Effects Of Alloying Elements On The Formation And Bonding Strength Of New Alumina-Forming Austenitic Heat-Resistant Steel/Oxide Layer Interface

Environmental pollution and energy shortage have been regarded as two main factors that restrict the sustainable development of the economy.The clean and efficient coal burning technology urgently requires the thermal power unit materials turn to large capacity ultra-supercritical units,which is a serious challenge for the oxidation resistance and high temperature mechnical properties of the heat-resistant steels used in ultra-supercritical units.The new Alumina-Forming Austenitic?AFA?heat-resistant steel with excellent high temperature oxidation resistance through the formation of protective Al₂O₃ and Cr₂O₃ scale and creep resistance via the dispersive precipitation of nanoscale NbC-type carbide strengthing will be the high temperature candidate materials of the next generation of ultra-supercritical units.High temperature oxidation resistance is the key factor to determine the durability of AFA steels.It is closely related to the structure,composition and bonding properties of the oxide layer.AFA steels with special chromia and alumina composite oxide layer are expected to be used for ultra-supercritical unit greater than 650°C.At present,microscopic formation mechanism of the oxide layer,effects of alloying elements on the formation of the oxide layer,and the interfacial bonding strength have not been explicitly explained.In this paper,we have studied the formation mechanism and bonding strength of the oxide layer in new AFA steels using the first-principles methods combined with experimental methods,and got some valuable conclusions.The main contents are as follows:?1?Optimization design and preparation of three new alumina-forming austenitic heat resistant steels based on the Fe-22Cr-25Ni?wt.%?for 700°C ultra-supercritical unit.Combined with the experimental data,a systematic investigation regarding the structure stability,thermodynamic property and mechanical property of these new steels was carried out by employing the first-principles method.A comparison of the mechanical properties presented that with increasing Al content,the plasticity was further improved without reducing the strength.First-principles calculation results showed that as Al existed in Fe-Cr-Ni alloy system as solid solution,the new structure was stable especially under high temperature.The solution of Al may improve the plasticity of the system not at the expense of obvious strength drop,particularly in case of alloying with Al+Si.?2?Oxidation experiments at 800°C for different times were carried out to analyze the effect of oxidation time on the structure and composition of the oxide layer.With increasing Al content,the oxidation resistance significantly improved.For 1.5Al steel,oxide layer had a multilayer structure with an outer Cr₂O₃ and an inner Al₂O₃ owing to the diffusion of Cr and Al from the matrix to the oxide layer.In order to explain the formation of Al₂O₃ scale,the diffusion behavior of Al atom from the Fe matrix to Cr₂O₃ bulk was examined.And the impacts of alloying elements?Cr,Ni,Mn,Si?on the formation of Al₂O₃ were also studied.The results indicated that the Al atom originating from the Fe-based matrix prefered to diffuse into the Cr₂O₃ slab,thereby resulting in the formation of the Fe/Al₂O₃/Cr₂O₃ construction,which agreed with the experimental behavior.Moreover,introductions of Cr,Ni,Mn,and Si could slow down the diffusion of Al and result in a slower growth rate of Al₂O₃.Furthermore,proper coordination of Si and Al could improve the bonding of the Fe/Al₂O₃/Cr₂O₃interface,thus improve the bonding of the oxide scales.?3?The tensile test at the atomic level of Fe/Cr₂O₃ and Fe/Al₂O₃/Cr₂O₃interface was carried out by focusing on the strain-stress relationships,interface structure and interatomic interaction.The deformation analysis revealed that the two interface structures had undergone homogeneous deformation and unhomogeneous deformation stages.The interfacial bonding strength of the austenite/oxide layer was stronger than that of the matrix,so the fracture occurred within the austenite matrix.The formation of Al₂O₃ in Fe/Al₂O₃/Cr₂O₃interface was conducive to strengthening the interaction between Fe and O,Al and O,thus improving the interfacial bonding strength.?4?This part investigated the segregation behavior of alloying additives X?X=Cr,Ni,Mn,Cu,Si,Al,V,Ti,Mo,W,Nb,Hf,Pt,Y,Ce?on the Fe/Cr₂O₃and Fe/Al₂O₃/Cr₂O₃ interfaces as well as the effects of these additives on the interfacial bonding strength.The results indicated that Cr,Cu,Si,Al,V,Ti,Mo,W,Nb,Hf,Y,Ce atoms were easily segregated at the Fe/Cr₂O₃ interface,while Ni,Mn and Pt were easy to dissolve into the matrix;Ni and Mn further improved the stability of the matrix.In alumina-forming austenitic steel,the introduction of Al could restrain the segregation of Cu to the interface and make it dissolve into the matrix,thus improved the stability of nanoscale Cu-rich precipitation phase.The interfical bonding strength of Fe/Al₂O₃/Cr₂O₃ interface was stronger than that of Fe/Cr₂O₃ interface,and the influence of the alloying additives on the interfical bonding was more remarkable.The introduction of Si and Al to Fe/Cr₂O₃ and Fe/Al₂O₃/Cr₂O₃ interfaces could improve the bonding strength through strong interactions between Si/Al and O.Thus,Si and Al could improve the bonding of surface oxide scales and enhance oxidation resistance.?5?Density functional theory was employed to study the segregation behavior of oxidizing medium S and the effect of S on the bonding of Fe/Cr₂O₃and Fe/Al₂O₃/Cr₂O₃ interface,as well as the effects of alloying additives Hf,Pt,Y and Ce on the segregation behavior of S and the interfical bonding of S-doped interface.The calculation results suggested that S prefered to occupy the interstitial site near the interface,further degraded the interfacial bonding dramatically.In alumina-forming austenitic steel,the introduction of Al could reduce the segregation rate of S to the interface;further weaken the adverse effect of S on the interfical bonding.Hf,Pt,Y and Ce could reduce the segregation rate of S to the interface.Hf?Ce?and S could form Hf-S?Ce-S?bond,then enhance the bonding of the S-doped interface,further alleviated the detrimental effect of S to some extent.