Study On Aging Precipitation Behavior And Corrosion Resistance Of 00Cr17NbCu Ferritic Heat-Resistant Stainless Steel

Ferritic heat-resistant stainless steel is widely used in boiler pipes,automobile parts and other fields due to its advantages of low price,good oxidation resistance,good corrosion resistance and high thermal conductivity.In recent years,with the continuous improvement of service life and exhaust temperature of automobile exhaust system,more demanding requirements have been put forward for service conditions of the hot end parts of exhaust system.Alloying is a common and important method to improve the properties of the hot end parts of automotive systems.In this paper,00Cr17NbCu ferritic heat-resistant stainless steel was studied.Optical microscope,scanning electron microscope and transmission electron microscopy were used to express the microstructures and characterization.The change of precipitated phase,the orientation and the growing rules of precipitated phase were observed in detail,the effects of aging treatment on microstructure evolution of00Cr17NbCu ferritic heat-resistant stainless steel were investigated.To study the influence of different aging processes on corrosion resistance of 00Cr17NbCu ferrite heart-resistant stainless steel and its corrosion mechanism,electrochemical experiments such as dynamic potential polarization curve and AC impedance spectrum were used.The semiconductor characteristics and composition of passive film on the surface of ferritic heat-resistant stainless steel were studied by Mott-Schottky curve and X-ray photoelectron spectroscopy?XPS?.The effect of aging treatment on microstructure of 00Cr17NbCu ferrite heart-resistant stainless steel was researched.The results show that after different aging treatment,four different types of precipitated phases were found,they are?-Cu,NbC,Fe₃Nb₃C and Fe₂Nb-type Laves phase respectively.As the aging time was 2 h,?-Cu grows gradually in the shape of fine particles and grew in the direction of rod-shaped with the aging temperature increasing.When the aging temperature reached 850?,the amount of?-Cu decreased.With the aging temperature increasing,the carbide and Laves phase precipitated and grew in the grain boundaries and the matrix.At 850?,the Nb-phase precipitated in large quantities and coarsened seriously,while the Cu-rich phase precipitated less.At 750?isothermal aging treatment,?-Cu was first precipitated and each precipitation gradually became coarser with the extension of aging time.The coarsening rate of Fe₂Nb is higher than that of NbC in 750?isothermal aging treatment by building a coarsening model.The microhardness of the sample decreased with the aging temperature increasing,and the hardness reached the maximum at 650?due to the action of precipitation hardening.With the increase of temperature,the precipitation particles coarsened,when the aging temperature increased to 850?,the hardness was the lowest.With the extension of aging time,the microhardness of the sample first increased and then decreased,and finally tended to be stable.As the aging time was 5 h,the microhardness of the sample reached the peak,which is 46%higher than that of the solid solution state.With the extension of aging time,the microhardness began to decrease and finally stabilized.The 750?isothermal aging treatment can be divided into three stages:under aging?0⁵ h?,aging peak?5 h?,over aging?5³0 h?.The effect of aging treatment on corrosion resistance of 00Cr17NbCu ferrite heart-resistant stainless steel was researched.The results show that with the increased of aging temperature,the corrosion resistance of the sample decreased.When the aging temperature was 850?,the concentration of a large number of Nb-phase in the steel leads to the uneven distribution of Cr in the matrix,and the sample shows the worst resistance to Cl^-corrosion.In the 750?isothermal aging,the corrosion resistance decreased in the early stage of 0² h aging,and the corrosion resistance was improved by further extending the aging time to 30 h.In the early aging period?0² h?,the formation of a large number of small?-Cu and Nb-containing precipitation caused the potential difference around to reduce the corrosion resistance.As the aging time is prolonged,more Cu was dissolved in the form of CuCl^2-on the surface to inhibit the Cl^-corrosion.The passive film formed on the surface of 00Cr17NbCu ferrite heart-resistant stainless steel presented the characteristics of p-n type semiconductor.The passive film is mainly composed of Fe and Cr oxides.