Study On Very High Cycle Fatigue Behavior Of FV520B-I Steel Used For Centrifugal Compressor Impellers

Centrifugal compressors are widely used in engineering fields due to their high efficiency,large flow rate and high compression ratio.In practical engineering,low stress amplitude,vibration and high-speed centrifugal load make fatigue failure become the main failure mode of centrifugal compressor impeller.More and more fatigue tests show that with the decrease of load and the prolonging of life,the crack origins of the impeller migrate from the surface to the internal defect,where the very high cycle fatigue(VHCF)failure behavior happen.Martensitic stainless steel FV520B-I is commonly used in the process of centrifugal compressor impellers'manufacture.Studying on its failure behavior in VHCF regime(>10~7cycles)is significant for the design,manufacture,reliability analysis,life and strength of prediction of compressor,and it also has some reference meaning for defect controlling in steel smelting.This paper calculated the assembly stress of the centrifugal compressor impeller,the stress under the conditions of airload,stalling and surge by simulation software.The results show the stress at the joint between the impeller root,the cover plate and the chassis under normal workload condition is highest,where cracks initiate and fatigue behavior happens.In blade,tensile(I)type cracks are easier to initiate and then that lead to fatigue failure through the calculation of stress of blades including cracks and stress intensity factor in the crack tip.Then the sizes of samples in ultrasonic fatigue test were designed and calculated based on the ultrasonic fatigue vibration theory.The VHCF test of FV520B-I steel was carried out on the ultrasonic fatigue testing machine.The influences of inclusions,GBF and other characteristic regions around the crack origin on the VHCF life and strength,were analyzed based on the ultrasonic fatigue test data.The life probability accumulation model was established considering two failure modes of FV520B-I steel and the P-S-N curves of FV520B-I were obtained by fitting the experimental data.Based on the P-S-N curves of two failure mechanisms,the fatigue strength of 10~7 and 10~9 cycles was predicted and the results were consistent with the experimental data.Third,the distribution of the inclusion size and the maximum inclusion size in the target volume were predicted by the SEV method and GPD method.And then the maximum inclusion size was used in the strength prediction.The maximum likelihood function logarithm method was used to calculate the 95%confidence interval of the maximum inclusion size,in the meanwhile,the 95%confidence interval of fatigue strength could be obtained by strength equation proposed by Murakami,but the results show that the model was conservative.Finally,based on the work of Yang Z G and Liu Y B,the stress intensity factor of internal cracks was corrected by considering the influence of hydrogen element on very high cycle fatigue strength.The stress intensity factor is introduced into the Paris equation to describe the internal crack growth process of FV520B-I steel.Fitting the parameters of Paris equation with experimental data,the VHCF life equation of FV520B-I steel could be obtained by integration.Based on the inclusion size distribution function and the formula of VHCF life of FV520B-I steel,the probability density function of stress and life can be obtained and the reliability of FV520B-I steel could be calculated under given inclusion size distribution.