Mechanical Properties And Failure Behavior Of Two Ferritic Stainless Steels At High Temperature

Due to its high temperature strength,excellent high temperature oxidation resistance and low price,ferritic stainless steel has gradually replaced traditional cast steel as the main steel used in automobile exhaust system.In this paper,the tensile and fatigue tests of 441 and444 LM ferritic stainless steels used in hot ends of automobile exhaust systems were studied at700 ?.The deformation behaviors and failure mechanisms during the high temperature tensile and fatigue processes were also discussed.The results will provide a reliable theoretical basis for the application of the ferritic stainless steels in the high temperature components of automobile exhaust system.Tensile test results at 700? show that the tensile yield strength,ultimate tensile strength and elongation at break of 444 LM ferritic stainless steel are slightly higher than that of441.The fracture mode of the both ferritic stainless steels are typical ductile fracture.The size and depth of dimples of 444 LM ferritic stainless steel are greater than that of 441.The results of energy dispersive X-ray spectroscopy(EDS)analysis show that the chemical compositions of precipitates at the bottom of dimples is similar to the(Ti,Nb)(C,N)carbonitride in the as received samples.The grains near the fracture of both ferric stainless steels are elongated,and the aspect ratio of the grains increases from 1.6 in the as-received condition to 2.6.A large number of small-angle grain boundaries formed in the grains near the fracture the closer the distance to the fracture,the higher the proportion of small-angle grain boundaries in the grains.The thickness of the oxide layer of 444 LM ferrite stainless steel is about a half of that of 441.Compared with 441 stainless steel,the oxide layer formed on the surface of 444 LM stainless steel is more compact and has better bonding strength with the substrate.The fatigue life curves at 700? show that the fatigue limits of 441 and 444 LM ferritic stainless steels are 50 MPa and 80 MPa,respectively.Fracture surface characterization shows that the damage behaviors of both ferritic stainless steels are the synergy effects of fatigue and creep.With the extension of fatigue cycle,the effect of creep on the fracture gradually increases.The grains near the fatigue fracture of both ferritic stainless steels are elongate along the loading direction.The closer the distance to the fracture,the more obvious the elongation of grains.The statistical results of grain size show that after fatigue test,the grain size of both ferritic stainless steels has moderately increased and the grain size of 441 stainless steels increases more obviously.Transmission electron microscopy results demonstrate cellular dislocations,dislocation networks and sub grains formed in the two ferritic stainless steels after fatigue deformation.The dislocation pile up at the grain boundary resulting in stress concentration at the grain boundary.A small amount of large precipitates in the grain interior and at the grain boundary of both ferritic stainless steels hinder the migration of grain boundary at high temperature.The tiny precipitates of Titanium nitride and Niobium carbide in the grain interior and near the grain boundary of 444 LM ferritic stainless steels have a significant pinning effect on the migration of grain boundary.