| Attributed to its high abrasion resistance, red hardness, good toughness, thermal fatigue resistance, harden ability, melting resistance and other properties, hot-work die steel is widely used in metal molding, hot extrusion and hot forging and other industries. Because of its good combination of strength and toughness,3Cr2W8V steel is the most widely used hot-work steel both at domestic and abroad. The chemical composition (wt.%) of 3Cr2W8V as follows:C0.30～0.40, Si≤0.40, Mn≤0.40, Cr2.20～2.70, W7.50～9.00, V0.20～0.50, p≤0.030, S≤0.030, and balanced Fe.In the article, the rotation accelerated shot peening equipment (RASP) was used to execute surface nanocrystallization processing for 3Cr2W8V hot-work die steel. Optical microscope (OM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize the microstructures of the sample; Nano-indentation apparatus was used to measure micro-hardness from the treated surface to the interior, with a variation of distance; Friction and wear behaviors of the sample were also investigated before and after the RASP treatment, combining scanning electron microscopy (SEM) observation. Main conclusions are as follows:(1) RASP treatment 45min introduce severe plastic deformation onto the sample surface, and random distributed nano-grains are formed on the surface of in the 3Cr2W8V steel and the mean grain size is about 30nm in the top surface. Nano-gradient structures were formed with the depth changed from the surface to the interior, as strain and strain rate changed. TEM microstructure observation showed that the grain refining dominated by dislocation behavior. Dense dislocation walls and dislocation tangles were first formed in the area closed to the interior, and then dislocation walls were the principle structure close to the surface. Parallel sub-grain appeared in order to decrease the deformation energy, and nano-grains were formed in the top surface.(2) Compared with the original matrix micro-hardness (about 7.3Gpa), RASP treatment significantly increase the surface micro-hardness to about 9.4Gpa for the RASP 45min sample, and the micro-hardness slowly decreases with the increase of distance from the surface, until reach the same as the substrate.(3) Friction and wear experiments were also performed for the RASP 45min sample in different conditions:one is different load and the other is same load with different depth from RASP-treated surface. It is found that surface nanocrystallization increased the friction and wear properties of the die steel under low and medium load, and significantly reduces the friction coefficient of the material. Under the condition of dry friction, the surface average friction coefficient of RASP processed die steel was 0.625, less than that of the matrix 0.888, while the wear volume and wear rate decreased by 77.6% and 53.6% respectively.(4) Research shows that the RASP treatment time has a great influence on the resultant microstructures and micro-hardness of the die steel. The growth of treatment time can contribute to grain refinement, but also can increase surface temperature, which will lead to recovery and recrystallization, therefore, nano-grain will grow. Experiment proved that 45-minutes treatment can obtain the finest grains in the sample surface, that is, the best treatment result. |
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