Research On Pressurized Metallurgy Manufacturing,Microstructure And Properties Of High Nitrogen Stainless Bearing Steel

With the rapid development of aerospace engine,the bearing steel in service could no longer meet the requirement of high performance,long service life and high reliability.The high nitrogen stainless bearing steel exhibits excellent performance in hardness,toughness,corrosion resistance and fatigue resistance,which has been successfully applied in the aerospace field overseas.However,there is still an extinguish gap between China and developed countries in the quality and stability of aerospace bearing steel,and the research of advanced bearing steel is seriously delayed,which has become a barrier restricting the development of aerospace engine in China.This study focuses on the pressurized melting process,hot deformation,heat treatment as well as the influence of alloying elements on the microstructure and properties of high nitrogen stainless bearing steel to support the relative research,and promote the development of high-performance aerospace bearing steel in our country.The conclusions are listed as follows:The thermodynamic and kinetic analyses together with the experimental research of vacuum carbon-deoxidization were conducted.Combined with Ni-Mg alloy and rare earth treatment,the oxygen,sulfur and inclusion controlling technology during the pressurized induction melting stage was preliminarily mastered.Besides,by analyzing the influence of melting pressure and casting pressure on the nitrogen content in ingots,the method to precisely control the nitrogen content through pressure control in stages was preliminarily developed.The high nitrogen stainless bearing steel with uniformly distributing nitrogen,low oxygen and sulfur contents as well as fine and dispersed inclusions was obtained.The constitutive equation of high nitrogen stainless bearing steel at the peak stress was established,and the apparent activation energy was 503.5kJ·mol^-1,which exceeded those of the traditional martensitic stainless steels.The quantitative relationships between initial critical stress and peak stress of dynamic recrystallization(DRX)as well as Zener-Hollomon parameter were established.With the increase of deformation temperature,the size of DRX grains rised.When the strain rate was in the range of 0.01?1s^-1,the increase of strain rate restained DRX,and when the strain rate reached 10s^-1,the DRX was promoted and the grain size was enlarged.At low deformation temperature,the precipitation of M₂₃C₆ and M₂N along the grain boundary would pin the grain boundary and suppress the DRX.Besides,the tangling and stacking of dislocation around the precipitates would restrain the dislocation movement.However,increasing the deformation temperature would dissolve the precipitates,and eliminate their influence on the DRX.With the increase of austenitizing temperature,the content of Cr-rich M₂₃C₆ and M₂N decreased,while the content of retained austenite increased,and the elements in the matrix distributed more uniformly.Since the Cr-depleted region was wider,and Cr-depletion was more serious in the vicinity of M₂₃C₆,metastable pitting preferentially initiated around M₂₃C₆.Raising the austenitizing temperature could reduce the pitting initiation sites,enhance the thickness of passive film,promote the enrichment of Cr₂O₃,Cr³⁺ and CrN in the passive film,and increase the content of solid-solution nitrogen in the matrix.Therefore,the pitting potential was enhanced,the propagation rate of pits was decreased,and the repassivation property and corrosion resistance were enhanced.With the increase of tempering temperature,the size of precipitates in high nitrogen stainless bearing steel increased.The precipitates were fine and close to each other at 550?,and the segregation of Cr and C was observed.At 650?,the precipitates grew up and the space between them increased,and Cr was enriched in these precipitates.When rising the tempering temperature,the hardness decreased at 300?,and the secondary hardening occurred at around 500?,then the hardness decreased again.With the increase of tempering temperature,the pitting potential of high nitrogen stainless bearing steel first increased and then decreased.In addition,the tempering temperature also had a significant influence on the stability of passive film.The stability and corrosion resistance of passive film was deteriorated at 450?,while improved at 650?.Based on the results of this thesis,the optimum heat treatment process was first austenitizing at 1020?,then sub-zero treatment at-80?,and finally tempering at 300?.Mo-alloying could reduce the formation energy,bonding energy,density of state of high nitrogen stainless bearing steel,and alleviate the lattice distortion,thereby enhancing the structure stability and improving the solubility of nitrogen in solid solution.Additionally,Mo-alloying could decrease the content of Cr-rich M₂N,and thus fully play the beneficial effects of nitrogen on corrosion resistance,and alleviate the detriment effect induced by Cr-rich precipitates.The synergistic effect of N and Mo could improve the passive film of high nitrogen stainless bearing steel,and significantly enhance its corrosion resistance.The substitution of nitrogen for carbon could prevent the precipitation of coarse carbides along the grain boundary,and transform the precipitation type from M₂₃C₆ to M₂₃C₆+M₂N,and finally to M₂N.Meanwhile,the partial replacement of carbon by nitrogen first increased then decreased the effective grain size,retained austenite content and fraction of twin martensite,while the variations of precipitation content and dislocation density are opposite.The partial substitution of carbon by nitrogen reduced the strength of 0.35C-0.37N steel,while obviously improved its impact toughness and intergranular corrosion resistance,and the fracture mode was transformed from typical brittle fracture into mixed ductile and brittle mode.The further replacement of carbon by nitrogen increased the strength of 0.20C-0.54N steel,but deteriorated the impact toughness and intergranular corrosion resistance.Nitrogen existed in the form of nitrogen atom and Cr-rich M₂N in high nitrogen stainless bearing steel.Nitrogen could restrain the formation of ?-ferrite,and refine the primary austenite grains.With the increase of nitrogen content,the content of precipitates was first reduced and then increased,and the dominant precipitates variated from M₂₃C₆ to M₂N.The increase of nitrogen content could enhance the hardness and strength of high nitrogen stainless bearing steel,while first increase and then decrease the impact toughness.Moreover,increasing nitrogen content could improve the stability and corrosion resistance of the passive film.The combined effects of precipitation content and type,passive film as well as nitrogen in solid solution induced the first increase and then decrease of pitting corrosion resistance with the increase of nitrogen content.Based on the results of this thesis,the high nitrogen stainless bearing steel with medium carbon and nitrogen ratio exhibited the best performance,and the optimum mass fraction of nitrogen was about 0.4%.The novel duplex process of pressurized induction melting and pressurized electro-slag remelting(PESR)was originally proposed to produce high nitrogen stainless bearing steel,and the preliminary exploration was conducted.The PESR ingot with compact solidification structure,non-existent nitrogen pore,high nitrogen content and low oxygen and sulfur contents was obtained.The PESR process could reduce the proportion of large-size inclusion in the steel,and transform the rare earth oxysulfides and MgO into Al-Mg-O inclusions.Furthermore,the impact toughness of high nitrogen stainless bearing steel was significantly improved,while the hardness and strength were rarely influenced.