| Large centrifugal compressors are widely used in machinery,petroleum,chemical industry,refrigeration and many other important fields.The impeller is the core component of the compressor,and its reliability directly affects the normal operation of the compressor impeller.The impeller is subjected to complex load during service,so it is ease to cause impeller blade fatigue fracture,vibration cracking and other failure.Furthermore,make the whole compressor failure.Therefore,how to effectively improve the performance of impeller or remanufacturing impeller becomes the core issue which aimed at improving the overall performance of the compressor.In this paper,the laser shock processing technology is used to strengthen the surface of the KMN-steel which commonly used in the impeller.The fatigue limit of KMN-?steel before and after laser shock is fast predicted by Luong method,and the fatigue life under two load conditions is also examined.Besides,the strengthening mechanism of laser shock processing and fatigue failure mechanism under two kinds of loading conditions are explored.Finally,the results showed that:(1)The surface microhardness of the specimens strengthened by laser shock enhanced from 340 HV to 390 HV,and the residual compressive stress was more than 600 MPa,the compressive stress layer reached 2mm,and the compressive stress decreased gradually from the surface to centripetal part.The microstructure showed that KMN-I steel formed a surface nanocrystalline after laser shock processing.(2)The fatigue strength of KMN-I steel was improved by laser shock processing.The fatigue limit of KMN-I steel was predicted by Luong method increased about 20 MPa after laser shock processing which under stress ratio R=-1;and under the average stress of 700 MPa,the fatigue strength increased from 960 MPa to about 1000 MPa.Secondly,the fatigue limit obtained by strain and temperature rise rate is close to the fatigue limit obtained by infrared thermography,which indicates that the three signal parameters can be used to predict the fatigue limit of metal.(3)In the process of metal fatigue,the variation of surface temperature and temperature rise rate is similar to that of strain change,which can be used to characterize the fatigue process of metals instead of the traditional stress-strain model.Modify the Morrow formula and Coffin-Manson formula respectively which contain the average stress,the stress-temperature curve and the stress-temperature rate curve,the temperature-cycle life curve and the temperature rise rate-cycle life curve were obtained based on temperature and temperature rise rate.The traditional stress-strain curves and strain-cycle life curves are extended.(4)The location of crack initiation was changed after laser shocked.There are two positions can be initiation cracks,one is the edge,the other is near the center of the side surface.There is a small crack propagation platform in the fatigue zone,and the fracture zone is larger,which is a typical equiaxed dimple shape,and the fatigue failure mode is ductile fracture.Secondly,the fracture occurred in the junction of the no impact zone.The stress concentration moves to both sides of the central part,and the fatigue life is prolonged. |
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