| UIT is relatively new in the field of surface deformation strengthening technology. Now, UIT has maturely been applied to welding joint treatment for improving the fatigue resistance of weld metal. However, there is not so many researches about the recovery behavior of components and parts after UIT. In order to determine the stability and the reliability of the strengthening effect of UIT after long-term service, the paper made a research on the recovery behavior of AISI304 stainless steel samples after UIT under the conditions of various heat treatments, by simulating the working environment.At first, the study improved and optimized the structural parameters of UIT devices. The lab re-optimized the size of head of 2mm impact pin based on finite element simulation, considering its plastic deformation affected the uniformity of strengthening in UIT process. Besides, the lab studied the factors of influencing the amplitude of impact pin. which included the mass of impact, the friction between guide sleeve and impact and the elasticity of springs. The result revealed how the amplitude of impact pin changed with the changing of the factors.The next research studied macroscopic feature, microstructure, martensite content and residual stress of UIT samples after various heat treatment conditions that included 250℃ x20h 350℃ x20h 350℃ x30h, by using the devices such as surface profiler, micro-hardness tester, microscope and x-ray diffraction, etc.The results showed that the surface roughness of AISI304 stainless steel before and after UIT were the same degree. Plastic deformation, nano-crystallization and strain-induced martensite transformation were caused by the high frequency impact of impact pins in the material surface. The microstructure of the outermost layer of materials was still unrecognizable after heat treatment, and the quantity of deformation twins in the subsurface layer decreased significantly. The degree of recovery was more obvious with higher temperature and longer time. The martensite content was over 90%after UIT, and decreased to 80%after heat treatment. No significant change of martensite content was found between different heat treatment conditions. The martensite content of the samples after all heat treatment conditions tended to be almost the same at the depth of 80μm.The maximum value of the micro-hardness of samples after heat treatment was 430HV0.1N at the depth of 70μm. The micro-hardness value of the outermost layer dropped from 450HV0.1Nto 340HV0.1N after heat treatment, which indicated the heat treatment softened the strengthened layer which was less 80μm far from surface, but it had no significant influence in the deeper area. After UIT, the residual stress was about 790MPa at the surface, and the maximum value was about 900MPa. According to the contrast experiment, the stress relaxation was more serious when the temperature and time of heat treatment increased.According to the result, the properties of the strengthening effect still reserved after heat treatment. Thus, the strengthening effect of UIT was worthy of trust, because the recovery and stress relaxation of the metals after UIT was a slow process under long-term service. |
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