Cyclic Creep Behaviors Of Ultra Pure Ferritic Stainless Steel B409 And B429Mo

Compared to the traditional austenitic stainless steel,the ferritic stainless steel is characterized by the higher resistance to high temperature,stress corrosion cracking and thermal fatigue.Moreover,it has a relatively low economic cost due to the low nickel content.The ferritic stainless steel has been used widely for the automotive exhaust system.Due to the frequent start-running-parking processes at the long-term elevated temperature,the automobile exhaust system suffers to the combined action of constant and cyclic loading,thus the cyclic creep damage directly influences the service life of such component.So far the researchers in China and abroad have done a lot of studies on the cyclic creep of austenitic stainless steel,but there are few reports about the cyclic creep behavior of ferritic stainless steel with body-centered cubic crystal structure.Especially,few microscopic mechanisms of the cyclic creep for ferritic stainless steel were studied.In this dissertation,the behaviors of the static creep and the cyclic creep of two ferritic stainless steels B409 and B429 Mo were investigated.Fracture surfaces of two ferritic stainless steels after the static and cyclic creep were observed using SEM to reveal the fracrure mode of the static and cyclic creep and microstructures of two ferritic stainless steels after the static and cyclic creep were observed using TEM to reveal the deformation mechanism of the static and cyclic creep,in order to provide a reliable theoretical foundation for the rational exploitation and safe handling.The results of the static creep tests indicate that the static creep behaviors of the B409 and B429 Mo ferritic stainless steel are affected by the imposed stresses.With increasing the imposed stress,the minimum creep rates increase and the rupture lives decrease.In the same condition,the B429 Mo stainless steel has higher creep resistance than the B409 stainless steel.The relationship between the minimum creep rate and the imposed stress of the B409 and B429 Mo stainless steel obeys Dorn law and the stress exponents are 2.34 and 4.22,respectively.The relationship between the creep rupture life and the minimum creep rate of the B409 and B429 Mo stainless steel satisfies Monkman-Grant relationship.The SEM analysis on the static creep fracture surface of the B409 and B429 Mo stainless steels show that the fracture mode of the B409 stainless steel is transgranular and intergranular mixed fracture because there are the creep cavities and characteristic of intergranular rupture in creep region and the fracture mode of the B429 Mo stainless steel is transgranular fracture due to the creep cavites in the creep region after the static creep.The results of the cyclic creep tests indicate that the B409 and B429 Mo ferritic stainless steels show cyclic creep retardation.The imposed stress and the hold time directly affected the cyclic creep behaviors of the B409 and B429 Mo stainless steel.With the same hold time,the minimum creep rates increase with increasing the maximum imposed stresses.With the same maximum imposed stresses,the minimum creep rates increase with increasing the hold time.Under the same condition,the B429 Mo stainless steel has higher cyclic creep resistance than that of the B409 stainless steel.The two stainless steels show the larger apparent stress exponents in the cyclic creep than those in the static creep.The relationship between the cyclic creep rupture life and the minimum creep rate of the B409 and B429 Mo stainless steels can be described by Monkman-Grant relationship.The SEM analysis on the cyclic creep fracture surface of the B409 and B429 Mo stainless steels reveal that the fracture modes of the B409 and B429 Mo stainless steel are both transgranular fracture.The creep cavities and characteristic of steps in the fatigue crack propagation region show that the cyclic creep fracture is the result of the fatigue-damage and the creep-damage interaction.With increasing the hold time,the creep cavities of fracture surface in the two stainless steel increase and the effects of the creep-damage increase.The TEM analysis show that the subgrain and dislocation networks are prevailing after the static creep fracture of the B409 and B429 Mo ferritic stainless steels.For the B409 stainless steel,there are dislocation networks in the subgrain and dislocation pile-ups at the grain boundary which lead to stress concentration in grain boundary.There are few second-phase particles precipitated in the grain boundary,which lead to the poor grain boundary strength.The dominant static creep deformation mechanism of the B409 stainless steel is boundary sliding.For the B429 Mo stainless steel,there are glide bands in the subgrain and many second-phase particles precipitated in grain boundary which hinder the boundary sliding.The dominant static creep deformation mechanism of the B429 Mo stainless steel is dislocation climbing.After the cyclic creep of the B409 stainless steel under 650?C and different hold time,the main dislocation patterns are sub-grain and dislocation networks.The dominant cyclic creep deformation mechanism of the B409 stainless steel is dislocation slip and cross slip.With increasing the hold time,the second-phase particles precipitated decrease and dislocation patterns in the subgrain of the B409 stainless steel evolve into dislocation networks.The dominant cyclic creep deformation mechanism of the B429 Mo stainless steel is dislocation slip and cross slip.With increasing the hold time,dislocation patterns in the sub-grain of the B429 Mo stainless steel evolve into the slip bands.Therefore,the hold time in the maximum imposed stress is the major factor which affects the cyclic creep deformation.With the hold time increases,the effect of the creep in the cyclic creep deformation is more prominent.