Hot End Of Automobile Exhaust System With 15 Cr Ferritic Stainless Steel Precipitates Behavior And Its Effect On High Temperature Mechanical Properties Research

Car exhaust manifold is the first component that the engine exhaust pass,and in a high temperature environment for a long time.The exhaust manifold must have excellent high temperature properties and thermal fatigue properties, when the higher temperature strength,the better thermal fatigue, the longer service life.The precipitation behavior and high temperature mechanical properties of15Cr type ferritic stainless steel which had applications in the automotive exhaust were studed. To do this, Three components of test steel was chosen:0.014-0.53Nb steel,0.014C-0.53Nb-0.1Ti steel and0.0022C-0.48Nb steel.After the solid solution and high temperature aging,in order to analyze the high temperature mechanical properties, the above three kinds of specimen is high temperature tensile, The phase analysis to determine matrix and precipitate phase Nb content after the long aging, Precipitated phase changes in the aging process by XRD, SEM and TEM analysis of three specimens.the results found:(1)With longer aging time,the0.014-0.53Nb steel and0.0022C-0.48Nb steel high temperature yield strength decreased faster,but0.014C-0.53Nb-0.1Ti steel at elevated temperature yield strength changed little.after900℃aging500h the high temperature yield strength of the Ti-containing steel was significantly higher than the other two steels.(2) After aging500h at900℃:For0.014C-0.53Nb steels, M6C (Fe3Nb3C) is the main precipitated phase and solute Nb decline to0.097wt%from0.247wt%; For0.014C-0.53Nb-0.1Ti steels, Laves (Fe2Nb) is the main precipitated phase and solute Nb decline to0.161wt%from0.295wt%; For0.0022C-0.48Nb steels, M6N (Fe3Nb3N) is the main precipitated phase and solute Nb decline to0.179wt%from0.224wt%. During aging solid solution Nb content decreased for the three kinds of steel. Due to the generated M6C (Fe3Nb3C) phase and M6N (Fe3Nb3N) phase,0.014C-0.53Nb steel and0.0022C-0.48Nb steel strength lower with the solution Nb content to reduce. To contain the Ti steel, although solid solution Nb decreased but the intensity was very small,indicating that Laves (Fe2Nb) phase is the phase of a wholesome high-temperature strength.(3) Under high temperature,the grain boundaries become the weak link, however, the precipitates in the presence of grain boundaries play a role which drag the grain boundaries and strengthen grain boundaries.After aging500h at900℃, the distribution and morphology of precipitation at the grain boundaries ware vary for three kinds of steel:M6C (Fe3Nb3C) along the grain boundaries coarsening and become discontinuous distribution. Laves (Fe2Nb) phase on the grain boundaries coarsening and become continuous distribution.A large number of M6N (Fe3Nb3N) phase distributed along the grain boundaries;The morphology of M6C (Fe3Nb3C) and M6N (Fe3Nb3N) on the grain boundaries was cellular,which a side parallel to grain boundary,while the other side showed a hemispherical growth toward a grain of internal.Laves phase in the grain boundaries were lenticular growth,show the characteristics of both sides of the thin and middle of thick.lt is because of morphology of three precipitated phase at the grain boundaries and coherent relationship with the grain boundary affects the strength of the grain boundary, causing the yield strength changes in the aging process.