| At present,the use of stainless steel is increasing year by year.Austenitic stainless steel is the main consumable.Austenitic stainless steel is widely used in today’s society because of its excellent performance.Many properties of austenitic stainless steel are related to its solid phase transformation,such as 304 austenitic stainless steel under general casting conditions,the primary phase is ferrite structure.The morphology of primary ferrite determines the corrosion resistance of austenitic stainless steel.In this paper,the growth mechanism of primary ferrite formed during directional solidification is studied.As the price of nickel continues to rise,the economic benefits of ferritic stainless steel are increasingly prominent,and the use of ferritic stainless steel is increasing year by year.The ferritic stainless steel containing Ti and Nb studied in this paper can reduce C and N elements in ferritic stainless steel and improve the properties of ferritic stainless steel by adding stabilizer elements Ti and Nb in ferritic stainless steel.Firstly,304 austenitic stainless steel was obtained by directional solidification experiment.The initial ecological ferrite structure and morphology were determined by OM and SEM observation.Then,the orientation relationship between primary ecological ferrite and austenite was studied by EBSD method.For two kinds of ferritic stainless steels under annealing and casting conditions,OM method was also used to observe the metallographic structure and TiN precipitates in solidified structure.Among them,for annealed ferritic stainless steel,the effect of texture on the properties of steel was studied in detail.By means of SEM and EBSD,it was found that the texture in annealed ferritic stainless steel was almost uniform recrystallization texture of gamma fibers.The orientation relationship between TiN and ferrite in annealed ferritic stainless steel and the same kind of steel was also proved.The behavior of the solidified tissues was consistent.In addition,three different orientations of annealed ferritic stainless steel grain are found to have different corrosion sensitivity.Corrosion experiments are carried out.It is found that ND and TD directions have better corrosion resistance than RD directions,and TD directions have higher mechanical properties than RD directions.For the ferritic stainless steel under casting conditions,it is melted by directly melting annealed ferritic stainless steel and adding a certain amount of ferrotitanium.Observing the precipitates of TiN in its metallographic structure,we can see that the size of TiN increases with the increase of Ti content.In addition,a series of electrochemical experiments,such as polarization curve test and electrochemical impedance spectroscopy(EIS),were carried out to investigate the effect of Ti and TiN on the corrosion properties of ferritic stainless steel under casting conditions.According to the pitting location and self-corrosive current density in the polarization curve,it can be concluded that the addition of Ti improves the corrosion resistance of the material.This conclusion is also confirmed by the results of EIS test of the selected R(QR)equivalent circuit. |
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