The Second Precipitation Behavior Of Fe-Cr-Al-N High Aluminum Stainless Resistant Steel

Lightweight high temperature resistant steel of containing aluminum has great application prospect in many fields,such as aerospace,marine,automobile manufacturing,energy,medical equipment and so on.Aluminum is easy to form compounds with other elements of the active elements in the steels,such as FeAl and Fe₃Al,The precipitation behavior of phases has prominent influence to the property of steels,so the precipitation behavior of the second phases has prominent engineering value and academic meanings in steels.The precipitating behaviour was investigated by combining XRD,SEM,DSC in the process of heating with thermodynamic calculation using FactSage,the first principles.The temperature,type and maximum possible precipitation of the second phase in steel were calculated by FactSage thermodynamic calculation.Based on the first principles,the effect of Cr content on the stability and mechanical properties of FeAl ordered phase was investigated.After heat treatment,the precipitation temperature,species,morphology,size and distribution of the second phase were investigated by DSC,XRD,optical microscope and SEM.The hardness and corrosion resistance of sample steel were tested in this paper,Results are as follows:Possible phases of Fe-Cr-Al-N were FeAl?Fe₃Al?Fe₄N?Fe₃C?Al₉Cr₄?Al₈Cr₅?AlN and Cr₂₃C₆;Combining the theoretical calculation with experimental results show that the second phase were FeAl and AlN,and The others were not detected because of instability or too little.The precipitation temperature of FeAl and AlN were 570??1620?respectively by thermodynamic calculations,and the decalescence peaks appears in the range of 561?to 740?in the DSC curve of FeAl.AlN appears from liquid at 1620?by thermodynamic calculations.The first principles show that the stability of FeAl can be raised by doping Cr,because it can form two additional peak at 72.24⁷1.24 eV and 44.09⁴2.46 eV in the total density of states of FeAl,and the strength of the peak?charge density?electron transfer?the interactions between the electrons all above-mentioned increased with the increase of Cr.The propertise such as strength?shear strength?deformation resistant of FeAl increase by doping Cr,and the FeAl strength of increase gradually with the increase of Cr,however,the shear strength and deformation resistant firstly increased and then decreased with the addition increase of Cr,reaching the highest values in 18.75%Cr.the brittleness of FeAl can be improved within 18.75%range,however,the brittleness of FeAl can be reduced by doping 25%Cr.In summary,it is easy to form FeAl,because of improving the phase by doping Cr.The matrix of Fe-Cr-Al-N steel is ferritic,and their compact structure and little defects such as porosity,crack,shrinkage cavity,but the defects of Fe-Cr-Al-N include rough grain?segregation along grain boundaries,and the diameter of grain up to500⁶00um.The second phase AlN is distributed in the matrix.be,and the result of XRD indicates that there are many the phases of FeAl.FeAl was not observed by the metallographic microscope and SEM,because the space structure of FeAl and the matrix is body centered cubic structures.The matrix solid solution state of Fe-Cr-Al-N is ferrite,and their compact structure.Some small particles were observed in the matrix and the results of the spectra show that the small particles are poor aluminum phase.After steel is treated with solid solution in the air,aluminum is burnt out in the surface layer and the oxide film cracked and flaked off,leading to serious dealuminizing phenomenon in steel.After aging treatment at 550??600??650,the results show that the matrix aging state of Fe-Cr-Al-N is ferritic matrix,and their compact structure.The results of energy spectrum analysis and XRD show that FeAl was detected only at 550?aging treatment,so the optimal aging temperature of FeAl precipitation phase is 550?.Some small particles were observed in the matrix and the results of energy spectrum analysis showed that the small particles was rich chromium phase or poor chromium phase,which indicated that the matrix existed segregation after aging treatment.After aging treatment,the surface of aluminum still did not precipitated at 550?and 650?aging treatment.The hardness of the sample was changed after different treatment.The hardness of cast,solid solution state,550??600??650?ageing states were 204.84HV,176.12HV,253.84HV,212.35HV and 210.83HV respectively.The hardness of the steel is reduced by the solid solution treatment,due to no martensite phase transition occurs and the phases were dissolved into the matrix..The hardness of steel is improved by aging treatment,because of the phases were precipitated after aging treatment.the corrosion potential of 550??600??650?aging states and cast were-0.741V?-0.736V?-0.637V?-0.575V respectively.It shows that the cast steel displayed good corrosion resistance,and the corrosion resistance from large to small in turn is cast?650??600??550?aging states.The corrosion resistance of the steel is reduced by aging treatment,because of the phases were precipitated,the corrosion resistance of550?aging treatment was the worst,deducing that there were many phases precitated in the 550?.