Study On High Temperature Oxidation Behaviors Of Ni-cr-fe Based Alloys And Evolution Regularity Of Pre-oxidized Film In Oxidation/carbonation Critical Conditions

Ni-Cr-Fe based alloys were widely used as high temperature oxidation-resistantmaterials in ethylene cracking industry due to the fast formation of dense chromiumoxide layer on the surface in oxidation environment. With continuingimprovement ofthe ethylene crackingprocess, the growingcomplexityof raw materials and the depthof cracking, the ethylene cracking temperature were increased. Betteroxidation/carburization-resistant properties for ethylene cracking tube materials arerequired. The chromium oxide can protect alloys from oxidation and carburization attemperature below 1000℃. When the temperature excess 1000℃, chromium oxidecan be easily decomposed to volatile CrO₃, CrO₂ phases, which could loss theprotectiveproperty.AsingleCr₂O₃protectivefilm couldnot meet therequirements ofmodern industrial technology. When alloying elements were added, more stablecompositeoxidefilm canbeformedonthe alloysurface,whichis aresearchfocus onanti-carburizationmaterials.In this paper, alloys were cast by adding Si and Mn elements to Ni-Cr-Febased alloys. The effects of alloy elements and the pre-oxidation conditions(temperature and oxygen partial pressure) on oxide film composition andmicrostructure were systematic studied. The stabilities of MnO,Cr₂O₃and MnCr₂O₄spinel were also analyzed in the carbonaceous environment. The thermodynamicproperties and carburization resistance of composite oxide film on the alloy surfacewere also studied. The oxide film structure and surface morphology werecharacterized using thin film X-ray diffraction (XRD), scanning electron microscopy(SEM),energyspectrumanalysis(EDS)andothermethods.(1) The influence of pre-oxidation temperature and oxygen partial pressure onthe microstructure of the oxide film was studied. When oxidation of Ni-Cr-Fe based alloys occurat lowertemperature,moredeveloped oxides forms intheeutecticregion,mainly because of the laminar structure of eutectic areas. The phase boundaries canprovide fast diffusion channel for alloying elements, therefore, the surface oxidemainlybased on feather-like growth in this region. With the increase of pre-oxidationtemperature, diffusion of alloying elements within the austenite phase increased,mainly through the defect of Cr₂O₃ film in austenite areas, so some rod-like Mn-Crspinel were formed in this area. It was found that oxygen partial pressure of thesystem has an important effect on the microstructure of oxide film. At lower oxygenpartial pressure, a thick and porous structured oxide film was formed, and thedominant phase of oxide film was Cr₂O₃. With the increases of oxygen partialpressure, the dominant phase of oxide film gradually transformed to composite oxidefilm. The micro-structure of oxide film becomes dense and thinner. A maximumcontentofspinelphasewasobtainedinthemiddleoxygenpartialpressure.(2) The growth kinetics of oxide film for Ni-Cr-Fe based alloy is closely relatedto the microstructure of the matrix and the corresponding Si content. During theoxidation process, a continuous amorphous layer of SiO₂ sub-layer in the low Sicontent alloys can not be formed, resulting in a thickand uniform oxide layer formedon the surface; while a continuous SiO₂ amorphous layer can be formed on thesub-surface in the higher Si content alloys, which can block the outside diffusion ofalloying elements from the substrate, resulting in a thinner outer oxide film. Moredeveloped oxide formed above this region due to a discontinuous of SiO₂sub-films.Higher diffusion coefficient of Mn ion is the reason of Mn-Cr spinel forms at theoutmost of the film. For lower Mn containing alloy, small particle Mn-Cr spinelwould to be formed. With increases of Mn content, the morphology of Mn-Cr spineltransforms tolath-shaped. LowerSi andhigherMn content ofthealloyresultedinthestellatemorphologyofMn-Crspinelonthealloysurface.(3) Study of MnO₂, Cr₂O₃ and MnCr₂O₄ spinel powder sample in carbonaceousenvironment shows that MnO₂ would transfer to MnO after high temperaturecarburization, and MnO shows a good stabilityin carbonization process. Cr₂O₃ wouldtransfer to Cr₇C₃, Cr₂₃C₆ and Cr₃C₂ in the carburization process, which shows the weight loss first and weight gain later. The micro-structure was also porous structure.MnCr₂O₄ spinel shows excellent stability and no structure transformation during thewholecarburization process.(4) The carburization resistance of composite oxide film for different alloysafter pro-oxidation were also studied. MnCr₂O₄ spinel in composite oxide filmcould hinder the diffusion of C to matrix, and delay the carburization of Cr₂O₃.Thermodynamics forming and diffusion dynamics of the composite oxide filmwere calculated, which confirm that the composite oxide film was in the order ofMnCr₂O₄, Cr₂O₃and SiO₂ from outmost to inner substrate.