Effect Of Al And Si Elements On The Microstructure Evolution And High-temperature Oxidation Behavior Of Ferritic Heat-resistant Stainless Steel

The X10CrAlSi18 ferritic heat-resistant stainless steel has excellent thermal conductivity and good high-temperature resistance for gas corrosion,and it is compounded with Al and Si elements to assure its exceptional resistance to high-temperature oxidation.This steel is a novel material that is employed in the structural connectors of supercritical and ultracritical power plant boilers.However,there is no theoretical basis for the effect of Al and Si elements added to steel on phase change,microstructure,and precipitates management during the preparation stage.Meanwhile,there is a paucity of analytical data on the effect of Al and Si elements on the ductile-brittle transition temperature and the formulation of processing parameters.Furthermore,much work remains to be done on the complex film forming process of Al and Si components,as well as the systematic assessment of oxidation resistance during application.In light of the aforementioned issues,this paper first investigates the impact of Al and Si elements on the evolution of the solidification microstructure,then elucidates the mechanism of Al and Si elements on the ductile-brittle transformation behavior of this steel,and finally clarifies the mechanism of Al and Si elements on the high-temperature oxidation behavior of this steel.The above research results in the precise control of the solidification structure during the preparation process,the optimization of subsequent processing parameters,and the practical evaluation of high-temperature oxidation performance.It provides theoretical and technical support for resolving key technical issues in the preparation,processing,and application processes,as well as for realizing large-scale production.Based on the calculation of JMatPro thermodynamic simulation software,the equilibrium solidification structure of X10CrAlSi18 heat-resistant stainless steel with different Al and Si contents is composed of ferrite,M₂₃C₆phase andα-Cr phase.The initial precipitation temperature of austenite gradually decreased as the Al and Si content increased,as did the range of austenite precipitation temperature and the maximum precipitation amount,while the content of Al and Si have little effect on the precipitation law of M₂₃C₆phase.The M₂₃C₆phase is primarily composed of Cr,Fe,Mn,and C elements,and as temperature decreases,the mole fraction of Cr element increases,the mole fraction of Fe element decreases,the mole fraction of Mn element increases first and then decreases,and the mole fraction of C element remains essentially unchanged.The microstructure evolution of X10CrAlSi18 heat-resistant stainless steel during solidification was studied by means of high temperature confocal microscopy,and the effect of Al and Si content on the solidification structure and precipitates was elucidated.The results show that the Al and Si contents have a significant impact on theδ-ferrite nucleation temperature,δ→γphase transition temperature,and volume fraction ofγ-austenite/martensite and M₂₃C₆carbides in the final solidification microstructure.Since both Al and Si are ferrite forming elements,theδ-ferrite phase region will expand during the solidification process,which is conducive to the precipitation ofδ-ferrite crystallization,resulting in an increase in the nucleation temperature ofδ-ferrite.The increase of Al and Si content can narrow the free energy difference betweenδ-ferrite andγ-austenite,weakening the driving force ofδ→γphase transformation,which not only decreases theγ-austenite transformation temperature,but also inhibits the martensite transformation in the subsequent cooling process,resulting in a decrease ofγ-austenite/martensite volume fraction.The M₂₃C₆carbides are mainly distributed in theδ-ferrite andγ-austenite/martensite phase boundaries,which are not favorable for M₂₃C₆carbide nucleation when the volume fraction ofγ-austenite/martensite is reduced.The ductile-brittle transition behaviors of X10CrAlSi18 heat-resistant stainless steel for various Al and Si content were investigated by a series of temperature Charpy impact tests at temperatures ranging from 20 to 90°C.The results show that with the increase of Al and Si content,the impact absorption energy at each impact test temperature gradually decreases,the ductile-brittle transition temperature increases,and the number of cleavage planes in the fracture increases.Due to the increase of Al and Si content,the ferrite grain size increases,and the M₂₃C₆carbide gradually changes from a small-sized strip or small-sized massive to a long strip or large massive.The large ferrite grain size and the long strips or large massive of M₂₃C₆carbides are the main factors that deteriorate the impact toughness.The high-temperature oxidation resistance behavior of X10CrAlSi18 heat-resistant stainless steel with different Al and Si contents from 700°C to 900°C in air was studied by the static discontinuous oxidation weight gain method.The results show that the oxidation weight gain and oxidation rate gradually decreased with the increase of Al content at each oxidation time,which show a good resistance to high-temperature oxidation.This is mainly attributed to the increased Al content,which forms a continuous and dense Al₂O₃oxide at the beginning of oxidation and can effectively hinder the outward diffusion of other ions in the matrix.However,with the increase of Si content,the oxidation weight gain and oxidation rate gradually increased in each oxidation time,indicating that the high-temperature oxidation resistance decreased.This is because the increase of Si content will increase the content of SiO₂in the initial stage of oxidation.Due to the difference in chemical bond types between Al₂O₃and SiO₂,the radius of Al and Si will change during the formation of oxides,thus reducing the density of the oxide film and weakening its protective effect of the oxide film..The increase of Si content also enhances the number of M₂₃C₆carbides in the matrix and reduces the adhesion between the oxide film and the matrix.While the precipitated M₂₃C₆carbides consume a large amount of Cr and Mn concentration in the nearby matrix,which inhibits the formation of protective oxide films of Cr₂O₃and Mn₂O₃at the early stage of oxidation,resulting in a deterioration of high-temperature oxidation resistance.