Investigation Of 0Cr18Ni9 Stainless Steel Supersonic Fine Particles Bombaeding(SFPB) Surface Nanocrystallization's Mechanism And Property

Surface nanocrystallization(SNC)is a newly developed technique to form a nano-structured surface layer inbulk material.This technique will promote the global properties of materials through modifying the microstructure and property of the surface layer,it will also significantly optimize the traditional surface treatment processes;Meanwhile,this technique will provide with ideal specimen to systematically study the relationship between technique,structure and property in nanometer regime.In this paper,a widely used 0Cr18Ni9 stainless steel with a very low stacking fault energy was selected to achieve SNC by means of Supersonic Fine Particles Bombarding(SFPB); subsequently,SNC was achieved through conducting low temperature gas nitriding on the SNC samples.The microstructures of the final products were characterized by means of X-ray diffraction(XRD),scanning electron microscope(SEM)and transmission electron microscope (TEM),the properties including hardness,wear resistance and corrosion resistance were also examined;the grain refinement mechanism during SFPB and the SNC mechanism were proposed based on the experimental result.The following summaries were obtained:1.SNC was achieved on the 0Cr18Ni9 stainless steel by means of SFPB.A nano-structured surface layer with a thickness of about 30μm was developed after 30 minutes' SFPB,and the grain size in the top surface layer is about 18 nm;with the increment of treating duration the grain size decreases continuously then approaches to a unchanged value;the grains change from equiaxed nano-scale with random orientations in the top surface layer into irregular sub-micron scale with less random orientations at subsurface layer,and the grain refinement process becomes less uniform also. 2.A strain-induced martensite transformation occurs during the SFPB.The amount of martensite in the top surface layer increases with treating duration till nearly 100%volume martensite phase was detected after 30 minutes' treatment;with depth increment,the microstructure changes from the martensite phase to a mixture of the martensite and the austenite phases with a little larger size.3.The predominate deformation manner of low stacking fault energy 0Cr18Ni9 stainless steel during SFPB is deformation twin,the following microstructure characters were identified along depth:surface nano-scale grains,mufti-directional deformation twins,single-directional deformation twins,planar dislocation arrays and stress-free matrix.The grain is subdivided into various structural elements through intersection of deformation twins,which is different from that for materials with medium-high stacking fault energy,in which dislocations behavior was believed to induce grain refinement.This subdivision process took place on a finer and finer scale with approaching to the treated surface,the random-oriented nano-sized grains in the top layer may be related to the multidirectional repetitive loading.4.SNC accelerated the transportation of Nitrogen in 0Cr18Ni9 stainless steel sample and significantly optimized the low temperature gas nitriding process,a much thicker nitrided layer was obtained compared to the coarse-grained counterpart nitrided under the same conditions;the phase constituent of the nitrided layer is nano-sized expanded austenite and expanded martensite, and the sizes of these two phases increase along depth and with good hardness distribution.5.The SNC 0Cr18Ni9 stainless steel sample shows increased surface hardness which is over 2 times higher than that of original coarse-grained one.The SFPB shows excellent wear-resistance under 10N for 30minutes;and SNC samples exhibited excellent polarization behavior in the 3.5% NaCl solution,while had badly pit corrosion resistance.