The Study Of Nanocrystallization Mechanism And Microstructure And Performance On The Surface Of 304 Austenitic Stainless Steel Under The Ultrasonic Impact Treatment And Recrystallization Treatment

As a new method of surface treatment technology, ultrasonic impact treatment is similar to the shot-peening treatment. In the process of ultrasonic impact treatment on the surface, the impact of high frequency vibration of the needle is putted on the material surface. And the severe plastic deformation happened on the surface of the specimen, on the other hand, there are a lot of dislocations and deformation twins and other products of deformation appearing on the surface.While, the interaction between dislocation and dislocation?twin results and twin results in dislocation wall and dislocation cell appearing on the surface of sample, therefore, strain hardening happened on the surface.To inquiry the mechanism of nanocrystallization of surface, we use the technology of UIT which applys on the surface of the 304 Austenitic Stainless Steel. According to SEM(scanning electron microscopy).TEM(transmission electron microscopy)?Microhardness?X-Ray diffraction equipment and other laboratory apparatuses, we make researches on self-nanocrystallization mechanism of 304 stainless steel under different passes. In addition, after ultrasonic impact treatment , experiment of annealing. The result is that the hardness and properties such as wear resistance get increased by producing a nano-crystal layer on the surface of 304 stainless steel. In addition, it can also strengthen the surface of 304 stainless steel by introducing compressive residual stress on the surface to improve the fatigue crack growth properties. Meanwhile, the martensite phase transition induced by deformation and the interaction between twin are other two methods to improve the properties of 304 stainless steel. In addition, on the basis of UIT, make the experiment of annealing. The results showing that the recrystallization refine of 304 stainless steel occurred after 400 ? annealing,at the same time, with the annealing temperature increasing, the grain size on the surface of sample gradually increase. And the deformation twin on the surface decreased, the effect of the interaction between twins grasually decreasedAccording to the experiment, the nano-crystal organization can be produced while treating the 304 Austenitic stainless steel by ultrasonic impact treatment. Meanwhile, it also produces the residual compressive stress.The main method of plastic deformation of surface grain is twin. In the meantime, large amount of dislocation and deformation twin crystal appear on the surface of material,at the same time, the martensitic phase transformation happened due to deforming. With the increase of deformation, the volume of Martensite of surface is gradually increasing, the grain size becomes smaller, and the depth of nano-crystal layer?hardness increases with the increase of UIT passes.Besides, the ultrasonic impact treatment introduces compressive residual stress in the surface, and improves the fatigue life of the material. Comparing to the sample that has been treated by UIT, the hardness of surface of the specimen after annealing decreases a little. Recrystallization annealing occurred under the condition of 400?, which, the grain size has got decreased. After this time, with the increase of temperature, the grain size begins to grow up, and the residual stress gradually releases; the volume of austenite of the surface is increasing, and the transmation of martensite to austenite happened.