Phase Precipitation And Deformation Mechanism Of FeCrAl Stainless Steel

FeCrAl stainless steel is an ideal material for the production of automotive exhaust gas purifier carriers due to its excellent high-temperature oxidation resistance and low thermal expansion coefficient.However,due to the high Al content in FeCrAl stainless steel,the solidification forming control is difficult and the forging rolling is easy to crack in production.These problems seriously restricts the development and production of automotive exhaust gas purifier in China.In view of these,the phase precipitation and deformation mechanism of FeCrAl stainless steel were studied on the following aspects in this thesis:(1)Equilibrium solidification phase transition,(2)Precipitation mechanism of AlN during nonequilibrium solidification,(3)Precipitation behavior of ?' phase,(4)Dynamic recrystallization behavior,(5)Dynamic strain aging behavior.The phase transitions and precipitation behavior of FeCrAl stainless steel during equilibrium solidification and cooling were studied using phase diagrams derived from Factsage calculations together with results from thermal expansion experiments.Based on the Factsage calculations,a complete equilibrium phase transition path was determined.According to the vertical sections and isothermal cross-sectional graphs,the precipitation of(Fe,Cr)7C3 is mainly affected by Cr,Al and C content.A decrease in Al content or an increase in Cr content can reduce the phase stability range of(Fe,Cr)7C3.An increase in A1 content can reduce the phase stability range of ?.The average linear expansion coefficient of FeCrAl alloy is decreased by the presence of the ?' phase and increased by the precipitation of(Fe,Cr)23C6.The effect of cooling rate on the evolution of AlN inclusions precipitated during nonequilibrium solidification in FeCrAl stainless steel was investigated using unidirectional solidification experiments and thermodynamic and kinetic calculations.The number and size of AlN inclusions precipitated under different cooling rates were examined with the Feature function of the field-emission scanning electron microscope.The Ohnaka diffusion-AIN growth coupling model and finite difference method of solute microscopic segregation-AIN growth coupling model were set up to determine the mechanism of AIN particle growth.The results showed that AlN precipitates in the mushy zone.The size of AlN particles decreases and the number of AlN particles increases with increasing cooling rate,whereas the volume fraction is essentially unchanged.The nitrogen content varies significantly with the cooling rate when AlN particles start to precipite during solidification.Increasing the cooling rate and reducing the nitrogen content in the molten steel can reduce the AIN particle size in FeCrAl alloys as the growth time decreases.During solidification,the segregation of nitrogen,aluminum and chromium in the liquid phase decreases with the increase of the solid phase fraction,while that of aluminum and chromium in the solid phase increases.The segregation of nitrogen in the solid phase first increases and then decreases after the AlN inclusion begins to precipitate.The precipitation behavior of ?' phase for FeCrAl stainless steel aging at 475? was studied by thermodynamic analysis,three-dimensional atom probe tomography and nano indentation experiment.Atom probe tomography and thermodynamic analysis show that the separation of a phase and ?' phase at the nanometer scale is the main mechanism leading to the isothermal aging strengthening of FeCrAl stainless steel at 475?.The ?' phase precipitated from FeCrAl stainless steel is a dispersive spherical particle.The content of Cr in the central region of ?' phase increased with the aging time at 475? and Al atoms were strongly distributed in the Fe-riched phase.The rare earth element La can be enriched around the ?' phase during the aging process and the precipitation of ?'phase can be inhibited.The nano-indentation experiments showed that the hardness and elastic modulus of FeCrAl stainless steel increased with the aging time at 475?.To investigate the dynamic recrystallization(DRX)behavior of as-cast FeCrAl stainless steel,a series of compression tests were carried out on a Gleeble-3500 thermal simulator.By regression analysis of the true stress-true strain curves,the apparent activation energy for FeCrAl stainless steel was estimated to be 300.19 kJ/mol,the constitutive equation was developed successfully,and some thermophysical parameters were identified and expressed as the function of Zener-Holloman parameter.The kinetic model of DRX was proposed with the modified Avrami type model.According to analysis of the kinetics model and microstructure evolution,the volume fraction of DRX grains increased with the increase of strain;at a fixed deformation temperature,the deformation strain required for the same amount of DRX volume fraction increased with increasing strain rate;the size of DRX grains increased with the increase of temperature or the decrease of strain rate.Microstructure observation also indicated that the continuous dynamic recrystallization was the predominant nucleation mechanism for DRX in FeCrAl stainless steel.Monotonous deformation behavior of a ferritic FeCrAl alloy at a temperature range from room temperature to 600? was studied using the uniaxial tensile tests.The results showed that dynamic strain aging(DSA)occurred at a strain rate of 3.333×10-4s-1 over the temperature range from 200? to 400?.Negative strain rate sensitivity of ultimate tensile strength(UTS)was observed as the characteristic manifestation of DSA performed in FeCrAl alloy.It was also found out that,type A,A+B,B,C+D and C serrations appeared in turn in the stress-strain curves tested at a constant strain rate with the increase of temperature,and the decrease in strain rate shifted the DSA temperature range toward the lower temperature region.Statistical method was used on stress drops with DSA to investigate the character of serration.From the activation energy calculations with McCormick's model,the solute atoms responsible for DSA were identified to be substitutional aluminum atoms.