Research On The Precipitated Phase Of 25Cr-5Ni-Mo-N Duplex Stainless Steel

In this study, the influence of aging temperature and aging time on the precipitated phase of 25Cr-5Ni-Mo-N duplex stainless steel was researched by means of X-ray diffraction, SEM/EDS, TEM and metallographic microscope. The effect of the precipitated phase on corrosive property was also investigated by double loop electrochemical potentiokinetic reactivation (DL-EPR) and polarization curve. After solution treatment at 1100℃for 1h, some samples were aged at 550℃,650℃,750℃,850℃,950℃and 1050℃for 3h, respectively, in order to study the effect of aging temperature on the microstructure and intergranular corrosion resistance. The others were kept at 750℃for 3h,6h,12h,24h,48h,72h and 96 respectively to analyse the impact of aging time on the microstructure and corrosive property.The results show that:Compared with the specimens without aging,aging at 550℃,950℃and 1050℃, the a phase precipitates after aging heat treatments during 650-850℃for 3 hours, and the a content is the most at 750℃followed by 650℃and 850℃. However, theχphase only precipitates at 750℃. The M23C6 carbides only precipitate when aging from 650℃to 950℃. Theσphase is nucleated preferentially atα/γinterfaces and dislocations piled up, then grows toward a ferrite. In the initial stage of the ferrite decomposition, the M23C6 particles pre-form atα/γinterface by eutectoid decompositionαâ†'M23C6+γ2, and grow into a ferrite with the austenite constantly until the eutectoid decomposition (αâ†'M23C6+γ2) finished. Thenσphase nucleates selectively at the high energyα/γinterface as well as on the fully completing M23C6 carbides precipitation by the second eutectoid decomposition (αâ†'σ+γ2), and grow into the a ferrite.The dual phase scale of the specimens is not the primary factor influencing intergranular corrosion resistance, while theσcontent is conclusive. The specimens without aging,aging at 550℃and 950℃show little susceptibility of intergranular corrosion due to withoutσandχphases precipitation, and the the ratio (Ir/Ia)×100% of the specimen aging at 1050℃is zero. The susceptibility of intergranular corrosion is biggest for aging at 750℃followed by 650℃and 850℃corresponding to theσcontent. So, the heat treatment should avoid the sensitizing temperature range (650-850℃), and the second solution treatment at 1050℃can be choosed to improve the intergranular corrosion resistance. The M23C6 carbides,σandχphases precipitate after aging at 750℃for 3-96h of 25Cr-5Ni-Mo-N duplex stainless steel. Besides, theσcontent and austenite content increase with the increasing aging time, while the ferrite content reduces with the increase of aging time. Theσphase assembles and grows up continuously with the prolonging of the time. The content of autenite and ferrite are linear with the aging time. The precipitated rate of a phase become faster and faster with the increasing time for 3-24h, and it reaches the highest-level between 12-24h. On the contrary, the precipitated rate ofσphase decreases with the increasing time for 24-96h.The corrosion resistance is worse with the increase of aging time. Specimens reveal local intergranular corrosion while aging from 3h to 24h after electrochemical test. Nevertheless, the corrosion resistance drops off precipitously after aging for 24, it represents as a sharp decrease of pitting breakdown potential. Consequently, it suffers from pitting corrosion, which is related to the growth rate ofσphase. Theσcontent gets rise dramaticlly after aging from 12h to 24h. The corrosion resistance shows a downward trend while aging from 48h to 96h, and the difference is so slight because of the very small variation of theσcontent and the distribution characters ofσphase.