Failure Analysis Of Cracking In HP-40Nb Ethylene Cracking Furnace Tubes

Cracking furnace tube is one of the key parts in petrochemical processing industry,which is produced by centrifugal casting.By means of macroscopic examination,metallographic analysis,scanning electron microanalysis and its accessory energy dispersive spectrometer and electron back scattering technology scattering technology)and other characterization means and relevant mechanical performance test methods,the relationship between the mechanical properties and microstructure evolution of the HP-40Nb（25Cr35Ni Nb）ethylene cracking furnace tube,which failed to swell and crack after21 900 h in service,shorter than service time of design for 43 800 h,were basically researched.The results are shown:The metallographic structure of HP40Nb centrifugal cast tube is mainly composed of equiaaxial crystals on the inner wall and columnar crystals on the outer wall,and their wall thickness accounts for 47%and 53%respectively in the furnace tube.The microstructure is composed of a large amount of austenite+a small amount of eutectic carbide Cr₇C₃ and(Nb C+a small amount of Cr₂₃C₆)distributed in network.Both the inner wall and outer wall of the cracking furnace tube are oxidized.Cr-containing carbides dissolve slowly at service temperature to provide a steady supply of metal Cr atoms in the oxidation process,resulting in Cr-poor zone on the inner and outer surface of the furnace tube and near the surface of the opening crack.Two oxides,Si O₂ and Cr₂O₃,mainly exist in the Cr-poor zone.The oxidation layer on the inner wall is evenly distributed,and the distribution of Si O₂ and Cr₂O₃ is network.The outer surface oxide layer is completely covered on the surface,showing a sparse porous distribution,and a small number of holes are included in the oxide layer,which does not play a protective role,on the contrary,the oxide layer generated in the service process reduces the effective wall thickness of the cracking furnace tube.After 2.5 years service,the grain boundary coarsening occurred seriously in the furnace tube microstructure,and the skeletonlike carbide changed to network,and the skeletonlike carbide Cr₇C₃ changed to more stable Cr₂₃C₆ under the high temperature service environment.EBSD results show that the carbide generation on the outer wall of cracking furnace tube changes from skeleton Cr₇C₃ to network Cr₂₃C₆ at high temperature.After carburizing,the primary carbide Cr₂₃C₆at the grain boundary is gradually transformed into Cr₇C₃,and the transition starts from the side with high carbon concentration in the process.Matrix in the process of service will intracrystalline secondary carbide precipitation Cr₇C₃,serving the secondary carbides in high temperature environment with concentration of C rise will occur Cr₇C₃→Cr₂₃C₆ transformation process.The failed HP40Nb ethylene cracking furnace tube has significant creep deformation at the fire facing surface.Coke is attached to the inner wall at the bulging position.The thickness of the inner wall carburizing layer reaches 61%of the wall thickness of the furnace tube.After service,the mechanical properties of HP40Nb ethylene cracking furnace tubes decline,and the microhardness decreases from the inner wall to the outer wall along the radial distribution.The fracture analysis shows that there are a large number of creep voids and cracks in the inner and outer walls of the cracked furnace tubes,and the voids are mainly distributed near the primary carbide.There are a lot of spherical precipitates and banded coke on the fracture,and the fracture type is brittle fracture.Therefore,the main causes of early failure of furnace tube are local high temperature,long time high carbon activity and small proportion of columnar crystal area.