Study On Microstructure And Properties And High Temperature Oxidation Behavior Of High Aluminum Ferrite Heat Resistant Stainless Steel

At present,in order to save energy,reduce emissions and protect the environment,thermal power generation improves the thermal-electric conversion efficiency of thermal power units,forcing the generating units to face more demanding working conditions and placing higher requirements on the performance of boiler steel materials.High-aluminum ferritic heat-resistant stainless steel has become one of the materials of choice for steel used in super（super）critical thermal power units due to its advantages of low thermal expansion coefficient,high thermal conductivity,resistance to thermal fatigue damage,and low cost.Its typical representative aluminum element in X10CrAlSi18 steel grades improves the oxidation resistance of the material,due to the inclusion of easy-form aluminum and silicon oxide,and promotes the coarsening of ferrite grains at high temperatures,reducing the mechanical properties and performance of the material.Therefore,study the occurrence state of aluminum in ferritic heat-resistant stainless steel and determine the optimal chemical composition range;study the influence of different heat treatment systems on the structure and performance,determine the best heat treatment process;by controlling the reasonable grain size,suppression The precipitation of Al compounds andσequal brittle phase makes the steel plate have the best toughness and high temperature mechanical properties matching.The work has important academic research and practical application value.In this paper,based on the composition of X10CrAlSi18 ferritic heat-resistant stainless steel,two test steel materials with aluminum content were prepared through composition control design,smelting and rolling.With the help of optical microscope（OM）,scanning electron microscope（SEM）,energy spectrum analyzer（EDS）,X-ray diffractometer（XRD）,tensile test and other methods,the heat treatment process system and aluminum element on heat-resistant stainless steel were studied The effect of microstructure and mechanical properties;at the same time,a high-temperature oxidation test was carried out on the material.Through the research and characterization of oxidation kinetics curve,oxide film surface morphology and phase composition,oxide film cross-section morphology and element distribution,etc.The effect of oxidation temperature and aluminum on the oxidation behavior of new heat-resistant stainless steel materials.The main conclusions are as follows:1)The thermodynamic equilibrium phase diagrams of the two test steels were simulated by JMat-Pro software.The results show that the main precipitates of ferritic heat-resistant stainless steel during the solidification process are M23C6 carbide,M7C3carbide,Sigma phase and Alpha-Cr.Experimental analysis found that the hot-rolled microstructure of ferritic heat-resistant stainless steel is mainly composed of ferrite phase and carbide aggregated into a network.2)By annealing the ferritic heat-resistant stainless steel at 790℃～910℃,it was found that its microstructure is mainly composed of ferrite phase,intermetallic compounds and granular or lath M₂₃C₆ carbide;annealing at 850℃for 30min After that,the microstructure is uniform,the ferrite grain size is fine and uniform,the precipitation phase is less,and it has the best comprehensive mechanical properties:the tensile strength is 664.551MPa,the yield strength is 371.224MPa,and the elongation is31.65%.3)After the content of aluminum in ferritic heat-resistant stainless steel is increased from 0.63%to 1.06%,the diffusion of carbon atoms is suppressed,and the network carbide is dissolved into island-like granular carbide;the ferrite grain recrystallization process is promoted To refine the grain.Therefore,the high aluminum test steel has excellent strength and hardness,and the plasticity change is not obvious.4)High temperature oxidation resistance is an important indicator to evaluate the performance of heat-resistant stainless steel.Experiments show that the average oxidation rate of two types of aluminum-containing ferritic heat-resistant stainless steels at 700℃,800℃,and 900℃is less than 0.1g·m^-2·h^-1,and their oxidation resistance levels are all completely anti-oxidation materials.The oxidation weighting curves all conform to the parabolic law of oxidation.The lower the oxidation temperature,the greater the aluminum content.The smaller the weight gain during the same oxidation time,the lower the oxidation rate,and the higher the oxidation resistance of the material.5)The analysis of the cross-sectional morphology of the oxide film shows that both ferrite heat-resistant stainless steels with aluminum content form a thin oxide film,which is mainly composed of five oxides of MnCr₂O₄,Fe Mn₂O₄,Fe₂O₃,Cr₂O₃ and Al₂O₃;the oxide film is the most The inner layer oxide is mainly composed of Al2O3and Cr2O3 oxides,and the outer layer is mainly composed of MnCr2O4 and FeMn2O4spinel structure oxides;there are granular or strip-shaped inner oxides generated inside the oxide film,and the inner oxides are mainly Al₂O₃.