An Investigation On High Temperature Oxidation Resistance Of Medium-Chromium Alloyed Ferritic Stainless Steel

With the rapid development of automobile industry,the demand of automotive steel including ferritic stainless steel(FSS)shows a multiple increasing trend.The 444 FSS as the material to manufacture the automobile exhaust manifold is widely applied in hot end of the automobile exhaust system.With Euro V Emission Standard generally implemented in Europe,our country also began to implement corresponding levels of emissions standards in 2010,which directly lead to the exhaust temperature rising fast.The local temperature even reached as high as 1100?,which required materials with better high temperature oxidation resistance and heat corrosion performance.In view of the abundant rare earth resources in our country,it is feasible to make good use of rare earth metals' "reactive element effect" to improve the performance of high temperature oxidation of the ultra-purified FSS.Based on this,in this paper the effect of the addition of W and the rare earth element Ce on mechanical properties and on the high temperature oxidation resistance of 444 FSS were stuied.The main conclusions are drawn as follows.(1)The effects of different contents of W and rare earth elements on mechanical properties of 444 FSS were studied at room temperature and elevated temperatures.The room temperature tensile test results showed that the addition of rare earth elements can improve the elongation of the material while the strength remained the same.However,addition of rare earth element Ce together with 1.OVwt%W will reduce its elongation sharply,which can not meet the service requirement.Under the tensile condition at elevated temperature,the addition of rare earth element Ce has little effect on the tensile strength and the high temperature elongation may be slightly reduced.However,the addition of W can improve the high temperature tensile strength but also can slightly decrease the high temperature elongation.(2)The isothermal oxidation kinetic curves of the experimental steels were measured after oxided at 950-1100?.The addition of rare earth element Ce can significantly improve the oxidation resistance at 1050 ? and 1100? as well as reduce the reaction rate at the beginning of oxidation.Moreover,the oxidation weight gain of the experimental steels which were added rare earth element together with 0.5wt%W oxided at 950? and 1000 ? for a long time is lighter than the experimental steels which were added rare earth element only.In addition,adding rare earth element together with 1.0wt%W can significantly reduce the oxidation rate in the early stage of the oxidation at 950 ? and 1000 ?.However,its oxidation resistance is worse than that of the experimental steels which were added rare earth element only when the temperature reached 1050 ? and 1100 ?.(3)The oxide film morphologies of the experimental steels at various oxidation temperatures for different times were observed and analyzed.The addition of rare earth element Ce can make the oxide particles on the surface of oxide film smaller and more compact,and almost avoid the oxide film spallation after oxidized at 950-1050?.In addition,the oxide film of the experimental steels containing rare earth element and 0.5wt%W has no obvious difference as compared with the steel only containing rare earth element.However,the oxide films appeared obvious spalling phenomenon during cooling when rare earth element and 1.Owt%W were added to the experimental steels.XRD phase analysis indicates that the oxide films of the experiment steels are mainly composed of Cr2O3 and spinel MnCr2O4.However,when the experimental temperature is close to 1050?,more Fe3O4 in the original Cr2O3 layer will be generated when W is added up to 1.Owt%.(4)The cross-section morphologies of the oxide film were analyzed.Addition of rare earth element Ce could reduce the thickness of the oxide film and the oxide film combine firmly with matrix.The cavities at oxide film/matrix interface are filled with the precipitated Laves phase Fe2(Nb,Mo)and Cr2O3 protective film is generated which can inhibit the expansion of cavity as well as reduce the number and size of defects at oxide film/matrix interface.Moreover,1.Owt%W addition can form a large number of Laves phase and cavities which may significantly reduced the adhesion strength between oxide film and matrix.(5)The grain boundry can be effectively pinned by Laves phase Fe2(Nb,Mo)precipitated at grain boundary and the grain size is well controlled.The addition of elements Ce and W can reduce the amount of solid solution of Laves phase Fe2(Nb,Mo)at 1000-1050 ?.As a result,the studies suggested that addition of 0.05wt%Ce or 0.05wt%Ce together with 0.5wt%W can significantly improve the oxidation resistance of materials and meet the requirement for mechanical properties at room and elevated temperatures.Moreover,the adhesion strength between the oxide film and the substrate can be improved and the oxidation reaction rate reduced.This can be utilized at service temperature of 950-1100?.