| SAF2906Cast Hyper Super Duplex Stainless Steel is a new grade of steelwhich has high Cr, N content、high strength and excellent corrosion resistance. Butfew researches have been conducted concerning the characteristics of precipitatesand their effects on corrosion resistance. In the thesis, the effects of aging treatmenton precipitates and the corrosion resistance in artificial sea water, hydrochloric acidand nitric acid solution after aging treatment of SAF2906have been studied by OM,SEM, XRD, TEM and Electrochemical experiments.During the350-550℃aging treatment for120min after solution treatment at1050℃,the secondary austenite (γ2,martensitic type) precipitate increases with theincreasing temperature between350℃and470℃and reach the maximum at470℃.The γ2precipitation decreases when the temperature comes to500-550℃; thesecondary austenite (γ2,martensitic type) precipitation reaches the maximum at550℃when aging at450-550℃for240min. When the aging time comes to600minbetween450℃and550℃, dispersed phase (chromium-rich α’ phase) precipitates inthe original ferrite when aging at450℃, the number of α’ phase increases with theincreasing temperature from450℃to470℃、500℃, and only Cr2N nitrideprecipitates when the temperature comes to550℃.When aging in the700-950℃for30min, only secondary austenite (γ2,widmanstic type) is found to precipitate when aging at700℃. σ phase precipitates inthe ferrite/austenite grain boundary when aging at750℃.When aging between800-830℃, the main precipitations are in the form of γ2＋σ eutectoid.Whentemperature comes to850-870℃,σ phase and the γ2precipitates directly from theferrite.Aging at900-950℃the Cr2N nitride is found in addition to σ phase.The σphase increases with the increasing temperature when aging at750-850℃.The σphase precipitation fluctuates in the range of870-950℃and reaches the maximum at900℃. Aging in the830-950℃for4-30min, σ phase precipitates at830℃for8min,but it only takes4min to precipitate when aging at850℃since phase transformationdriving force increases. With extension of the aging time, σ phase precipitationincreases and reaches the maximum at900℃for30min when aging between830-950℃.Aging at350-470℃for120minthe secondary austenite (γ2,martensitic type)precipitation makes the passive film more heterogeneous and is vulnerable tocorrosion; the secondary austenite (γ2,martensitic type) precipitates most at470℃and so the passive film protection declines worst. the corrosion current density reachthe highest at the same time.Aging at700-950℃for30min, the passive filmprotection reduced when secondary austenite (γ2, widmanstic type) precipitate at700℃, the Cr depleted zone was formed when σ phase precipitates at750-950℃which decline the corrosion resistance, the maximum of σ phase precipitate at900℃lead to the corrosion resistance decline worst.Aging at450℃for600min, α’ phase which is rich in Cr elements precipitates,which leads to the decline of passive film protection and corrosion resistance. Thecorrosion resistance deteriorates with the increasing aging temperature.when aging at550℃for600minthe precipitate of Cr2N nitride makes the corrosion resistanceworst.The aged specimen follows the same corrosion law in1mol/L hydrochloric acidsolution, artificial sea water and1mol/L nitric acid solution. Among the polarizationcurves in three solutions, passivation region in1mol/L hydrochloric acid is thenarrowest, the corrosion current density is the highest and the capacitive loopdiameter is the smallest; followed is the1mol/L nitric acid solution, whose corrosionresistance is the best in the artificial sea water. |
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