High Temperature Oxidation Behavior Of Ferritic Stainless Steel For Hot End Of Exhaust System

Ferritic stainless steel has been used in automotive exhaust systems due to its low thermal expansion coefficient,good thermal conductivity and oxidation resistance.However,as the exhaust emission standards are gradually increased,the operating temperature of the exhaust system is getting higher,and the working environment is becoming more stringent.As the highest temperature component in the exhaust system,the exhaust manifold will increase its operating temperature from the current950°C to above 1050°C.At this time,it is important to investigate the oxidation behavior of ferritic stainless steel in the actual service environment.In this paper,synthetic automobile exhaust or air was used to simulate the service environment of components such as exhaust manifolds.The oxidation behaviors of ferritic stainless steels 441,444 and F3?new development,compounding0.056 wt.%Ce and 0.58 wt.%W on the basis of 444?in synthetic exhaust gas and air environment were studied by cyclic oxidation weight gain method and SEM,EDS,XRD,Raman and other analytical methods.The effects of temperature,alloying elements and oxidizing atmosphere on their oxidation performance were described.The oxidation behavior of ferritic stainless steel 441 in the synthetic automobile exhaust environment in the range of 900¹050°C was studied.The results showed that the upper limit service temperature of 441 ferritic stainless steel in automotive exhaust environment is about 900°C.The oxide film formed at this temperature is a two-layer structure containing the outer layer of?Mn,Cr?₃O₄ spinel layer and the inner Cr₂O₃ layer.When the oxidation temperature rises to 950°C and above,iron oxide nodules appears.The oxide nodules is distributed along the initial oxidation layer,the outside is mainly iron oxide,and the inside is Fe-Cr oxide.When the oxidation temperature reaches 1050°C,more voids are formed inside the oxide nodules.The growth of the Fe-Cr oxide leads to the occurrence of breakaway oxidation and changes the oxidation kinetics curve from a parabolic law to a straight line.The effect of alloying element Mo on the oxidation performance of ferritic stainless steel in synthetic exhaust environment was studied.It was found that after oxidation at 1050°C for 100 h,the addition of 2 wt.%Mo makes the 444 stainless steel have better oxidation resistance than 441,and the precipitated Laves phase is changed from Fe₂Nb to?Fe,Si?₂?Nb,Mo?,hinder the diffusion of cations from the substrate.In addition,the 444 stainless steel oxide nodules also have an incompletely failed chromium oxide layer,which inhibits the growth of pure iron oxide and delays the occurrence of breakaway oxidation of stainless steel.The effect of composite addition of alloying element W and rare earth Ce on the oxidation performance of ferritic stainless steel under synthetic exhaust gas environment was studied.It was found that,the addition of Ce and W makes the F3stainless steel have better oxidation resistance than 444,and The W element appears in the precipitated Laves phase.The addition Ce and W further improves the adhesion of the oxide film to the matrix,and also promotes the formation of a complete and dense?Mn,Cr?₃O₄ spinel layer on the outermost layer of the oxide film.Effectively inhibits the evaporation of chromium oxide.The effect of synthetic exhaust/air environment on the oxidation performance of444 and F3 stainless steels was investigated.Compared with the air environment,The oxidation rate of both stainless steels is accelerated in the synthetic exhaust environment.For 444 stainless steel,when breakaway oxidation happens in the air environment,the oxide nodules appears and the oxide film is largely peeled off.Larger volume of the oxide nodules is formed in the synthetic exhaust environment.For F3 stainless steel,the oxide film formed in the synthetic exhaust environment is flatter than air and the thickness is increased.