| The surface structure and properties of a material may influence the material service life directly, so it is very meaningful that using surface strengthening technologies to enhance the surface properties. Ultrasonic Impact Treatment (UIT) is a new surface deformation strengthening technology, and had been widely used on prolonging the fatigue durability of welded structures. The safety problem of nuclear power station is drawing wider attention with the high-speed development of nuclear power industry. Ultrasonic Impact Treatment was used to strengthen the surface properties of304austenite stainless steel which is used on nuclear main pump, in order to extend the service life of the material and ensure the safe and continuous running of nuclear power station.The original HJ-Ⅲ ultrasonic impact device was improved and the problem that the ultrasonic impact head was too sensitive to additional load was solved by using the guidepost and springs. After improved, the ultrasonic impact gun can cooperate with other equipments conveniently to realize mechanization automatic operation and gain continuous, uniform and efficient surface hardening treatment.AISI304austenite stainless steel samples were impact treated by the improved ultrasonic impact gun which was fixed on the lathe saddle that can move along the axis at different speeds. The three-dimensional profilometer, microhardness tester, optical microscope, scanning electron microscope (SCE), electron probe microanalysis (EPMA), X-ray diffractometer, friction wear tester, electrochemical workstation and other devices were used to observe and test the structure and properties of the samples before and after impact treated.The experiment results showed that:the surface roughness of UIT samples is better than the surface that obtained by wire electro discharge machine, when the impact duration was6min/cm2, the surface roughness is the best and Ra is0.582μm. The surface layer of AISI304austenite stainless steel samples were severely deformed after ultrasonic impact treated, and nanostructured surface layers were fabricated accompanied with the formation of deformation induced martensite. With the prolonging of the impact durations the average crystallite dimension was getting smaller, and the volume fraction of deformation induced martensite was becoming higher. For24min/cm2UIT sample, the grain size was the smallest17.3nm and the quantitative of martensite was the maximum value100%. Compared to the sample without UIT, the stainless steel surface hardness markedly improved. The surface michardness of the UIT samples is about450HV which is1.7times of the original one, which is about265HV. Hardness decreased increasing with depth until the matrix level. The influence of impact duration to the depth of hardened layer was the same as the influence to the depth of plastic deformation layer, the deformed and hardened layer increased with the impact duration, but the growth rate speed to drop. Beneficial compressive residual stresses were introduced in the surface layer by ultrasonic impact treatment, the maximum value was about-850MPa, and the depth of residual compressive is more than1000μm. The wear resistance of AISI304austenite stainless steel was improved in certain extent after UIT, friction coefficient became small, and wear scar became shallow than the untreated one. The electrochemical corrosion performance in boric acid had no evident change before and after impact treated. |
PDF Full Text Request