Fabrication,microstructure And Properties Of Powder Metallurgy High-N Ni-free Austenitic Stainless Steel

High nitrogen nickel free austenitic stainless steel?HNASS?exhibits many advantages.It not only overcomes the problems that the nickel?Ni?element in traditional austenitic stainless steel is harmful to the human body and the high cost of nickel element,but also has perfect corrosion resistance,superior combination of strength and toughness and good processability.Thus,HNASS was widely used in structural and engineering applications,including medical implant materials,marine engineering?marine pipelines?,equipment,and so on.Based on the requirements of low cost and high performance of the HNASS and the technical characteristic of powder metallurgy?PM?,this paper prepared both dense and porous high nitrogen austenitic stainless steel by powder metallurgy method,designed and optimized its process method,composition,organization,and performance.?1?Dense high-N Ni-free austenitic stainless steel was prepared by powder nitriding and spark plasma sintering?SPS?using low nitrogen CrMnstainless steel powder as the received material.Explored the effects of powder nitriding temperature?700-900??on the nitrogen content and phase constituent of the nitrided N-containing CrMnstainless steel powders,while N-containing stainless steel powder with high and controllable nitrogen content?0.68-2.89 wt.%?was obtained.Then,dense high-N austenitic stainless steel with FeCrMnN system was prepared using SPS,and the effect of nitrogen content on the phase constituent,evolution of microstructure,corrosion resistance,and mechanical properties were studied.The quantitative analysis results of each strengthening mechanism showed that the solid solution strengthening mechanism and the grain boundary strengthening mechanism dominate and significantly improve the mechanical properties of the powder metallurgical high-N Ni–free austenitic stainless steel,the precipitation strengthening effect is not obvious and the contribution to performance is not significant.?2?Dense high-nitrogen nickel-free austenitic stainless steel was also prepared by powder nitriding and SPS using medium-nitrogen CrMnModuplex stainless steel powder as the original material.Explored the effects of powder nitriding temperature?600-775??on the nitrogen content and phase constituent of the nitrided N-containing CrMnstainless steel powders,while N-containing stainless steel powder with high and controllable nitrogen content?0.75-2.62 wt.%?was obtained.Then,dense high-N austenitic stainless steel with FeCrMnMoN system was prepared using SPS,and the effect of nitrogen content on the phase constituent,evolution of microstructure,corrosion resistance,and mechanical properties were studied.The results showed that the dense high-N austenitic stainless steel has very good yield strength and tensile strength.As the N content increases,the corrosion resistance of the alloys increased first and then decreased,while the nitriding temperature is 650?,the alloy possessed the best corrosion resistance.?3?Porous high-nitrogen nickel-free austenitic stainless steel was prepared by compression molding and gas-solid nitriding sintering using the medium-N CrMnModuplex stainless steel powders as the received materials.The effects of sintering temperature,the content of pore former and compacting pressure on its porosity,pore morphology,microstructure,compressive properties,and elastic modulus were studied.The potentiodynamic polarization behavior of the as-fabricated porous HNASSs in the 0.9 wt.%Na Cl solution was also explored.Results showed that as the porosity increases the corrosion resistance of the as-fabricated porous HNASSs gradually decreases.With the increasing of gas-solid nitriding sintering temperature,its annual corrosion rate and corrosion current density decreased,the corrosion resistance of the as-fabricated porous HNASSs increased gradually.?4?Explore the effect of hydrogen reduction pretreatment on the gas-solid nitriding and sintering process of the as-received N-containing CrMnduplex stainless steel.The effects of reduction temperature and reduction dwell time on the surface and inner phase constituent of the as-fabricated porous HNASSs were investigated.Based on the qualitative and semi-quantitative analysis of the X-ray diffraction?XRD?and X-ray photoelectron spectroscopy?XPS?,explore the evolution of oxides during the reduction process.Explored the effect of reduction treatment on nitriding and analyzed the chemical state of nitrogen in samples with and without reduction pre-treatment.Based on thermodynamic theory,studied the thermodynamic stability of various oxides,and explore the thermodynamic problems in the reduction process.Results showed that the reduction process parameters have significant effects on the content of nitrides precipitate.With the increase of the reduction time,the reduction process is more complete,the content of CrN precipitates increases slightly,while the content of the Cr₂N precipitates decreases slightly.The effect of reduction temperature on the content of nitrides is more complicated,with increasing reduction temperature,the content of nitrides first increased,then decreased and finally increased again.This paper comprehensively studied the preparation,microstructure and properties of the dense high-N Ni-free austenitic stainless steel with FeCrMnN system and dense and porous high-N Ni-free austenitic stainless steel with FeCrMnMoN system,it not only brings a new method for the preparation of higher performance engineering high-N Ni-free austenitic stainless steel,but also provide a new choice for medical implant materials,but also provides a theoretical reference and a new way for the preparation of high-performance powder metallurgical high-N Ni-free austenitic stainless steel.