Research On High-temperature Fatigue Behavior Of Several Ultra-purified Ferritic Stainless Steels

It has become a theme in modern automotive industry to improve the fuel efficiency,purify the exhaust gas,lighten the weight,control the cost and extend the servic life.The traditional materials used in the automotive exhaust system cannot meet the developing requirements of modern automotive industry.And the austenitic stainless steels have been widely used in the automotive exhaust system due to their excellent high-temperature perperties.In recent years,the ferritic stainless steels,characterized as the higher high-temperature strength,excellent thermal fatigue property,good resistance to high-temperature oxidation and stress corrosion cracking,have been greatly developed and applied in the automotive exhaust system instead of austenitic stainless steels.The hot end components in the automotive exhaust system,which are close to the engine,often suffer the high temperature environment from 600oC to 1000oC.The loading situations for the hot end components in the operating process of the vehicles are more complex.The load change and mechanical vibration,which are produced during the start-up and shut-down of the vehicles,will make the hot end components suffer the cyclic loading.While the vehicles are continually running,the hot end components are subjected to nearly steady loading for a period time.Therefore,the hot end components will suffer the high-temperature fatigue damage and creep damage,as well as oxidation damage induced by the high-temperature exhaust gas.Obviously,it is necessary to investigate the high-temperature fatigue,creep and oxidation damage mechanisms of the ferritic stainless steels used as the hot end components.And to understand above issues is the premise to solve the core problems in utilizing the ferritic stainless steels as the materials of hot end components.Hence,three ferritic stainless steels,including B429,B429Mo and B441,were chosen as the research objects,and the cyclic deformation behaviors of three ferritic stainless steels at 800oC were investigated under such test conditions as pure fatigue with no dwell time in air,fatigue-creep interaction with 10s dwell time in argon and fatigue-creep interaction with 10s dwell time in mixed gas.In addition,the corresponding deformation and fracture mechanisms were revealed.And thus,it is expected that the reliable theoretical basis for the further engineering application of three ferritic stainless steels in automotive exhaust system can be provided.The results of high-temperature fatigue tests show that under the loading condition with no dwell time at 800oC,the fatigue limits of B429,B429Mo and B441 steels are 30MPa,35MPa and 25MPa,respectively.In the higher stress zone,the cyclic deformation resistance and fatigue life of B429 steel are higher than those of B429Mo steel,while in the lower stress zone,the cyclic deformation resistance and fatigue life of B429Mo steel are higher than those of B429 steel.At all adopted maximum cyclic stresses,the cyclic deformation resistance and fatigue life of B441 steel are lower than those of B429 and B429Mo steels.Under the fatigue-creep loading condition with 10s dwell time in argon,the cyclic deformation resistance and fatigue life of three steels remarkably decrease,compared with those under the fatigue loading condition with no dwell time.Under the condition of fatigue-creep interaction,the B429Mo steel exhibits the highest cyclic deformation resistance and the longest fatigue life,and the B429 steel has the lowest cyclic deformation resistance and the shortest fatigue life.Compared with the cyclic deformation resistance and fatigue life under the fatigue-creep loading condition with 10s dwell time in argon,the cyclic deformation resistance and fatigue life of three steels further reduce under the fatigue-creep-oxidation loading condition with 10s dwell time in mixed gas.Under the condition of fatigue-creep-oxidation interaction,the cyclic deformation resistance and fatigue life of B429Mo steel are the highest,and the cyclic deformation resistance and fatigue life of B429 steel are yet the lowest.The results of the microstructural observation indicate that under the fatigue loading condition with no dwell time,the cyclic deformation in both B429 and B429Mo steels occurs in a planar slip manner,while the cyclic deformation in the B441 steel occurs in a wavy slide manner.In the higher stress zone,the solid solution strengthening effect of Nb element improves the cyclic deformation resistance and fatigue life of B429 steel.In the lower stress zone,the solid solution strengthening effect of Mo element enhances the cyclic deformation resistance and fatigue life of B429Mo steel.At the lower maximum cyclic stresses,the Fe₃Nb₃C phase precipitates in both B429 and B441 steels,while is not found in the B429Mo steel.The precipitation of Fe₃Nb₃C phase may weaken the solid solution strengthening effect of Nb element,and has the detrimental effect on the high-temperature fatigue performances of ferritic stainless steels.Under the loading conditions with 10s dwell time in argon and in mixed gas,the sub-grain microstructures are observed in all three steels.The formation of sub-grains leads to the obvious decrease in the cyclic deformation resistance and fatigue life of three steels.In addition,Cr element can increase the resistance of dislocations slip through restricting a particular slip system,which results in an improvement of cyclic deformation resistance and fatigue life of the B441 steel,compared with those of the B429 steel.The results of the observation on the mophologies of fracture surfaces reveal that under various loading conditions at 800oC,the cracks initiate transgranularly on the free surface of fatigue specimens and propagate in a transgranular manner.The fracture mode of three steels does not change due to the creep damage induced by dwell time.However,the creep damage can accelerate the propagation process of fatigue cracks.In addition,the high-temperature oxidation damage cannot also change the fracture mode of three steels.But the occurance of breakaway oxidation will further accelerate the propagation process of fatigue cracks.