Research On Effect Of Cr Content On Microstructure And Properties Of Super Austenitic Stainless Steel 654SMO

As a new generation of high alloy and high corrosion resistant stainless steel,super austenitic stainless steel has excellent corrosion resistance and comprehensive mechanical properties.It is an ideal substitute material for some nickel-based alloys and titanium alloys and has great potential for various applications in extremely harsh environments.However,compared with conventional stainless steel,super austenitic stainless steel starts relatively late and still faces many bottleneck problems in the process of research and production.The data of relevant research is too scarce to realize industrialization,especially for the high-grade 654SMO.Therefore,strengthening the research on 654SMO has important significance to the development of super austenitic stainless steel.In this research,the effect of Cr content on microstructure and properties of super austenitic stainless steel 654SMO was investigated systematically.Microstructure characteristics of 654SMO with various Cr contents were analyzed by optical microscope(OM),field emission scanning electron microscope(SEM)and transmission electron microscopy(TEM)Corrosion resistances were evaluated by electrochemical workstation.Mechanical properties of 654SMO after solid solution and aging treatment were measured by tensile and impact testing machine.The main conclusions are obtained as follows:Thermo-Calc calculation results showed that the main types of precipitates are ? phase,Cr2N and M23C6.With the Cr content increasing,both the precipitation temperature range and nucleation driving force increased,promoting the precipitation of ? phase.However,the change of Cr content has no obvious effect on the precipitation of Cr2N and M23C6.With the Cr content increasing,the Cr activity increased,while the Mo activity decreased.This promotes the formation of Cr-rich phase and inhibits the formation of Mo-rich phase.During the isothermal aging process,when the aging time was less than 2 h,the nucleation number and total precipitate content of 654SMO-Cr23.5 were the largest.However,the size of the precipitates was small.They were mainly spherical and short rod-like precipitates and dispersedly distributed in the matrix.In addition,many Cr2N and R phase formed at the grain boundaries and within the grain.With the aging time extending,the precipitates in 654SMO-Cr23.5 grew not as distinctly as those in 654SMO-Cr24.5 and 654SMO-Cr25.5.The large precipitates in 654SMO-Cr24.5 and 654SMO-Cr25.5 were main needle-like and rod-like ? phase.After 6 h aging,the total precipitate amounts of 654SMO-Cr24.5 and 654SMO-Cr25.5 were larger than that of 654SMO-Cr23.5.Thus,increasing Cr content promotes the formation of ? phase and suppresses the precipitation of Cr2N and R phase.Electrochemical test results indicate that,with the increment of Cr content,the breakdown potential increases slightly and the current density at the same potential reduces significantly in green death solution.That is,the pitting corrosion resistance of the steel has been improved.The DL-EPR results show that increasing Cr content enhances the intergranular corrosion sensitivity,which makes the steel more prone to intergranular corrosion.When aged for less than 2 h,the intergranular corrosion sensitivity of 654SMO-Cr23.5 is the strongest due to its highest content of precipitates.Subsequently,the total amount of precipitates and the DOS value of 654SMO-Cr23.5 were lower than those of654SMO-Cr24.5 and 654SMO-Cr25.5.This could be attributed to the slow growth of precipitates in 654SMO-Cr23.5after 6 h aging.Despite slight improvement in pitting resistance,the steel is more susceptible to intergranular corrosion with the increment of Cr content.The solid solution sample possesses excellent strength and toughness.After aging treatment,the microcracks preferentially formed near the hard and brittle secondary phase.As a result,the tensile strength and elongation of the steel reduced,and the yield strength of the steel increased.The fracture mode changed from ductile fracture to brittle fracture.The increase of Cr content has little effect on the strength and toughness of solid solution steel.With the aging time extending,the Cr content has a significant effect on the type and morphology of precipitation phase,leading to an increased deterioration effect on the mechanical properties.Compared with the fine near-spherical precipitates in 654SMO-Cr23.5,the sharp slender needle-like ? in 654SMO-Cr25.5 has much stronger deterioration effect on the mechanical properties of the steel.