Effects Of Annealing On The Microstructure And Properties Of 316L Stainless Steel With Nanostructured Surface Layer

Surface nanocrystallization (SNC) technique can be used to synthesizenanostructured surface layer on the engineering metallic materials throughsevere plastic deformation, therefore the surface (and globe) properties andbehaviors can be enhanced significantly for the materials. However, due to theexistence of large amount of non-equilibrium grain boundaries in thenanocrystalline materials, grain growth usually occurs with the increment ofthe environmental temperature, that may induce sharp reduction of theproperties of the nano-crystalline materials, so the heat stability of thenanocrystalline materials is one of the key-factors for the development of theSNC technique. Therefore, a 316L stainless steel was selected to be treated bymeans of the surface mechanical attrition treatment (SMAT) technique in thiswork, and a nanostructured surface layer was obtained. Then, vacuumannealing was carried for the SMAT sample at different temperatures fordifferent durations. The structures and the properties were analyzed for theSMAT sample before and after the annealing by means of the X-ray diffraction(XRD),transmission electron microscope (TEM), hardness and residual stresstesting. The conclusions can be summarized as follows: After the SMAT, agradient structure can be obtained for the AISI 316L stainless steel, of whichthe grain size increases gradually from nano-scale in the top surface tomicro-scale in the matrix. While the annealing temperature is lower than 773K, no obvious changes can be observed for the gradient structure and the grainsize. With the further increment of the annealing temperature to 973 K,recrystallization occurs in the gradient structure, and grain growth can befound in the nanostructured surface layer. However, the grain growth rate isvery low, and the grain size in the nanostructured surface layer is still in the — II —退火对 316L 不锈钢表面纳米化结构和性能的影响nano-scale. Residual stress is introduced to the SMAT sample, which will bereleased during the annealing process, and that may induce martensitetransformation in the nanostrucured surface layer and the adjacentsubmicro-grained layer. The surface hardness of the 316L stainless steel isenhanced significantly after the SMAT (from 2.6 Gpa to 4.3 Gpa), and thehardness decreases gradually along the depth. While the annealingtemperature is lower than 773 K, the hardness distribution along the depthalmost remains unchanged. With the further increment of the annealingtemperature to 973 K, the release of residual stress and grain growth willinduce sharp drop for the hardness along the depth of the sample. The effectsof the annealing duration on the structure and properties of the SMAT sampleis less important compared with the annealing temperature.