| The UNS S32707 hyper-duplex stainless steel(HDSS)can meet the harsh service requirements of a deep-sea environment.The use of HDSS provides several advantages,such as thinner offshore engineering structures,weight reduction,a saving of raw materials,and cost reduction,and HDSS is considered to be an ideal material for use in deep-sea oil and gas exploitation equipment to replace nickel-based corrosion-resistant alloys.However,there are many problems in the preparation of the HDSS by traditional processing technology due to the high content of alloying elements and nitrogen in the HDSS.The high nitrogen HDSS powder was prepared by plasma rotating electrode atomization,and the near-net shape high nitrogen HDSS parts with complex shape,excellent mechanical properties,and corrosion resistance were prepared by selective laser melting,hot isostatic pressing and powder injection molding.This project studies on the key scientific issues in the integrated research and design system including the whole process of preparation,process,structure and performance,and is aims to resolve the problems such as high oxygen content and high impurity content of the gas atomized powder,and difficult to control the nitrogen content of powder and the sintered parts and dual phase micro structure in the preparation of powder metallurgy high nitrogen HDSS.The high efficiency preparation of parts with different sizes and the high efficiency utilization of powders with different particle sizes were realized.The main conclusions are as follows:Novel ferritic stainless steel with a relative density of 98.2%,advanced mechanical properties,and significant magnetic responses was prepared by a selective laser melting(SLM)process from the powder with a grain size range of 15 to 53 ?m with a laser power of 190 W,a scanning speed of 800 mm/s,a powder layer thickness of 0.03 mm,a hatch distance of 0.09 mm,a laser energy density of 87.96 J/mm3 and 67° of rotation of the upper and lower layers.The special scanning strategy makes the material present random grain orientation and weakened anisotropy.The rapid cooling after melting inhibits the formation of austenite,and the ferrite content reaches 98.5%.Printed parts prepared using SLM had excellent mechanical properties due to a large number of dislocations in the ferrite grains(dislocation strengthening),the nano-nitrides precipitated at grain boundaries(precipitation strengthening),and a 3.68?m average grain size of ferrite(fine grain strengthening).The measured tensile strength,yield strength,and the microhardness were 1493 MPa,1391 MPa,and 528.7 HV,respectively.Nitrides precipitated at ferrite grain boundaries lower the ductility and toughness of printed parts prepared by SLM.The measured elongation,the reduction of area,and the impact absorbing energy were 13.2%,24.1%,and 18 J,respectively.The high specific saturation magnetization of the SLM samples was mainly caused by an ultra-high ferrite content of 98.5%compared to forgings.The lower coercivity was due to the average 3.68 ?m grain size of the ferrite.The specific saturation magnetization and the coercivity were 106 Am2/kg and 1.79 mT,respectively.This provided a new approach for the near net shaping of structural and functional integrated soft magnetic materials with both strength and corrosion resistance.UNS S32707 HDSS prepared by the SLM process can obtain a nearly balanced two-phase structure of ferrite and austenite through the appropriate solution annealing process,which can significantly improve the ductility and toughness.A large number of dislocations formed during SLM are the main driving force of recrystallization during solution treatment.The nitrogen content reduced by 33%in the SLM process,resulting in the imbalance of the two phases and the reduction of the austenite content.The a phases were dissolved completely with solution treatment at 1100?.The grain size and the ferrite content increased gradually,while the austenite content decreased gradually,and the strength and hardness decreased gradually,while the elongation and section shrinkage increased gradually with the increment in solution annealing temperature.The samples with solution treated at 1150? for one hour had better comprehensive mechanical properties,the measured tensile strength,the yield strength,the elongation,the section shrinkage,the microhardness,and the impact absorption energy were 901 MPa,658 MPa,36.4%,48.4%,291.5 HV,and 132 J,respectively.The comprehensive mechanical properties are similar to that of seamless pipe in ASTM A790 standard,and superior to that of casting UNS S32707 HDSS.The samples with solution treated at 1100? for one hour had better pitting resistance.The pitting potential was 1196 mV,which is better than that of hot rolled plate.The critical pitting temperature was 45?.The surface layer of passive film is mainly composed of oxides of Fe and Cr and hydroxide compounds.Ni4+ adsorbed on the surface of passive film and protected the passive film.The inner layer of the passive film is mainly composed of metal elements,Cr2N and oxides of Fe,Cr,and Mo.Given a pressure of 150 MPa,the relative density of sintered parts is 99.9%after hot isostatic pressing sintering at 1200? for 3 h from the coarse powder with a grain size range of 53 to 150 ?m.The ? phase and nitride precipitated when the furnace cools down,and this significantly undermines the ductility and impact toughness.In contrast,the ? phase was dissolved completely with solution treatment at 1100? for 1 h followed by water quenching,which led to a notable increase in ductility.A duplex structure of ferrite and austenite was identified,and the ratio of ferrite to austenite was approximately 46.8:51.8.The measured tensile strength,the yield strength,the elongation,the hardness,the impact absorption energy,the pitting potential and the critical pitting temperature were 832 MPa,578 MPa,17.5%,256 HV,15.82 J,1169 mV,and 50? respectively.High-value utilization of the UNS S32707 HDSS coarse powder was realized.The UNS S32707 HDSS was prepared by powder injection molding and supersolidus liquid-phase sintering from the powder with a grain size range of 15 to 53 ?m,and the nitrogen content and the relative density of the sintered parts were 0.37%and 97.9%,respectively.The sintered parts can achieve full densification followed by hot isostatic pressing,and the sintering densification of coarse powder and precise control of nitrogen content were realized.The nitrogen escapes from high-nitrogen duplex stainless steel during vacuum sintering,resulting in a decrease in austenite content after solution treatment.The relative density increased gradually,whereas the nitrogen content of the sintered parts decreased gradually with the increase of sintering temperature under the nitrogen pressure of 0.02 MPa.The parts sintered at 1400? for 1.5 hours exhibited ideal sintering relative density and nitrogen content.After solution treatment at 1150?for 1 hour,the ratio of ferrite to austenite was approximately 46.8:51.8,the measured tensile strength,the yield strength,the elongation,the hardness and the pitting potential were 761 MPa,529 MPa,14%,321.4 HV and 1071 mV,respectively.The mechanical properties of the sintered parts at 1400? for 1.5 hours followed by hot isostatic pressing and solution treatment at 1150? for 1 hour are significantly improved compared with those without hot isostatic pressing treatment.The measured tensile strength,the yield strength,the elongation,the hardness and the impact absorption energy were 882 MPa,557 MPa,31%,276.8 HV,and 14 J respectively.The comprehensive mechanical properties are equivalent to that of casting UNS S32707 HDSS.This study provides a new method for the preparation of high-nitrogen HDSS with a complex shape and controllable nitrogen content by near-net shaping. |
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