Research On Aging Precipitation Behavior And Corrosion Resistance Of Hyper Duplex Stainless Steel UNS S32707

Combining with the good corrosion resistance of austenitic stainless steels and the high strength of ferritic stainless steels,duplex stainless steels have similar volume fractions of austenite and ferrite and exhibit excellent properties.Hyper duplex stainless steel(HDSS)UNS S32707 possesses excellent corrosion resistance and mechanical properties than other ordinary duplex stainless steels.It was developed to replace the expensive Ni-based alloys and titanium alloys partially.However,due to their higher alloy contents,hyper duplex stainless steels are very susceptible to secondary phase at higher temperature,which seriously reduces their mechanical properties.Moreover,the precipitation of secondary phases also causes Cr and Mo deletion in the surrounding matrix,which deteriorates pitting and intergranular corrosion of the steels.Therefore,it is necessary to research on the aging precipitation behavior and its influence on the corrosion resistance of the steels,which is able to provide guidance for the heat treatment,hot working processes and the application of UNS S32707.Aging precipitation behavior of UNS S32707 is systematically studied by Thermo-Calc software,optical microscope(OM),scanning electron microscope(SEM),X-Ray Diffraction?XRD)and transmission electron microscope(TEM).The pitting corrosion resistance of UNS S32707 is investigated by anodic polarization and electrochemical impedance spectroscopy test in 3.5%NaCl solution.The effect of aging time on the intergranular corrosion resistance of super duplex stainless steel UNS S32707 was investigated by dual-loop electrochemical potentiostatic reactivation(DL-EPR)test and microstructure analysis to clarify the relationship between aging precipitation behavior and intergranular corrosion resistance.The main conclusions are as follows:The results calculated by Thermo-Calc software show that a phase,HCP and M23C6 may precipitate in hyper duplex stainless steel UNS S32707 when being exposed at high temperature,?phase is the main precipitate,followed by HCP and M23C6 phase.The upper limit of precipitation temperature of a phase and nitride are about 1050 ? and 1180 ?.C and N inhibit the precipitation of a phase,and Cr and Mo promote the formation of ? phase.Combined with OM,SEM and thermodynamics calculation results,the solution treatment of UNS S32707 was carried out at 1200 for 1 h to obtaine homogeneous two-phase structure and make sure the secondary phase completely dissolve back into matrix.The decrease of N content could increase the ratio of a phase and decrease the proportion of y phase and cause the solution treatment temperature decreasing by 50 ?.Initial time-temperature-transformation(TTT)curves with 3%and 7%precipitation volume fraction are investigated by quantitative metallography method.The TTT curves are C-shaped and the“nose" temperature of precipitation is found to be 950 ?.The corresponding incubation periods are 100 s and 150 s,respectively.Aged at 950 ?,the a phase preferentially forms continuously along the a/a and ?/? boundaries and then penetrates into a grains via the eutectoid reaction(a ? 72 + ?).With the increasing of aging time,the volume fraction of precipitates in UNS S32707 increases,but the precipitation rate decreases gradually.When aged for 6 h,the ferrites were completely consumed.After being aged for 24 h,some a phase nucleate within primary y phase.Governed by the eutectoid reaction,a cellular structure consisting of lamellar a and 72 phase also forms at the initial stage of aging.With aging time prolonging,the cellular structure gradually grows by means of the migration of the cell boundary.During aging from 30 min to 6 h,the lamellar a phases in the cellular structure broke into small lamellas or into bending bars and then evolved into blocky shapes.Most of the rounded Cr2N particles form at the ?/?2 interface,favored by the diffusion of N from ?2 phase and the Cr diffusion from a phase and ?2 phase.A few rod-like Cr2N particles nucleate at ?/? boundaries after long term aging,due to the low energy interfaces between Cr2N and y phase.It should be noted that no Cr2N was founded in a phase,which can be mainly attributed to the rapidly consumption of a phase.The pitting corrosion resistance of UNS S32707 is investigated by anodic polarization and electrochemical impedance spectroscopy test in 3.5%NaCl solution.With the aging time increasing,the precipitation of ? phase and Cr2N lead the depletion of Cr and Mo surrounding matrix and ?2 and the formation of depleted zone,which provides favorable place for pitting initiation and deteriorates the pitting corrosion resistance of the steel.The effect of aging on the intergranular corrosion of UNS S32707 was studied by DL-EPR method.With the aging time prolonging,the Ir/Ia value of UNS S32707 increases,indicating the deterioration of the intergranular corrosion resistance.This is because two Cr and Mo riched phases(? phase and Cr2N)form in UNS S32707 when aged at 950?.The elements of Cr and Mo in the vicinity of the grain boundaries continuously diffuse to the grain boundaries to satisfy the nucleation and growth of the secondary phase,which results in the formation of Cr and Mo depleted zone and the decrement of intergranular corrosion of UNS S32707.After aging for 6 h,the cellular structure disappears and the sensitivity of intergranular corrosion decreases rapidly.When aged for 12 h and 24h,the increasing of the a phase at the y/y boundaries and the precipitation of the ? phase and Cr2N within the primary austenite result in the obvious increment of intergranular corrosion resistence.However,when the aging time prolonged to 48 h,the diffusion rate of Cr and Mo atoms from the matrix to the depleted zone is greater than the consumption rate of Cr and Mo caused by the nucleation and growth of the precipitates,which leads to the decrease of the intergranular corrosion resistence.