Cavitation-Erosion Behavior Of Laser Cladded Low-Carbon Cobalt-Based Alloys On 17-4PH Stainless Steel

Cavitation is a microscopic,transient and complex failure mode.It is one of the main causes of damage to the intake side of the final stage blades in the low-pressure cylinders of steam turbines.Therefore,it is of great significance to study the mechanism of cavitation-erosion and explore cavitation-resistant materials for the steam turbine industry.Based on the actual operating conditions of steam turbines,this research analyzes the cavitation phenomenon inside the droplet when it hits the blade.Then comparing the water hammer pressure generated by the droplets and the impact load generated by the microjet reveals the main failure mechanism of last stage turbine blades.In this research,the cavitation-erosion resistance of Stellite 21,named C12 and Stellite 728,named C14,in this research,which all contains low carbon,prepared via laser cladding on 17-4PH stainless steel substrate,is studied in 3.5 wt.%Na Cl solution.Using the electrochemical corrosion test,the mechanism of synergistic interaction between cavitation and electrochemical corrosion is investigated.For comparison,17-4PH stainless steel substrate,wrought Stellite 6B and laser cladded Stellite 6 are studied under the same condition.The surface morphology and microstructure change of the specimens after the cavitation-erosion test are analyzed with opticl metallographic microscope?OM?,scanning electron microscope?SEM?,energy dispersive spectrometer?EDS?,X-ray diffraction?XRD?and 3D surface profiler.From the accumulated weight loss,weight loss rate,eroded surface morphology,incubation period,and life prediction criteria at different stages of cavitation,the cavitation-erosion mechanism of the laser cladded C12 and C12 alloys and the cavitation failure modes of the tested materials are analyzed.The cavitation strengthening mechanism of the materials is further explored by studying the effects of surface roughness,hardness,microstructure and solution constituents on cavitation-erosion.The research tasks can be detailed as follows:?1?From literature review,it is known that cavitation-erosion occurs when liquid droplets hit a solid surface.The water hammer pressure can be calculated and the impact pressure of micro-jet is analyzed when the bubbles collapse.It is shown that the impact pressure of the droplets hitting the blades is far less than the yield strength of the turbine blade material,but the micro-jet pressure in the cavitation-erosion process is far greater than the material yield strength.Therefore,the demage of the last stage blades in the low pressure cylinder of steam turbines is mainly caused by cavitation-erosion.?2?Low-carbon cobalt-based alloys are characterized by low carbon content and also contain a certain amount of Mo element.The strengthening agents of these alloys include Co₇Mo₆ and Co₃Mo intermetallics and minor carbides.Compared with C12,the Mo content of C14 is doubled,the microstructure of laser cladded C14 is refined with the increase of Mo content.C14 also contains a small amount of Nb element,which enhances the hardness of the alloy by forming Nb C strengthening phase.?3?From the analyses of the accumulated weight loss,weight loss rate,surface morphology,incubation period and life prediction for 17-4PH substrate,wrought Stellite6B,laser cladded Stellite 6,laser cladded C12 and C14 in different stages of cavitation-erosion,it is found that laser cladded C12 and C14 alloys have the best cavitation-erosion resistance.With the increase of Mo content and the addition of Nb element,the cumulative weight loss rate of the C14 specimen is only 8.23%of the17-4PH substrate,and the weight loss rate is about 1.012 mg/h,with the longest incubation period?35 min?under the same test condition.The surface morphologies of the C12 and C14 specimens different stages of cavitation-erosion are relatively smooth,and the roughness values keep a low growth rate.?4?The effects of surface roughness,hardness,microstructure and solution constituents on the cavitation-erosion performance of the specimens are investigated.The results show that high negative Rsk?Skewness?value is beneficial for forming water film to buffer the cavitation-erosion damage;compared with deionized water.The corrosion in 3.5 wt.%Na Cl solution cooperates with the cavitation micro jet,which significantly shortens the incubation period of the specimens in the cavitation-erosion process and accelerates the surface damage.In addition,the cavitation-erosion process can cause work hardening of the specimen surface,thus improve the cavitation-erosion resistance.Moreover,the phase transformation of fcc to hcp in the cobalt-based alloys also favors cavitation-erosion resistance.?5?The study of the failure mechanism of the tested materials due to cavitation-erosion hasfound that cavitation-erosion occurs preferentially at the grain boundaries,and the cavitation-erosion resistance of the grains depends on its orientation,which reflects the anisotropy of mechanical properties of polycrystals.The microstructural analyses reveal that the grain size of laser cladded C14 alloy is obviously reduced due to higher Mo element,and the cavitation-erosion resistance is then further improved.