| As an emerging structured material,high-nitrogen and Ni-free austenitic stainless steel has excellent mechanical properties and biomedical potential.However,the high content of nitrogen,which replaces the nickel element in traditional stainless steels,makes it easier to precipitate nitrogen compounds during thermal processes,readily resulting in intergranular corrosion(IGC)and stress corrosion cracking(SCC),which seriously affects practical engineering application of the steel.For this kind of face-centered cubic metallic materials with low stacking fault energies,some researchers have improved their corrosion resistance by traditional thermal-mechanical process.However,this common method of grain boundary engineering(GBE)optimization still exhibits some limitations.For this reason,the researchers further put forward some methods on the surface grain boundary engineering(SGBE)optimization of 304,316 and other traditional austenitic stainless steels;however,these reported methods are still difficult to be applied to practical engineering materials due to their low efficiency or poor controllability,and also there is as yet no report on the SGBE of high-nitrogen and Ni-free austenitic stainless steel.Therefore,in the present work,Fe-18Cr-16Mn-2Mo-0.85N high-nitrogen and Ni-free austenitic stainless steel was selected as the target material to find out the SGBE optimization processes by using surface spinning strengthening(3S)technology and subsequent heat treatment.The effects of SGBE optimization on the second phase precipitation behavior and room-temperature tensile properties of the experimental steel were studied.On this basis,the effect of SGBE optimization on anticorrosion properties of the experimental steel was studied by intergranular corrosion tests and slow-strain-rate stress corrosion tests.The present work is expected to provide a novel pathway to improve the anticorrosion properties of high nitrogen Ni-free austenitic stainless steel.The experimental steels were processed by 3S and then annealed at different conditions.It can be found that different optimization degrees of GBE layers could be formed near the surface region after 3S followed by annealing at 1050℃ for a certain time.With the increase of annealing time,the thickness of GBE layer increases,but the matrix structure will become obviously coarsening under overlong annealing time or overhigh temperature.Therefore,annealing at 1050℃ for 60 min after 3S was selected as the optimal SGBE process in the experimental steel.In this case,the fraction of special boundaries increases from 48.3%to 69.1%,and some large-sized twin related domains(TRD)are simultaneously formed.Analyses of the microstructures of GBE layer demonstrate that the formation of GBE layer is closely related to the 3S deformation-induced planar-slip dislocations in a great number and some stacking faults,which can promote the formation of numerous annealing twins by reacting with the migrating GBs during the subsequent annealing,thus increasing the fraction of special boundaries.The results of tensile tests at the room temperature show that SGBE almost has no effect on the strength but slightly increases the ductility of the experimental steel.The microstructure and composition of the precipitates at GBs of the experimental steel after sensitization are analyzed,and it is found that higher fraction of special boundaries in the GBE layer can greatly inhibit the intergranular precipitation of Cr2N phase.The results of intergranular corrosion tests show that the GBE layer exhibits a better intergranular corrosion resistance.The main reason is due to the fact that the Σ3 boundaries instainless steel are quite different from other special boundaries and random high-angle grain boundaries(RHAGBs),and they are difficult to be corroded and present an excellent intergranular corrosion resistance.The blocking effect of Σ3 boundaries in GBE layer on the connectivity of RHAGB networks can effectively prevent the occurrence of corrosion behavior.At the same time,the formation of TRD delays the propagation of corrosion cracks along the grain boundaries and prevents a large number of grains from falling off.In a word,the GBE layer near the surface can effectively improve the intergranular corrosion resistance of the experimental steel.Low-strain-rate stress corrosion tests in NACE A solution at 50℃ demonstrate that,compared to the non-GBE sample,the SGBE sample exhibits a better resistance to stress corrosion cracking.Observations of the surface morphology of the tested samples indicate that a greater amount of Σ3 boundaries in the SGBE sample can inhibit the initiation of corrosion cracks on the surface,and effectively delay the crack propagation along grain boundaries,thus improving the resistance to SCC of the experimental steel. |
