The Damage and Phase Structure Transformation of Pyrolysis Furnace Radiant Section Tube after Service
Author:Guo Jing Feng
Pyrolysis furnace is an important device in the production of ethylene,which is welded by Fe-Ni-Cr alloy centrifugal casting tubes.The radiant section tube is the key component in pyrolysis furnace,which is under the highest service temperature and harshest service environment of pyrolysis furnace.Therefore,the pyrolysis furnace tube commonly refers to the radiant section tube.High temperature,oxidation,carburization,creep,etc.bring about various damages to the furnace tube,which have a great influence on the service life of the pyrolysis furnace.At present,Cr25Ni35Nb and Cr35Ni45Nb type alloys are the most widely used pyrolysis furnace tube materials in China.It’s important for the safe operation of pyrolysis furnace tubes to study the damage of Cr25Ni35Nb and Cr35Ni45Nb type alloys after service.Although there are many researches on the pyrolysis furnace tube,the studies of the total damage of pyrolysis furnace tube,weld damage,carbide transformation and the furnace tube magnetic changes is still insufficient,and the in-depth analysis is urgently needed.In view of the insufficiency of the above-mentioned researches on the ethylene pyrolysis furnace tube,the combined long tube at radiant section(from inlet to outlet)of a liquid pyrolysis furnace after an eight-year service is studied in this paper.The combined long tube consists of one-pass tube,bent tube and two-pass tube,which is used to study the total damage of pyrolysis furnace tube.In addition,the weld joints and tubes of Cr25Ni35Nb and Cr35Ni45Nb alloys under different service conditions are selected for analysis,which are used to study the mechanism of carbide transformation,the factors of magnetic change and the damage of weld joint and base metal of pyrolysis furnace tubes after service.The main conclusions are drawn as follows:(1)The microstructure,oxidation damage and mechanical properties of the combined long tubes in the radiant section of the pyrolysis furnace are investigated in present research.It is found that microstructure deterioration occurred at the furnace tube.The oxide layer consisting of oxide film and Cr-depleted zone is formed at the inner and outer surfaces,and the mechanical properties at room temperature and high temperature declined.The microstructure deterioration and oxidation damage degree of the two-pass tube(Cr35Ni45Nb alloy)in the radiant section is greater than that of the one-pass tube(Cr25Ni35Nb alloy).In the two-pass tube,the highest volume of carbide is 45%,the maximum thickness of oxide layer at the outer wall is 1270 μm,and the maximum thickness of oxide layer at the inner wall is more than 700 μm.The high temperature creep properties of one-pass tube and two-pass tube decreased after service,but the creep properties of one-pass tube decreased faster than that of two-pass tube.(2)The damage of weld joint of the pyrolysis furnace tube under different condition and the combined long tube at radiant section is conducted in this research.It is found that the microstructure between new weld joint and base metal is obviously different,and the high-temperature creep strength of base metal is significantly higher than that of the weld.The high-temperature creep rupture time of base metal is more than 2 times that of weld joint.After a long-term service or aging treatment at high temperature,the microstructure of weld joint and base metal degraded,that lead to the decreasing of creep properties of weld joint and base metal.The creep properties of base metal decreased faster than that of weld joint.Therefore,the high temperature creep strength of weld joint and base metal gradually became uniform(the creep fracture time of the weld joint reached 85%of the base metal).(3)After service,the carbide transformation of pyrolysis furnace tube alloy is characterized by EBSD(electron backscatter diffraction).The microstructure of furnace tube alloy deteriorated after service,the M7C3 type carbide(M is mainly Cr element)and NbC type carbide are transformed into M23C6 type carbide(M is mainly Cr element)and G phase(Ni16Nb6Si7),respectively.In addition,many secondary carbides M23C6 precipitated at dendritic interior.The G phase(Ni16Nb6Si7)nucleated at dendritic interior and the edge of carbides,which mixed with M23C6 type carbides during growing.In carburization zone,the M23C6 type carbide(M is mainly Cr element)changed into M7C3 type carbide(M is mainly Cr element).The M7C3 type carbide began nucleate at the edge of the bulk M23C6 type carbide.Then,with the diffusion of C atoms,the bulk M23C6 carbide continuously transformed into M7C3 type carbide until the whole bulk M23C6 carbide completely changed into M7C3 type carbide.(4)The mechanical properties of Cr25Ni35Nb and Cr35Ni45Nb type alloys carburized and non-carburized(microstructure deterioration)pyrolysis furnace tubes are analyzed in this paper.The hardness of carburized tube is higher than non-carburized tube.In addition,these two kinds furnace tube have the similar room-temperature tensile properties.The high temperature tensile strength of the carburized furnace tube is similar to non-carburized tube,but the elongation of carburized tube is about 50%of non-carburized tube.The creep properties of carburized tube is obviously higher than that of non-carburized tube.The creep rupture time of carburized tube is dubble of that of non-carburized tube.(5)Based on the analysis of hysteresis loops and saturation magnetization along the wall of pyrolysis furnace tube of Cr25Ni35Nb and Cr35Ni45Nb type alloys after service,the Cr-depleted zone and carburization zone,which are generated by oxidation and carburization respectively,lead to magnetic transformation of the pyrolysis furnace tube.However,the thickness of oxide layer and the volume fraction of carbides have no relationship with the magnetism of the serviced furnace tube.There is a high correlation between the furnace magnetism and the Cr content of the matrix of the furnace tube alloy,and there is irrelevant to Ni and Fe content.When the Cr content of the matrix is less than 18%(wt.%),the saturation magnetization gradually decreases with the increase of Cr content.When the Cr content in the matrix is higher than 18%,the matrix of the furnace tube is paramagnetic material;and the furnace tube is no magnetism.