| High strength stainless steel has been widely applied into high-tech areas such as aerospace and marine engineering due to its excellent toughness and corrosion resistance which is a result of the combination of strengthening method including low carbon martensite phase transformation and precipitation hardening.With the vigorous development of the marine science and technology,requirement to the corrosion resistance of high strength stainless steel is much higher.The precipitation which is rich of corrosion resistance elements and reversed austenite formed during tempering have observable effects on its pitting corrosion resistance.In this paper,the microstructure,pitting corrosion resistance of 13Cr15Ni4Mo3 N high strength stainless steel were studied by means of scanning electron microscope?SEM?,transmission electron microscopy?TEM?? FeCl3 immersion test,potential dynamic polarization.The semi-conductive performance of passive film formed on the surface after aging treatment was investigated through potential dynamic electrochemical impedance spectroscopy?DEIS?and Mott-Schottky curve test.The results showed that the amount of precipitation?M23C6 and ?-?Cr,Fe?phases?in13Cr15Ni4Mo3N increased with the rise of temperature when aged during the temperatures ranging from 350? to 450? after solid solution treatment and air cooling.And the content of reversed austenite remained around 5%,The pitting potential Eb100 decreased as well as the corrosion resistance.The tensile strength and yield strength increased and reached to the peak.While the yield ratio reduced along with the impact energy decreasing to the bottom when tempered at this range.On the other hand,the amount of precipitations merely changed with fine M2C??Cr,Mo?2N?phases dispersedly precipitated aged at the temperature ranging from 535 ? to 625 ?.The amount ofreversed austenite increased from 8.14% to 25.74% with the Eb100 and corrosion resistance improving.While the tensile strength,yield strength reduced,At the same time,the yield ratio increased along with the subtle change of impact energy.It could be seen that the optimum combination of mechanical property and corrosion resistance was obtained when aged at 350?for 4h.Furthermore,the results of microstructure controlling showed that precipitations weakened the pitting corrosion resistance while improved strength during aging treatment.The strengthening effect was reduced because of the coarsening of precipitations.Reversed austenite was adverse to the strength while helpful to meliorate the ductility and improve the pitting corrosion resistance.Besides,the results of electrochemistry test and morphology analysis showed that there was a content gradient of the corrosion resistance elements near the etch pit.Specifically,the content of corrosion resistance and carbon was higher.On the contrary,the content of corrosion resistance elements elements was comparatively lower in the etch pit.In other words,the corrosion occurred in the depleted areas of corrosion resistance elements.And the specimens would lose metallic luster after FeCl3 immersion.The extent of corrosion in the specimens aged at 350? was the lightest and still glossy.While the extent of corrosion in the specimens aged at 535? was comparatively heavier and completely lost the metallic luster.Meanwhile,the results of DEIS and Mott-Schottky curve test were fitting calculated.And it could be concluded that the passive film defects diffusion coefficient D0 changed along with the aging temperatures when immersed in 3.5%NaCl solution until the applied potential reached to the pitting potential.And D0 firstly increased and then reduced with the increase of aging temperature accompanied by the stability of passive film decreased at first and then increased.And it fitted well with results of polarization curve and dynamic potential electrochemical impedance spectroscopy.The compactness of passive film on the surface was also affected by the aging temperature. |
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