Research On Laser Repairing And Strengthening Technology Based On Steam Turbine Last Stage Blades (17-4PH) Materials

Laser cladding repair technology is one of the most advantaged technologies in the repair and remanufacturing field,which using high energy density laser beam to make the cladding and substrate form metallurgical bonding in order to repair the original dimension and to improve and strengthen microstructure and properties.In this paper,the 17-4PH martensitic precipitation hardening stainless steel material was chose as the research object against the background of the injury protection in steam turbine last stage blades,and the research of laser homoplasm repair technology has been developed.The parameters with optimal repair quality were obtained by theoretical analysis and experimental verification.The repairing quality of laser cladding was evaluated by testing performance of the optimal blade as-repaired layer compared with the base material.The microstructure and properties of17-4PH repairing layer has been improved and enhanced by using solid solution aging treatment and WC particles laser alloying.The mechanism on microstructure and properties of17-4PH layer in different solid solution aging temperature and WC content has been explored.The results show that the optimized repairing process parameter of laser cladding17-4PH are:laser power of 1500W,scanning speed of 5mm/s,powder speed of 14g/min,overlapping rate of 33%,and stratified thickness of 1mm.The damaged blade has been macro-defect-free repaired by layering and path planning with computer aided manufacturing software LMDCAM2.Thereinto,the Unidirectional cladding mode?"Z"glyph cladding?and interlayer reverse scanning have been adapt.The as-repaired layer is mainly composed of a large number of lath martensite and a small number of retained austenite.Because of the characteristics of rapid heating and cooling in laser cladding process,the microstructure of as-repaired layer is finer,and the average microhardness(342.1HV_0.5)is higher than that of the base material(322.6 HV_0.5).The average tensile strength of the repairing layer is 963.35 MPa,slightly higher than that of the base material,but the plastic of the as-repaired layer is lower than that of the base material.Compared with the base material,the wear resistance of the repairing layer has increased 25%.The corrosion resistance and liquid impact erosion resistance of the repairing layer are better than that of the base material.It indicates that the as-repaired layer can basically meet the requirements of the surface service performance of the blade.The solution and aging treatment was conducted to the repairing layer at three aging temperatures.The results show that the microstructure of the as-repaired layer after solution treatment at 1040?by 1h and aging treatment at 480?by 4h is mainly made of white quenched martensite and coarse-needle-like tempered martensite.The microstructure of the repairing layer after solution treatment at 1040?by 1h and aging treatment at 550?/660?by 4h are mainly made of sorbite structure which keeps the phase of original martensite.The repairing layer after aging treatment at 480?by 4h is the hardest(average microhardness reached 430.4 HV_0.5)and has the best wear resistant?1.56 times of the untreated layer?.The as-repaired layer after aging treatment at 550?by 4h has the best corrosion resistance,and the passivation region in electrochemical polarization curve is the most obvious.The repairing layer after aging treatment at 480?by 4h has the best liquid impact erosion resistance.3D surface measurement and software calculation shows that the liquid impact erosion resistance of repairing layer after aging treatment at 480?by 4h can be increased by 60%.After adding 30wt%WC,the layer is composed of a large number of?-Fe phases and a little?-Fe,M₇C₃ phases?M=Cr,Fe et al?,W₂C phases,WC phases and Fe₃W₃C phases.The average microhardness of the layer after adding 30wt%WC is 519.5 HV_0.5,about 1.6 times of the base material(322.6 HV_0.5).The corrosion resistance of laser cladding 17-4PH layer has improved with the increase of WC content.The results of 3D surface measurement show that the liquid impact erosion resistance of the layer with 30wt%WC is 2.1 times higher than that of the original layer.The lamellar structure of WC particles after water erosion appeared spalling in some degree and formed a pit under the large stress from water droplet impact.The dispersion distribution of WC particles can inhibit the plastic deformation on the material surface caused by tangential flow and reduce the damage area of the outside water erosion area.