| Various surface treatment techniques including electroplating, spraying, vapor deposition, chemical treatment have been developed to improve materials properties (strength, hardness, wear resistance, corrosion resistance) to meet engineering requirement. With the development of nano technology, if conventional surface treatment technique and nanometer technology are combined, this will provide a new development direction for realizing nanocrytallization of engineering materials. Various metal materials surface can be obtained a nanostructured layer by surface nanocrystallization treatment. Due to absence of a shape interface between nanostructured layer and substrate, it can not peel off with the change of environmental conditions during materials application. Therefore, some surface treatment techniques with high cost can be replaced with surface nanocrystallization.So far, various advanced techniques, such as surface mechanical attrition treatment, ultrasonic shot peening, ultrasonic cold forging technology, etc. have been developed to fabricate a nanostructured layer on the surface of engineering materials. These processes, however, suffer from either low processing efficiencies or complex process, or difficult treating workpiece with large dimension, all of which have hidered the widespread application of surface nanocrystallization technologies. Hence, it is necessary to develop a new surface nanocrystallization technology. In this work, we develop a novel and efficient surface nanocrystallization technique, fast multiple rotation rolling (FMRR), to achieve a nanostructured surface layer in low carbon steel and316L stainless steel surface, and effect of nanostructured layer on properties, thermal stability of nanocry stall ine and surface nanocrystallization mechanism were investigated in detail. The main contents are summarized as follow:(1) A novel and efficient surface nanocrystallization technique was developed with the aim to achieve a nanostructured surface layer in metal surface.(2) Low carbon steel and316L stainless steel were treated at different duration by the FMRR treatment. The micrstructure of treated samples was characterized. Grain size of sample of low carbon steel was refined to nanometer after treatment for30min, and the average grain size is about20nm. When the treatment duration increase to60min, the grain size further decrease to9nm. The316L stainless steel after treatment30min and60min, the results indicated that FMRR treatment can form defomation-induced α’-martensite on surface layer, and the volume fraction of α’-martensite is10%and20%, respectively. At the same time, the grain size is refined to about17nm and12nm, respectively.(3) Effect of nanostructured layer of low carbon steel on microhardess, strength and friction and wear properties was investigated. Experimental results indicated that the mierohardness of the treated sample is nearly twice harder than that of original sample. The yield strength (0.2%offset) is increased by24%, and the ultimate strength is increased by18.6%. At the same time, the plasticity is slightly reduced from26%to22%. In addition, the friction coefficient of treated sample decreases from0.52to0.30. The wear volume loss of the treated sample is lower than that of the original sample, indicating the wear resistance of the low carbon steel has been improved by fast multiple rotation rolling.(4) Effect of nanostructured layer of316L stainless steel on its properties was investigated. Experimental results indicated that the mierohardness of FMRR sample increases from190to530HV. The ultimate tensile strength of FMRR sample reaches a higher value of about608MPa, which is increased by22%compared with that of the original sample. The yield strength (0.2%offset) is increased by51%. At the same time, elongation for the sample (one side is NG structure and the other side is CG structure) is about38%. The corrosion tests of nanostructured layer indicated that the FMRR treatment reduced the corrosion resistance. Possible reasons leading to the decrease in corrosion resistance were the increase of interface, surface roughness, the volume fraction of a’-martensite, dislocation density.(5) The thermal stability of nanocrystalline layer for low carbon and316L stainless steel was investigated in details at different temperature. Experimental results indicated that nanocrystalline of low carbon steel has high thermal stability at400℃. In spite of the grain size slightly increases, the majority of grains are still nanocrystalline, ranging from30-60nm. With increasing of annealing temperature, the increase of grain size and the decrease of mean microstain occur for316L stainless steel. After annealing at500℃, the grains are still nanocrystalline. In addition, martensite transformation produces during annealing. The volume fraction of a’-martensite reaches a maximum value of92%for the annealed sample at500℃, which can be attributed to the stress relaxation.(6) For the low carbon steel, it is a body-centered cubic (bcc) metal material with high stacking fault energy. Its grain refinement mechanism may contain development of dese dislocation walls and dislocation tangles, transformation of dense dislocation walls and dislocation tangles into subboundaries with small misorientations, and evolution of subboundaries to highly-misoriented grain boundaries. Finally, grain size is refined to nano-scale. For316L stainless steel, it is a face-centered cubic (fcc) metal material with low stacking fault energy. Its grain refinement mechanism may dislocation produces on different slip plane and form twin crystal. These twin crystal tangles each other. Finally, grain size is refined to nano-scale.In this work, a novel surface nanocrystallization technique was developed, which has high efficiency and can treat a workpiece with large dimension. The nanostructured layer achieved by FMRR treatment can improve strength of materials. It provides a new approach to enhance service lifetime of conventional materials. In addition, the investigation of thermal stability of the nanocrystalline layer provides a reasonable temperature conditions for its application. Hence, this study has important significance in theory and practiceThis work was supported by the Natural Science Foundation of the People’s Republic of China (No.8171463,30800221and30870610), China Postdoctoral Science Foundation (No.200804401138), Shandong Province Post-doctoral Innovation Foundation (No.200902030) and Graduate Independent Innovation Foundation of Shandong University (no. yzc11053). |
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