Microstructures And Properties Of 316L Stainless Steel With A Gradient Nanostructured Surface Layer

Gradient structured materials have been widely studied in recent years due to their unique structures and excellent properties. It also provides a new way to develop new materials and reduces the cost of materials. One of the reasons that restrict the wide industrial applications of gradient structured materials is its processing technology.In this paper, the plastic deformation of 316L stainless steel was introduced by Rotation Accelerated Shot Peening (RASP), Hawking and Conventional Blasting, respectively. The surface morphology, phase changes, microstructures and mechanical properties of 316L stainless steel with gradient structure obtained by different processing technologies were systematically characterized by portable roughness instrument, X-ray diffraction analysis (XRD), optical electron microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), micro hardness tester and tensile machine. Boronizing behaviors of 316L stainless steel treated by RASP were investigated. Specific research conclusions are as follows:(1) The surface roughness of the treated sample produced by RASP increased to Ra≈32μm, Rz≈79μm, compared with as-received Ra≈0.58μm, Rz≈0.15μm, reduced to Ra≈0.08μm, Rz≈0.15μm produced by Hawking. It was only Ra≈1.5μm, Rz≈2.6μm produced by Conventional Blasting.(2) The deformation induced martensite phase transformation was observed in 316L stainless steel treated by RASP, Hawking and Conventional Blasting. The peak intensity of martensite decreased with the increase of depth from the surface.(3) The thickness of gradient layer produced by severe plastic deformation was thickened with the increase of RASP processing time, from 5min,30μm rised to 40min,1250μm, and Hawking,25um, Conventional Blasting,12μm, respectively. With the increase of depth from surface, the degree of deformation decreased gradually. The generation of high density twins proved that the grain refinement mechanism of 316L stainless steel was dominated by the dislocation motion and mechanical twinning.(4) The surface hardness increased up to 424.4HV,337.8HV and 273.3HV of treated 316L stainless steel by RASP, Hawking and Conventional Blasting, which means of 2.6,2, 1.7 times compared to the as-received sample (165.6HV), respectively. The hardness decreased with the increase of depth from surface.(5) The yield strength and tensile strength of 316L stainless steel treated by RASP, Hawking and Conventional Blasting reached up to 510.3MPa and 732.7MPa; 499.5MPa and 774.2MPa; 314.7MPa and 665.1MPa, which was about 1.25 and 1.75 times; 1.72 and 1.32 times; 1.08 and 1.13 times that of the the original sample (290.9MPa and 587.6MPa), respectively. Work hardening rate decreased with the increase of RASP processing time.(6) The thickness of diffusion layer of 316L stainless steel treated by RASP was about 11.4μm after boriding at 800℃ for 4h, which was nearly 2 times of the original sample without RASP treatment (5.8μm). The treatment time by RASP and thickness of sample have little effect on the thickness of the coating layer. No much difference in thickness of the boriding layers for all samples after boriding at 850℃ for 4.5h. The RASP process significantly improved the effect of low temperature boronizing process.