Study On Precipitations And Pitting Corrosion Resistance Of Hot Forming Duplex Stainless Steel In Marine Environment

SAF2507 SDSS（Super Duplex Stainless Steel）has the dual characteristics of including high content of Cr,Mo and nearly equaling two-phase（αphase andγphase）content.Additionally,it has super high strength,hardness and corrosion resistance,which is an ideal building material for ship and marine engineering.At present,the application of SAF2507 SDSS in ship and marine engineering is gradually increasing.However,the SAF2507 SDSS has high tensile strength and yield strength at room temperature.After the cold stamping forming,the workpiece is prone to lead to many defects,such as excessive rebound angle.This is difficult to ensure the accuracy of shape and dimension in the workpiece.These above defects can be effectively solved by using hot stamping forming technology.During the hot stamping forming process of the SAF2507 SDSS,it is easy to precipitate the precipitation of Cr-rich and Mo-rich in the microstructure due to the high degree of alloying.The content and chemical properties of precipitation can severely damage the surface passivation film,which reduces pitting corrosion resistance of SAF2507 SDSS.Therefore,it is of great practical significance to study on precipitation phase,precipitation mechanism,precipitation law and the effect of precipitation phase on pitting corrosion resistance of SAF2507 SDSS after hot stamping forming.In this paper,the SAF2507 SDSS sheet is used as the research object and the hot stamping forming experiment is conducted at 700℃¹100℃by medium-frequency induction heating furnace,hydraulic pressa and hot stamping forming die.After hot stamping forming test,using optical microscopy,scanning electron microscope and X-ray energy spectrometer to observe precipitation,measure the chemical content of precipitation and get the mechanism and rule of precipitation.At the same time,Through the JMatPro software to caculate the continuous cooling transition curve（CCT）and the isothermal transformation curve（TTT）to discuss the influence of the time and cooling rate of different stages of hot stamping on the precipitation.After metallographic test,the dynamic potential polarization curves and AC impedance spectra of SAF2507 SDSS in natural seawater at different hot stamping forming temperature were tested by electrochemical workstation.The pitting corrosion morphology was observed by scanning confocal morphology scanning electron microscopy to get the change rule of pitting resistance.Finally,the change rule of pitting corrosion resistance is explained from three aspects of hot stamping forming temperature,precipitation content and chemical composition.The main findings of this paper are as follows:（1）When the hot stamping forming temperature of SAF2507 SDSS is 750℃¹050℃,the precipitation of undeformed and deformed regions areχphase andσphase.The precipitation position isα/αgrain boundary andα/γphase boundaries.The precipitation temperature range ofχphase is 750℃⁸50℃,while the precipitation temperature range ofσphase is 750℃¹050℃.The precipitation mechanism ofχphase andσphase are bothαphase eutectoid decomposition.Theχphase is converted toσphase at 850℃.Theσphase content increased and then decreased.When the hot stamping forming temperature is 1050℃,theαphase and theγphase content are substantially equal,andα/γis 1:1.When the temperature is 1050℃¹100℃,there is no precipitation in the microstructure.Besides,theαphase content increased and theγphase content decreased,and the homologous transformation emerged occured between theαphase and theγphase.（2）When the hot stamping forming temperature of SAF2507 SDSS is 700℃¹100℃,the types,precipitation mechanism and precipitation rules of the precipitation in the undeformed and deformed regions are consistent.Although the deformation force promote theχphase andσphase nucleation and growth,but the cooling rate（v₁,v₂,and v₃）in the moving,forming,and pressure-holding quenching stage are greater than the critical precipitation cooling rate（v₀）of theχphase andσphase.The moving,forming and pressure-hardening phase time（t₁,t₂ and t₃）are less than the critical incubation time（t₀）of theχphase andσphase,therefore theχphase andσphase cannot effectively precipitate,and the solid phase transformation of the deformation zone is not the result of thermal coupling.Theχphase andσphase nucleation and growth only occured during the heat preservation phase.（3）When the hot stamping forming temperature of SAF2507 SDSS is 700℃¹050℃,theχphase andσphase of Cr-rich and Mo-rich accompany by the formation ofγ₂ phase（pitting resistant weak phase）which is depleted in Cr and Mo.γ₂ phase causes the self-corrosion potential of the material to decrease and then increase,the corrosion current density increases and then decreases,the pitting breakdown potential decreases and then increases,the AC impedance spectrum radius decreases and then increases,and the number of pitting holes increases and then decreases.The pitting resistance decreases and then increaes.SAF2507 SDSS has the worst pitting resistance when the hot stamping forming temperature is 950℃,and the pitting corrosion resistance is best when the hot stamping forming is 1050℃.When the hot stamping forming temperature of SAF2507 SDSS is 1050℃¹100℃,there is no precipitation phase in the microstructure,theα/γratio increases,and the partition coefficient of Cr and Mo decreases.Therefore the self-corrosion potential and the pitting breakdown potential decrease,the corrosion current density increases,the AC impedance spectrum radius decreases,the number of pitting holes increases and the density increases.The pitting corrosion resistance decreases again.