Effect Of Long-term Thermal Aging On Local Corrosion And Microhardness Of Duplex Stainless Steel

Austenitic-ferritic duplex stainless steel(DSS)is widely used in petroleum,chemical and nuclear energy fields because of its good corrosion resistance of austenite and excellent mechanical properties of ferrite.The ferrite in DSS plays the role of reducing weld hot cracking,improving stress corrosion resistance and improving material strength.However,researches show that thermal aging embrittlement will occur in ferrite phases when DSSs are exposed at 300-500°C for a long time.Thermal aging will deteriorate the corrosion resistance and mechanical properties of DSS,causing hidden safety hazards.There are many reports about thermal aging at home and abroad,and some beneficial results have been obtained.However,there are few reports on the local corrosion resistance of duplex stainless steel due to thermal aging from the aspects of composition and phase composition ratio.Three DSSs,Z3CN20.09M,SAF2205 and SAF2507 are adopted as studied materials in this paper.Three typical grades of DSS(different ratios between austenite and ferrite and different degrees of alloying)are subjected to thermal aging treatment with different parameters.The effect of thermal aging temperature on the microstructure of DSS has been studied by optical microscope(OM)and scanning electron microscope(SEM),transmission electron microscopy(TEM),and X-ray diffraction(XRD)methods.The electrochemical workstation and micro Vickers hardness tester have been used to study the effect of thermal aging temperature on the effects of local corrosion resistance and microhardness of steel,the study found:1)The change range of ferrite content of Z3CN20.09M,SAF2205 and SAF2507before and after thermal aging is 9.24-11.37%,49.26-51.79%and 50.30-51.76%,respectively.Cr-rich?'phase and Si and Ni-rich G phase precipitate from Z3CN20.09M at 350-500°C by spinodal decomposition.The thermal aging of SAF2205 and SAF2507 at 350-450°C results in the formation of Cr₂N and R phase(Fe₂Mo)in addition to?'phase and G phase due to their high degree of alloying.2)The increase of thermal aging temperature and time will affect the size and chromium content of?'phase.The average size of?'phase in Z3CN20.09M specimens increased from 7.41 nm to 18.55 nm,the chromium content decreased from 70.26%to63.87%,and the average size of?'phase in SAF2205 increased from 8.05 nm to 17.23nm.When the thermal aging temperature increased from 450°C to 500°C,the average size of?'phase of SAF2507 increased from 9.17 nm to 18.98 nm,and the chromium content decreased from 95.24%to 82.66%.3)The pitting resistance of DSS is related to the size and chromium content of?'phase.The larger the size and the higher the chromium content,the worse the pitting resistance.For Z3CN20.09M specimens,with the increase of thermal aging temperature and the extension of thermal aging time,the?'phase is gradually formed between 350-450°C,the chromium content is gradually increased,and the pitting resistance of the material is gradually decreased.Between 450-500°C,with the increase of temperature and time,the chromium content of?'phase decreased and the size increased.Because the chromium element does not form a new phase and G phase precipitates,the decrease of chromium content is small,bringing the gradual decrease of pitting resistance.For SAF2205 and SAF2507 specimens,the pitting resistance of the materials is the same as that of Z3CN20.09M during 350-450°C.But Cr₂N phases formed at 500°C and the healing will reduce the chromium content of?'phase,which will lead to the pitting resistance of the material decreasing first and then increasing.After Cr₂N phases are fully formed,the thermal aging time continue to be extended,the pitting resistance of SAF2205 and SAF2507 specimens will decline.The pitting resistance of solid solution SAF2507 specimens and SAF2507 specimens aged at350°C is the best due to the highest Cr and Ni content slowly thermal aging.Between450°C and 500°C,the degree of chromium segregation is the largest,so the pitting resistance of SAF2507 decreased most obviously followed by SAF2205 specimens,and the pitting resistance of Z3CN20.09M specimens decreased slowest.The pitting resistance of ferrite is lower than that of austenite in thermally aged specimens at 450°C and 500°C.Therefore,the locations of pitting are transferred from austenite phase to ferrite phase.The polarization peak of ferrite phase in sulfuric acid and hydrochloric acid solution is covered by the polarization peak of austenite due to thermal aging.The current density increased,and the selective corrosion phenomenon of austenite disappeared.4)The hardness of austenite has no obvious change during the thermal aging of DSS.The change of ferrite hardness is related to the chromium content of?'phase.The higher the chromium content,the higher the hardness.The decomposition rate of the three materials was slow at 350°C,and the degree of Cr segregation increased with ageing time,so the hardness increased.Because no Cr₂N precipitation in Z3CN20.09M specimens aged at 450-500°C,the dimension of?'phase becomes larger and the Cr content decreases with the increase of thermal aging temperature and time,so the hardness value of Z3CN20.09M specimens decreased.For SAF2205 and SAF2507specimens,with the increase of thermal temperature and time,the Cr content of?'phase decreased with the increase of?'phase length and the precipitation of Cr₂N,so the hardness of the two DSSs specimens also decreased.