Study On Microstructure And Properties Of Aluminum-Containing Ferrite Heat Resistant Steel

As energy drying up and people’s emphasis on environmental protection,in order to further improve the efficiency of power generation,reducing emissions of CO₂ and other harmful gases,save resources,all countries in the development of advanced ultra-supercritical power plant,in our country is expected to raising the generator operating temperature to 700℃,served such a harsh environment on key parts of the plant candidate materials put forward higher requirements.The ferritic alloys strengthened by coherent-coplanar Ni Al-type phase with the B2 structure,which is similar to theγ-γ′microstructure of nickel-base superalloys,are expected to be potential candidates for ultra-supercritical steam turbine applications.In this paper,JMat Pro software is used for thermodynamic simulation calculation.The different compositions of ferritic heat-resistant steels used Ni Al phase with B2 structure as the main strengthening phase are designed.Alloys are prepared by vacuum induction melting technology.The microstructure and properties of Fe-Ni-Cr-Al-Mo-Si alloy and Fe-Cr-Ni-Al-Mo-Ti alloy systems have been investigated by means of optical microscope,scanning electron microscopy,energy dispersive spectrum analysis,X-ray diffraction,hardness and tensile test methods,respectively.With the central of the effects of heat treatment on the microstructural evolution and properties of two kinds of alloys and the oxidation resistance of Fe-Cr-Ni-Al-Mo-Ti alloys at 900℃to carry out in-depth study,in this paper.The research results are as follows:The thermodynamic equilibrium phase diagram calculated by JMat Pro shows that the microstructure of the designed Fe-17Ni-15Cr-4.5Al-2Mo-2Si（mass fraction,wt.%,the same below）alloy at room temperature was mainly composed of ferrite phase and Ni Al phase,but austenite phase transformation occurs during cooling.However,Fe-13Cr-10Ni-5Al-4Mo-2Ti（wt.%）alloy has no austenite phase transition during the cooling process,and the microstructure at room temperature was mainly composed of ferrite phase,Ni Al phase and Laves phase.The cast microstructure of the Fe-15Cr-17.3Ni-4.37Al-2Mo-1.83Si（wt.%）heat resistant steel was prepared consists of ferrite,austenite distributed along grain boundary and cubic Ni Al-type precipitates with a size of around 430-480 nm homogeneously distributed in the ferritic matrix.The Ni Al-type precipitates can be refined to 50-100 nm after solution treatment at 1200℃for 0.5h followed by air cooling.The aging treatment following the solution treatment causes these precipitates coarse,and after aging treatment at 1000℃,the austenite phase is formed along the ferrite grain boundary.Tensile test results show that the alloy has no room temperature ductility in the condition of both solution-treatment and aging treatment at 750℃,but the room temperature ductility and tensile strength can be significantly improved after aging treatment at 1000℃because of the formation of austenite phase,the tensile strength reached 1096MPa and the yield strength was 860MPa.The creep test result under the condition of 700℃/150MPa shows that the steady-state creep rate of solid solution alloy is the lowest,reaching 6.83×10^-12/s.However,the formation of austenite phase is not beneficial for the creep resistance.The phase compositions of Fe-Cr-Ni-Al-Mo-Ti alloys was mainly ferrite,Ni Al phase,Laves phase and MX phases.Among them,the components of MX phase are complex,mainly consisting of complex precipitates formed by Ti,Nb,Mo,B,N,C and O.The addition of 0.05%C（wt.%）in the alloy can effectively prevent grain growth and bend grain boundary.However,the addition of Nb element would make the grain grow up.Replacing Ti with Nb would form a large number of spherical inclusions rich in Nb,Ti,C and O elements between the dendrites and grain boundaries,and precipitate a large number of irregular Laves phases in the alloy.The dissolution temperature of Ni Al phase and the precipitation of Laves phase increase with the increase of Cr content.Short-time aging at different temperatures shows that a large number of short rod Laves phases will be precipitated and coarsening of Ni Al phases in the alloy after aging at 600-800℃.After aging at temperatures above 900℃,there is almost no Laves phase precipitation in Nb-free alloys,Ni Al phase begins to dissolve gradually,and a large number of smaller secondary Ni Al phases are precipitated during air cooling.After long time aging at 700℃,in the Ni Al phase of the alloy would precipitated Ni₂Ti Al phase,the two phase to form a hierarchical structure.With the extension of aging time,Laves phase increase and grow up gradually in the alloy,the layered precipitates formed by Ni Al phase and Ni₂Ti Al phase gradually spheroidize.The static oxidation weighing method shows that the oxidation weight per unit area of the alloys increases and the average oxidation rate decreases with the prolongation of oxidation time at 900℃.Among them,Fe-13Cr-10Ni-5Al-4Mo-2Ti-0.005B,Fe-13Cr-10Ni-5Al-4Mo-2Nb-0.05C-0.005B and Fe-13Cr-10Ni-5Al-4Mo-2Ti-0.5Nb-0.05C-0.005B（wt.%）had smaller oxidation weight gain.After 100 hours,their oxidation weight gain were 0.40042mg/cm²,0.44358mg/cm² and 0.59004mg/cm²,respectively.Their antioxidant grade was completely antioxidant.The oxidation weight gain of Fe-13Cr-10Ni-5Al-4Mo-2Ti-0.05C-0.005B and Fe-9Cr-10Ni-5Al-4Mo-2Ti-0.005B（wt.%）were slightly higher,which were 1.48838 mg/cm² and1.69607 mg/cm²,respectively.The oxidation weight gain of Fe-5Cr-10Ni-5Al-4Mo-2Ti-0.005B（wt.%）was the largest,which was 7.10837 mg/cm².Its oxidation kinetics curve was linear,and its oxidation resistance was the worst.These three alloys are of oxidation resistance grade.The results of XRD and EDS analysis show that the main oxidation products on the surface of samples after oxidation for 100h were Al₂O₃,Ti O₂ and Al₂Ti O₅.Under the condition of 900℃oxidation,the less of Cr content in alloy,the easier Ti oxidation and Ti,Al complex oxides were precipitated in the alloy.Element C can promote Ti element to diffuse to the surface layer to form the oxide of Ti,while the addition of Nb has little effect on Ti oxide formation.When Nb element is used instead of Ti,only dark Al₂O₃ and a small amount of bright round and flaky NbO₂ were formed in the alloy.