an apparent decrease in material strength.In order to understand creeping property,three creep experiments was employed on new and old furnace tubes.Based on experiment result,we can know the degradation of furnace tubes.The creep longevity of furnace tubes was evaluated by Theta and Omega method,so that we can set up residual life prediction model.After the comparison,we found that Theta method,based on the proportional rise in the creep rate with strain,is more suitable to predict the creep longevity of 25Cr35NiNb alloy.Then Larson-Miller(LM),Orr-Sherby-Dorn(OSD)and Manson-Harferd(MH)models were established through the combination of conventional time-temperature parameter method(TTP)and particle swarm optimization(PSO).These three models were used for evaluating the HP40 heat resistant steel's creep rupture life.Subsequently,Z parameters were used to analyze the accuracy of these three models' forecasts.MH model was found to have highest accuracy by analyzing the forecasts of these three models.At last,the materials'creep rupture life was calculated to ensure the accuracy of the forecast results based on different reliability.So far,we have the preliminary understanding of the creep mechanism of alloy.On the basis of analyzing creep stress index and activation energy of the alloy,we worked out the creep mechanism of 25Cr35NiNb alloy in different conditions through its typical features.