Solidification Features Of FeCrAl Stainless Steel Used As Automobile Exhaust Purifier Carrier

With excellent oxidation resistance at high temperatures, relatively low thermal expansivity and heat capacity, FeCrAl stainless steel is ideal for metallic honeycomb substrate used as automobile exhaust purifier carrier. However, the deficient knowledge for the solidification features of this steel leads to the high cost of production, which seriously hinders the domestic populization and application of metallic automobile exhaust purifiers. In view of this situation, systematic research on these four aspects has been carried out:1) Determination of liquidus and solidus.2) Morphology and scale transformations of liquid-solid interface and the segregations in different freezing conditions.3) The phase diagram and phase transformations of multi-component system relevant to FeCrAl stainless steel.4) The effect of titanium on the solidification structures and high-temperature mechanical properties of FeCrAl ingots.1) To obtain the formulae suitable to calculate the solidus and liquidus temperatures of FeCrAl stainless steel, the liquidus and solidus temperatures were determined by differential scanning calorimetry (DSC) at different heating rates. The liquidus temperatures were also calculated by Thermo-calc software and empirical formulae, respectively. The comparison between the two calculations and DSC results indicated that certain formula can figure out the liquidus more accurately than Thermo-Calc software, with errors below 6℃.The comparison between Thermo-calc calculations performed under the Scheil model and solidus determined by DSC indicated that the solidus temperature could be well determined from solid fraction (fs) vs. temperature (T) curves at fs=0.99. Then the effects of different elements on the solidus temperature were analyzed. Finally, a formula proper to calculate the solidus temperature was obtained, with errors not more than 6℃.2) Morphology and scale transformations of liquid-solid interface in different freezing conditions were studied by Bridgeman directional solidification and optical microscope. The results indicated that, with the increment of cooling rate, the following transformations took place for the liquid-solid interface: planar interface(v’=0.012℃/s)â†'dendrite (v’=0.3℃/s)â†'cell(v’=0.6℃/s)â†'hexagon cell (v’=1.8℃/s). The secondary dendritic arm spacing could be well predicted by the formula which is generally suitable for binary alloy system. The aluminum segregations for different interface morphologies were studied by electron probe micro-analysis (EPMA). The results indicated that the aluminum partition coefficient, kAl, at planar interface was 0.95-0.97. No obvious aluminum segregations were found for other interface morphologies by elemental area scanning under EPMA.3)Vertical sections of Fe-(18～21)Cr-(3～5)Al-(0～0.03)C-(0～0.2)Si-(0～0.2)Mn multicomponent system relevant to FeCrAl stainless steel were calculated by Thermo-calc software and studied with literatures. The results were as following: The equilibrium phase compositions of FeCrAl stainless steel should be AlN, Two carbides of Cr7C3 and Cr23C6, two chromium-rich pahses of a’and σ. The precipitation of Cr7C3 mainly depends on C content and that of σ phase mainly depends on Si content. The equilibrium phase transformantion routes for FeCrAl stainless steel with a range of components were finally obtained. Accordingly, the equilibrium phase composition of FeCrAl stainless steel at room temperature should be aFe+AlN+Cr23C6+a’.4)The precipitation of TiN at freezing frontier for Fe-Cr-Al-Ti-N system was calculated by Thermo-calc software with Scheil module. Ingots with different Ti contents were melted and cast by vacuum induction furnace and the macrostructures were analysed. The results indicated that the center equiaxed grain ratio reached a maximum of 39.5% with 0.088%Ti. The ratio rapidly decreased with higher Ti content of 0.17%. The analysis of high-temperature tensile and fracture micro-analysis with SEM indicated that TiN inclusions in Ti-bearing ingots could induce crackle, which is not good to the improvement of high-temperature mechanical properties of FeCrAl ingots. Inclusion analysis by SEM indicated that Ti mainly exists in five types of inclusions:pure TiN inclusion, TiN-AIN composite inclusion, TiN-Al2O3 composite inclusion, TiS inclusion and TiS-oxide composite inclusion.