| Centrifugal compressors are the main power equipment used for gas transportation,which are widely used in the West-East gas pipeline,coal chemical industries,petrochemical industries and other important areas.With the continuous improvement of large centrifugal compressor rotating speed,the life of the impeller rotor served for several years or even more than a decade has been far more than 107 cycles and entered the regime of very high cycle fatigue.Therefore,to study the very high cycle fatigue behavior and mechanism of centrifugal compressor impeller material KMN and establish the residual life estimation method of centrifugal compressor impeller are not only the important basis to improve the long life fatigue design of the centrifugal compressor and ensure the safe service of the centrifugal compressor,but also of great theoretical significance for the remanufacturing critical threshold determination of the centrifugal compressor impeller.The author researched the influence of heat treatment,surface defects,specimen size and weld seam on the very high cycle fatigue behavior and mechanism of impeller material KMN,and established the static fatigue test system to simulate the actual conditions of impeller.Then the fatigue damage of centrifugal compressor impeller material KMN was detected using nonlinear ultrasonic testing,and the residual life estimation method of centrifugal compressor impeller was established.The very high cycle fatigue behavior and mechanism of impeller material KMN with different heat treatment,specimen size and welding condition were researched,and the very high cycle fatigue S-N curves were obtained,which were urgently needed by enterprises.The experimental results showed that the failure mechanism of KMN-I and KMN-S with different heat treatment was different in the very high cycle regime.For KMN-I,there was a slow decline after 107 cycles and fatigue fracture still occurred after 107 cycles.The fatigue cracks mainly initiated from the surface,and some internal fatigue cracks initiated from the inclusion MnS.While the traditional fatigue limit was observed in KMN-S and all the fatigue cracks initiated from the surface.For KMN-I large size specimens,the fatigue failure mechanism was similar to that of normal specimens in very high cycle regime.Fatigue fracture still occurred after 107 cycles and there was no fatigue limit.The surface fractures were mostly observed before 107 cycles 7 and a small amount of cracks initiated from internal non-metallic inclusions after 107 cycles.The very high cycle fatigue characteristic of KMN-I welding specimens was less obvious.The fatigue cracks initiated from surface before 107 cycles and initiated from internal matrix after 107 cycles.The formation mechanism of GBF area was explained by the theory of "dispersive decohesion of spherical carbide" using electron probe analysis.The enrichment of carbides leads to the initiation of the fatigue crack at the weak matrix of the material.The Murakami fatigue strength model was modified using fracture mechanics,and the modified model prediction was closer to the experimental results.The parallel layer model was used to predict the very high cycle fatigue life of KMN-I.It is proved that the initiation life of the very high cycle fatigue crack accounts for the majority of total fatigue life.The maximum strain of the impeller when the impeller resonated due to wake excitation in the actual working condition was simulated.When the strain rate changed in quasi-static test range(from 0.0003 s-1 to 0.1 s-1),the elastic modulus of impeller material KMN-I did not change obviously,which was consistent with the constitutive relation measured under static load.The vibration fatigue S-N curve of impeller material KMN-I was obtained.Compared with the very high cycle fatigue S-N curve,the fatigue strength of vibration fatigue S-N curve was lower under the same fatigue cycle.Fracture analysis showed that the fatigue cracks mainly initiated from the dislocation slip of the specimen surface and the characteristic of multi-source fatigue was observed in the low cycle fatigue regime.The static fatigue test system based on the vibration testing was established by designing the relevant fixture and accessories,which can complete static fatigue tests under the load of tensile,bending,torsion and their combinations within the frequency range of 5-4500Hz.The nonlinear ultrasonic testing was carried out on three sets of specimens with different fatigue loads(vibration fatigue test,bending fatigue test and tensile-bending fatigue test),and the nonlinearity parameter-fatigue cycles curves of the impeller material KMN-I were obtained.The experimental results showed that the nonlinearity parameter of the material increased first and then decreased with the increase of the fatigue cycles.The relationship between the propagation of the micro-cracks and the nonlinearity parameter was researched by microscopic analysis on the dangerous locations of the specimens.With the increase of the fatigue cycles,the microstructure of the material gradually deteriorated and then the micro-cracks initiated from the matrix.The nonlinearity parameter-fatigue cycles curve of bending fatigue test and tensile-bending fatigue test also showed a similarity to the vibration fatigue test,which proved that the nonlinear ultrasonic testing can be used to characterize the initiation of the micro-cracks in the early stage of fatigue life and the nonlinearity parameter can be used to reflect the fatigue damage and fatigue life of the material.The influence of tensile load and bending load on the nonlinearity parameter of the impeller material KMN-I was researched.The increase of the test load promoted the micro-damage inside the material,which led to the change of the nonlinearity parameter.The microscopic mechanism of the nonlinearity parameter was analyzed using dislocation string model.Based on the very high cycle fatigue S-N curve and?-N curve,the ?-S-N three-dimensional space model of the impeller material KMN-I was established.In order to solve the "double value problem" of ?-N curve,a double criterion method of impeller material fatigue damage was proposed combining hardness test.The internal fatigue cracks of the specimens were statistically analyzed,and the distributions and exremes of the fatigue cracks were calculated using logarithmic normal distribution and Gumbel distribution,respectively.The empirical formula of the impeller material KMN-I nonlinearity parameter and crack propagation dimension was established,and a method for estimating residual life of the impeller based on nonlinear ultrasonic detection was proposed.In this paper,the very high cycle fatigue behavior and mechanism of the impeller material KMN-I was researched,and the influence of heat treatment,surface defects,specimen size and weld seam on the very high cycle fatigue behavior and mechanism of the impeller material KMN-I was revealed,which has important guiding significance for long life fatigue design and safe service of the centrifugal compressor impeller.The fatigue damage of centrifugal compressor impeller material KMN was detected using nonlinear ultrasonic testing,and the residual life estimation method of centrifugal compressor impeller was established,which can provide theoretical support for the in-service detection of the centrifugal compressor impeller. |
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