| The fatigue failure of steam turbine blades is serious due to its working environment such as high temperature, high pressure and alternating load in the long-term time. Meanwhile, the inner wall of boiler, which is connected with turbine, is subjected to the high-temperature oxidation, and the formed shedding oxide (iron oxide) results in solid particle erosion to the nozzles and blades of steam turbine. The co-effectiveness of the two factors shortens the service life of turbine and raises the maintenance costs of turbine, affecting the reliability and economy of the operational generator units. Thus, the studies of proof coating on turbine, its fatigue failure and erosion wear have important theoretical significance and practical application value.The Cr3C2-NiCr cermet coatings were first fabricated by detonation spray (DS) and high velocity oxyen-fuel (HVOF) spray technologies, respectively. And then, the microstructure and micro-defects of the two as-sprayed coatings were observed and analyzed by means of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) as well as X-ray diffraction, optimizing the fabrication method of Cr3C2-NiCr coating. The routine mechanical properties of as-sprayed coatings, including the bonding strength of coating-substrate, the distribution of residual stress, microhardness and indentation fracture toughness, were respectively measured using static tensile test, X-ray stress apparatus and Vickers microhardness tester. Finally, the application performances of Cr3C2-NiCr coating were evaluated by means of high-cycle fatigue testing and solid paticle erosion testing.The experimental results are as follows:compared to HVOF coating, the microstructure of DS coating was denser with high content and uniform distribution of original carbides; the bonding strength of inter-splats was higher; the degree of oxidation and decarbonization and the content of porosity were lower; the NiCr binder with nanocrystalline and amorphous phases were newly formed, and the nano-size Cr23C6 carbide were dispersively precipitated from the binder. The bonding strength of DS coating-substrate was higher than 63MPa. The compressive residual stress was found on the surface of DS coating (23.35MPa) and at the interface of DS coating-substrate (280MPa), respectively. The microhardness and indentation fracture toughness on the cross section of DS coating were significantly higher than thoes on the HVOF coating. The fatigue testing has shown that the conditional fatigue limit and limited fatigue life of DS coating were significantly lower than thoes of the substrate, and the conditional fatigue limit decreased by 33.2%. The analysis on fatigue fracture has shown that the fatigue cracks of the substrate nucleated on the surface, and that of the coated samples nucleated on the position of Al2O3 inclusions, which were remained at the interface of coating-substate during the process of coating preparation. The erosion testing has shown that when the impacting angle of erodents was 24°, the solid particle resistance of two kinds of coatings (DS coating and HVOF coating) was superior to that of the substrate, and the DS coating had the lowest mass wear rate (the ratio of mass wear rate of DS coating to that of substrate was 1:19.60), which is expected to be applied to the turbine components such as the first stage nozzles of high-pressure zone and the first stage rotor blades of medium-pressure zone. At the same time, it is worth noting for the application units, when the rotor blades withstand heavy alternating loads in operation, it is important to improve the cleanliness at the interface of coating-substrate during the process of coating preparation, which will reduce the nucleation and propagation of fatigue cracks. |
PDF Full Text Request