Analysis Of Time-dependent Stress Distribution Of Ethylene Cracking Furnace Tube Under Multi-factor Action

As the core equipment of ethylene production,ethylene cracking furnace is developing towards high temperature and large scale.which requires higher performance for ethylene cracking furnace tubes,the core component of ethylene cracking furnaces.The long-term service of cracking furnace tubes under high temperature,high carbon potential and low oxygen pressure will inevitably lead to the influence of coking,carburizing,creep and other damage factors,which will cause their service life to be lower than the design life.In view of carburizing and creep is the main factor that causes the failure of the furnace tube,the carburizing kinetics of Cr25Ni35 Nb and Cr35Ni45 Nb alloys was studied and analyzed using high temperature vacuum supersaturated carburizing method.And then the two kinds of already/non-carburized materials are studied.Constant-load creep acceleration experiments were conducted to compare and analyze the effects of carburizing on the creep properties of materials.Finally,the time-varying stress field distribution of furnace tubes under the combined action of internal pressure and creep was analyzed in detail,and the effect of furnace tube geometry(pipe diameter and wall thickness)on the stress field is discussed.The research results have certain engineering reference value for the design of ethylene cracking furnace tube and its safe and reliable operation.The main contents of this article are as follows:(1)The vacuum carburizing test and carburizing kinetics analysis were performed on Cr25Ni35 Nb and Cr35Ni45 Nb furnace tubes by vacuum gas carburizing test.The results show that the carburization growth values of the two kinds of furnace tubes increase with the prolongation of carburizing time,but the carburization growth rate tends to be flattened as the carburization time gradually decreases.This is due to the increase in carbon concentration inside the material matrix and the decrease in the content of metal elements capable of forming carbides,which hinders the diffusion of carbon in the metal.The carburizing growth value of Cr35Ni45 Nb alloy is smaller than that of Cr25Ni35 Nb alloy,indicating that the moderate increase of Cr,Ni,Nb and other metal elements can improve the high temperature carburizing resistance of furnace tube materials.(2)Creep tests for Cr25Ni35 Nb and Cr35Ni45 Nb alloys under different temperatures and stress levels.The comparative analysis of the test results and the creep curve predicted by the modified ? projection method verifies the feasibility of the modified ? projection method in predicting the creep curve of the material.The creep curve equations for two kinds of furnace tubes at temperatures of 1000°C and stress levels of 10 MPa were predicted and the parameters of creep materials under the corresponding conditions were fitted to provide a numerical basis for the calculation of the stress field.(3)Creep tests were performed on Cr25Ni35 Nb and Cr35Ni45 Nb alloys after vacuum carburizing.The effect of carburizing on the creep properties of furnace tubes was investigated.The results show that the creep properties of the two materials decrease with the deepening of the carburizing degree,and the Cr35Ni45 Nb alloy is more resistant to high temperature creep and high temperature carburization than the Cr25Ni35 Nb alloy.(4)The finite element analysis software ABAQUS was used to analyze the stress distribution of the furnace tube under the action of internal pressure and creep,It was found that the maximum damage value of the furnace tube was eventually distributed along the outer wall of the furnace tube,that is,the first damage occurred at the outer wall.In addition,the influence of the tube diameter and wall thickness on the stress field of the furnace tube was also explored.It was found that the maximum stress of the furnace tube increases with the increase of the pipe diameter or the thinning of the pipe wall,and its service life decreases accordingly.