An Investigation On The Alloying Design And Performances Of New Heat-resistant Ferritic Stainless Steel

With the continuous upgrading of the standards for energy conservation and emissions reduction in automotive industry,the hot end of automobile exhaust system need to bear higher temperature.It is estimated that the temperature of the hot end of car exhaust system will exceed 1000?.A new generation of heat-resistant ferritic stainless steel with higher anti-oxidation ability to resist higher temperature oxidation corrosion is urgently needed,and is also the hot issue in automotive materials research.Under this background,this thesis deals with the cyclic oxidation resistance at elevated temperature by adding different contents and kinds of alloying elements W and Ce to the existing 444 ferritic stainless steel.Based on this,this study is to design and optimize the alloy composition of the ferritic stainless steel through the thermodynamic simulation calculation.The main contents and results are summarized as follows.(1)The high temperature cyclic oxidation experiment of some ferritic stainless steel containing Ce and W was conducted and the cyclic oxidation kinetics curves at 1050?for these steels were obtained.The results show that the addition of rare earth element Ce can significantly improve the oxidation resistance of the ferritic stainless steel.Weight gain rate per unit area of the ferritic stainless steels gradually was reduced with increasing the cycles of high temperature oxidation.Tungsten can affect the ability to resist high temperature cyclic oxidation.When the content of Tungsten is lower,the antioxidant properties will be further promoted.But the antioxidant properties will be deteriorated with increasing the oxidation cycles,while tungsten addition exceeds some certain content.(2)The surface morphologies of the oxide layer at 1050? for different cycles of oxidation were characterized for the ferritic stainless steels,and the adhesion ability between the oxide film and the substrate were analysed.The results show that rare earth element Ce contribute to produce more detailed oxide layer on the surface and this oxide layer can ensure that there will be no peeling for a long time.The structure of the main oxides in the oxide layer changed after W addition,and adhesion between surface oxide layer and matrix will be stronger.When W content exceeds a certain amount,the oxide layer of the steel will be very easy to crack and spall.In this case,it could not protect the matrix persistently and stably.(3)The sectional morphologies of the oxide layer at 1050? for different cycles of oxidation were observed and analyzed for these ferritic stainless steels.The results show that rare earth element Ce and tungsten can change the location and size of Laves phase.These changes will repaire some defects between oxide layer and matrix,and even cause the re-distribution of each element in oxide layer.(4)By using the thermodynamic software,calculations of phase diagram,precipitate phase and some performance were carried out and it was proposed that some amounts of tungsten and cerium was used to replace molybdenum in the steel.On this basis,the steel composition was optimized.The results showed that a moderate amount of W to replace Mo in steel can further improve the high temperature behavior of the ferritic stainless steel.The suitable addition of tungsten is 0.4%?0.5%.