| With the sharp increasing of automobile quantity,the negative impact on the environment is also increasing.Therefore,restrictions on exhaust emissions have been enforced in most country in view of preventing air pollution and to counter the problem of global warming.In order to purify automotive emission and improve fuel efficiency,the exhaust gas temperature is getting higher and higher,The exhaust manifolds,which are closed to engines,are usually exposed to temperature higher than 800? and subjected to the cyclic load in the exhaust systems.It has become critical for such components to possess the excellent high temperature strength,high temperature oxidation resistance and fatigue resistance.With the development of engine technology,exhaust manifold tend to work in the temperature above 1000? or even higher in the future.If the exhaust gas temperature reach 1050?,only a few expensive austenite heat-resistant stainless steel and superalloy can satisfy the required properties.At present,the domestic and foreign OEMs are seeking new ferritic stainless steel which can be used for the hot-end component of exhaust system.The purpose of this study is to develop heat-resistant ferritic stainless steel for hot-end component of exhaust.In this thesis,the effect of W and Ce on oxidation resistant,mechanical properties in room temperature and high temperature,impact toughness,and corrosion resistance of ferritic stainless steel were studied by modify the existing ferritic stainless steel(Fe-15Cr-0.5Nb?Fe-18Cr-0.5Nb and Fe-19Cr-0.5Nb-2.0Mo)with W and Ce.The result proved that the oxidation resistance,high temperature strength and corrosion resistance can be improved without deteriorating room temperature mechanical properties through W+Ce alloying.The effect of W and Ce on impact toughness and DBTT were also discussed.The main research work and results are as follows:(1)Effect of W and Ce on oxidation resistance of ferritic stainless steel(Fe-15Cr-0.5Nb.Fe-18Cr-0.5Nb and Fe-19Cr-0.5Nb-2Mo)at 1000? and 1050? were investigated through oxidation weight increasing method.Both W and Ce can improve oxidation resistance,but in different mechanisms,W enhance oxidation resistance by forming stable Laves phase in the interface of oxides and matrix metal,these Laves phase can hinder the ion diffusion,on the other hand,when the oxidation reaction were dominated by O2-diffusing into ferrite matrix,the grain boundary,which worked as diffusion channel of O2-can be blocked by Laves phase,thus prevent the occurrence of break-away oxidation.Oxidation resistance can be enhanced through Ce addition due to its reactive effect,Ce can promote the selective oxidation of Cr to form thin and dense oxide.(2)The positive effect of W on the oxidation resistance is not in proportional to W content.High W content causes deterioration of oxidation.W can increase the Cr content in the oxide and make the scale fragile,on the other hand,large amount of Laves phase dispersed in the interface of oxide and matrix metal,which reduce the adherence of oxide scale,result in oxide spallation.In this study,the oxidation resistance was deteriorated when W content is higher than lwt%.Ce can improve the trend of oxide spallation by suppress the precipitate of Laves phase in the oxide/matrix interface.(3)Both W and Ce increase the room temperature strength and the high temperature strength,but the effect of Ce is not as strong as W,W addition can improve anisotropy property of ferritic stainless 'steel,but decrease the average plastic strain ratio.During high temperature aging,W addition can significantly reduce the growth rate of ferrite grain,so that the solute strengthening effect caused by dissolve of Laves phase can compensate the soften effect caused by ferrite grain growth,thus ensure the high temperature strength stability of ferritic stainless steel.(4)Pitting corrosion resistance can be improved by both W and Ce addition,these W contained Laves phase formed in the steel have no negative effect on stress corrosion cracking resistance.(5)For Fe-18Cr-0.5Nb type stainless steel,lwt%of W addition have no negative effect on the ductile-brittle transition temperature(DBTT),but reduce the upper shelf energy,this is because W hindered the dissolve of Laves phase during annealing process,and these Laves phase tend to become crack initiation during impact test.Ce addition enhance the DBTT of ferritic stainless steel because of the formation of Ce oxides inclusion.When the steel annealed at 700?-950?,the room-temperature impact toughness was improved due to the recovery and recrystallization along with the dissolution of Laves phase,in this stage,W deteriorate the impact toughness and Ce addition can mitigate the negative effect.When the annealing temperature is higher than 1000?,the coarsening of ferrite grain leading to the higher DBTT and lower room temperature impact toughness. |
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