High Temperature Fatigue Behavior Of 429 And 429mo Ferritic Stainless Steel

Ferritic stainless steels are increasingly used for the automotive exhaust systems because of their excellent resistance to stress corrosion cracking, low coefficient of thermal expansion, good toughness, ductility, excellent resistance to high temperature oxidation, high temperature strength and low cost. Has been widely used in automotive exhaust system instead of austenitic stainless. Components in the exhaust systems are exposed to the temperature up to 1000oC depending on their position from engines, i.e. the temperatures in the exhaust systems vary from 100oC for tail end pipe to approximately 1000oC for exhaust manifold. Exhaust system components may be affected by exhaust gas flow or the bump of the road surface. Therefore, high temperature and mechanical vibration are the main causes for the life decreasing and the failure to cracking of the components in the exhaust systems. In this paper, 15 CrNbTi steel and 15 CrMoNbTi steel at 800oC under the condition the high-cycle fatigue behavior without hold times and under fatigue-creep interaction of fatigue behavior with hold times of 10 s were study. By investigating the high temperature fatigue behavior of ferritic stainless steels can provide a reliable theoretical basis for the design and safty use of ferritic stainless steels components.The results of fatigue tests show that the fatigue limits of 15 CrMoNbTi and 15 CrNbTi are 35 MPa and 30 MPa, respectively. 15 CrMoNb Ti steel possesses the higher fatigue limit than 15 CrNbTi steel. Indicate the addition of Mo can effectively improve the fatigue limit of ferritic stainless steel. In high stress region, at the same maximum stress, the fatigue life of 15 CrNbTi steel is higher than that of 15 CrMoNbTi steel, and in low stress region, at the same maximum stress, the fatigue life of 15 CrMoNbTi steels is higher than that of 15 CrNbTi steel. With the hold time of 10 s, because of fatigue-creep interaction, the high temperature fatigue life is greatly reduced.The observation of fatigue fractures indicates that the fatigue cracks of 15 CrNbTi and 15 CrMoNbTi stainless steels are initiated transgranularly at the free surface of fatigue specimens and propagated at a transgranular mode. Because of fatigue-creep interaction, A large number of creep holes were observed at the surface of ferrite stainless steel.Microstructure observation indicates that Laves phase was observed in 15 CrNbTi and 15 CrMoNbTi ferrite stainless steel. The coarse Laves phase will become the nucleation site of creep hole. Causing void nucleation and crack formation. The growth and expansion of creep voids will accelerated fatigue crack propagation. Obvious sub-grainboundary was observed in 15 CrNbTi and 15 CrMo NbTi ferrite stainless steel only under pure-fatigue loading conditions, The introduce of hold times causeing the formation of the low energy dislocation configuration. Leading to the decrease of the high temperature fatigue life of the ferritic stainless steels under fatigue-creep interaction conditions.