Research On Preparation And Properties Of Hyper Duplex Stainless Steel S32707

Having the significant higher alloy content than ordinary one,hyper duplex stainless steel S32707 possesses excellent corrosion resistance and mechanical property.Taking into account China's sustainable development strategy needs,such as deepwater oil and gas exploitation,flue gas desulfurization,new energy and other fields of development is imminent,and these just is play the advantage place of S32707.It is believed that this material can replace partial Ni-based alloys and titanium alloys which both are more expensive.Due to started late and blockaded by foreign countries,there is still a big gap between our country and other ones on the research and application of the hyper duplex stainless steel.There is no doubt that studying on its preparation and properties could provide theoretical foundation and technological guidance in application.In this paper,hyper duplex stainless steel S32707 was studied especially on its preparation,mechanical properties and corrosion resistance.Additionally,the effect of adding rare earth(RE)on S32707 was also studied.The main results are presented as follows.Using the 25kg pressurized induction furnace,The ingots were rolled which began with 1200? and ended in 1100?.The inclusions in the samples of S32707 without RE were mainly Al2O3 and a few compound ones like Al2O3-MnS.After the addition of 0.03%RE,the inclusions became fine and the number of them was decreased.After adding 0.06%RE,however,the inclusions became bigger and the number was increased.Treated by RE,partial Al2O3 turned to Ce2O3 and Ce2O2S.The best solubility system was heated at 1180? for 1h and the ferrite content in this steel grade was 49.8%.From the potentiodynamic polarization curves,it was confirmed that S32707 had outstanding corrosion resistance.It demonstrated that adding RE in steel improved its corrosion resistance.Analyzing the corrosion morphology,it was found that the corrosion resistance in ferrite phase was weaker than that in austenite phase,and the pitting of S32707 without RE would appear in the inclusions preferentially.The study of incubation period of pitting corrosion indicated that the current density,at the beginning,was sharply decreased over time because of the growth speed of passive film was faster than that of solubility.Iss(current achieved steady)was increased with the increase of the applied potential which meant the decline of the corrosion resistance of passive film.The study of EIS showed that with the increase of the applied potential,not only the resistance of passive film and the polarization resistance were gradually decreased,but also the passive film stability deteriorated.Under different potentials,after adding RE,Iss went down and the resistance of passive film and polarization both went up which illustrated the corrosion resistance was increased.The XPS analysis presented that the passive film forming in air was thicker.The insoluble Cr2O3 in the passive film restrained from dissolving.Via forming dense molybdenum oxides,the passive film prevented substances from passing through itself and decreased the corrosion rate effectively.By forming NH4+ and consuming H+,N improved the local corrosion environment.According to the results of the tensile tests,S32707 made by this experiment had better tensile properties than the standards of SANDVIK Company.Tensile properties of the steel with 0.03%RE were enhanced,but went worse after adding 0.06%RE.Changes of tensile properties were mainly caused by changes of inclusions.Low-temperature impact showed the ductile-brittle transition temperature of S32707 was about-44 ?.The changes of the fracture patterns of S32707 from low temperature to room temperature were clearage facet? quasl-clearage facet?mixture of quasl-clearage facet and dimple?dimple.Adding appropriate amount of RE could improve the impact toughness of steel.The fracture morphology of experimental steel was ductile fracture.