| The surface state of the material is usually the key factors which affect the service life, and poor performance or the presence of defects of the material surface will cause many mechanical components of early retirement, so material surface strengthening technology is more and more attention. Nuclear power equipment all the year round in high pressure, high temperature, radiation and other harsh conditions without intermission operation, to ensure that nuclear power equipment safety of causative long life, there are more stringent requirements on the material surface wear-resisting, corrosion resistance, anti fatigue performance. So in the field of nuclear power, the surface strengthening technology is referred to an extremely important position. Surfacing is the most commonly used surface strengthening method in the manufacturing process of nuclear power equipment. However, the surfacing layer is welded structure, its micro structure and properties are extremely uneven, especially it can not be avoid the residual tensile stress greatly in the surfacing layer that affect the quality of the welding parts and the service life of nuclear power equipment. How to apply effective means to improve the surfacing layer surface structure, improve the surface properties of the material, prolong the nuclear power equipment safety causative life is always an important subject in the field of nuclear technology research.Ultrasonic impact, as a new type of surface plastic deformation strengthening technology, has become widely used to improve the weld microstructure, eliminating welding stress and effective means. But the application of ultrasonic impact technology to the microstructure and properties of the surfacing layer has almost no precedent. In this study,304 surfacing layer that is the most commonly used in nuclear power equipment is research object, ultrasonic impact treatment techniques is research means, we can learn the effects of ultrasonic impact treatment on the surface state, microstructure, effects of microsegregation of surfacing layer, and the influences of ultrasonic impact treatment on micro hardness, residual should force, wear resistance corrosion resistance performance of surfacing layerThe experimental results show that before UIT, the surfacing layer has been milled, roughness Ra of surfacing layer reaches 0.4, after UIT, surfacing layer surface roughness Ra values did not significantly change, UIT can make the surface machining fine car to the grinding levels. And after UIT, surfacing layer of grain refinement, microstructure is more uniform and compact. In the process of ultrasonic impact with deformation induced martensite formation, the martensite content is the highest reached 99.1 percent; and after UIT, surfacing layer elements distribution becomes more uniform, caused by surfacing segregation phenomenon has been improved.After UIT, hardness of 304 stainless steel surfacing layer obviously increases, the maximum of hardness is 370HVN, compare with the original state of surfacing layer, the hardness value of the highest increase of 1.4 times, with the depth increasing.the hardness of surfacing layer gradually decreases; when the depth is more than 800 m, the hardness of surfacing layer values of two state tend to be consistent; after ultrasonic impact treatment, the original tensile residual stress in surfacing layer is eliminated, and the beneficial compressive residual stress has been introduced and the maximum compressive residual stress is-753MPa. welding residual stress increase with depth layer gradually decreases.in the 800 mum or less, there have compressive residual stress. After ultrasonic impact treatment, the wear resistance of surfacing layer has been improved, friction coefficient has changed from the original sample of 0.7 dropped to 0.6, wear scar depth also has been changed from about 6 μm reduced to about 5 μm, the corrosion resistance of surfacing layer in boric acid aqueous solution is not obvious after UIT. |
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