The Self Surface Nanocrystallization And Hybrid Surface Nanocrystallization Of AISI 304 Stainless Steel

Surface nanocrystallization (SNC), including surface self-nanocrystallization (SSNC) and hybrid surface nanocrystallization (HSNC), is a newly developed technique to form a nano-structured surface layer in bulk 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 AISI 304 stainless steel with a very low stacking fault energy (16.8 mJ/m2) was selected to achieve SSNC by means of surface mechanical attrition treatment (SMAT); subsequently, HSNC was achieved through conducting low temperature plasma nitriding on the SSNC 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 surface roughness, hardness, wear resistance and corrosion resistance were also examined; the grain refinement mechanism during SMAT and the HSNC mechanism were proposed based on the experimental result. The following summaries were obtained:1. Surface self-nanocrystallization was achieved on the AISI 304 stainless steel by means of SMAT. A nano-structured surface layer with a thickness of about 30 urn was developed after 15 minutes' SMAT, and the grain size in the top surface layer is about 10 nm; with the increment of treating duration the grain size decreases continuously then approaches to aunchanged 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 SMAT. The amount of martensite in the top surface layer increases with treating duration till nearly 100 % volume martensite phase was detected after 15 minutes' treatment; with depth increment, the microstructure changes from the martensite phase of about 10 nm 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 AISI 304 stainless steel during SMAT is deformation twin, the following microstructure characters were identified along depth: surface nano-scale grains, multi-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 surface layer may be related to the multidirectional repetitive loading.4. The SSNC AISI 304 stainless steel sample shows increased surface hardness which is over 2.5 times higher than that of annealed coarse-grained one, and the surface roughness is decreased continuously during SMAT till a very low value (Ra=0.15) is obtained. The SMAT shows less influence on the wear-resistance and corrosion-resistance under the present experimental conditions.5. SSNC accelerated the transportation of Nitrogen in AISI 304 stainless steel sample and significantly optimized the low temperature plasma nitriding process, a much thicker nitrided layer (10-12 jam) was obtained compared to the coarse-grained counterpart (4-5 urn) 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.6. The HSNC samples showed extremely high surface hardness, which is 8 times larger than that of annealed sample and doubles the value of plasma nitrided coarse-grained one, and the thickness of the hardened layer is much increased. The samples showed excellent wear-resistance against WC ball under the applied loads of 20 N for 30 minutes sliding without lubricant, the wear volume was one fiftieth relative to that of annealed samples and a quarter compared to that of the plasma nitrided coarsegrained samples, and the wear coefficients were decreased also. HSNC samples exhibited excellent pit corrosion resistance in the 3.5% NaCl solution.