Research On The Relationship Between GH738 Tissue And Ultrasonic Signal Based On Quantitative Metallography

GH738 is a phase precipitation hardening superalloy with excellent mechanical properties.It has high strength and toughness under high temperature,and has both anti-oxidation and anti-corrosion properties.It is mainly used for aero engine rotating parts.GH738 plastic forming process is good,different mechanical properties of GH738 forgings can be obtained by adjusting the temperature,time,deformation rate and other process parameters.In the GH738 die forgings,the mixed crystals and non-uniformity of the structure may occur in the forgings due to the uneven temperature and deformation,it may result in the degradation of the performance of parts and components and the occurrence of safety accidents.Therefore,it is necessary to establish a detection method that can be used to judge the microstructure of GH738 forgings without damage rapidly.In this paper,the research object is a certain type of aero engine guide vane GH738 stepped disk(different forging process parameters),(1)comprehensive analysis of the influences of the parts of the structure and residual stress under different process parameters and the shape of the disk on the parts,combined with metallographic experimental and ultrasonic testing requirements on the size of the specimen,designing a stepped disc cutting scheme and cutting;(2)Preparation of metallographic specimens for observation of microstructures,quantitative characterization of GH738 microstructure parameters based on quantitative metallography;(3)Using ultrasonic longitudinal wave echo method for ultrasonic detection,extracting a full-data ultrasonic A-signal of multiple echoes,calculating the ultrasonic attenuation coefficient and speed of sound,and intercepting the backscatter between the primary bottom wave and the surface wave with the primary bottom wave,the frequency offset and nonlinear coefficient of the bottom wave can be obtained by the signal’s Fourier transformed and calculated based on the initial wave detection frequency due to the difference in the microstructure of the tested sample;(4)Analyzing the grain size based on the Pearson correlation coefficient,crystal grain shape change and ultrasonic detection signal eigenvalue relationship,establish GH738 grain size/grain shape ultrasound evaluation method.The results show that:(1)There is a strong correlation between the frequency deviation of the bottom wave,the attenuation coefficient and the grain diameter increased with the increase of the grain diameter.The frequency deviation threshold(2MHz)and the attenuation coefficient of the bottom wave can be determined.Threshold(0.25dB/mm,0.05dB/mm)for identification of coarse crystals(diameter greater than 150μm)and fine crystallites(diameter less than 20μm);(2)One-time bottom-wave frequency offset is available for grain shape characterization The parameter increased with the increase of the roundness of the grain shape parameter;(3)When the grain size of the GH738 is less than 150 μm,the sound speed grows with the grain size(6015m/s@9μm~6040 m/s @150μm).When the grain size is larger than 150 μm,the sound speed decreases as the grain size increases.The research’s results provide a data reference for the non-destructive evaluation of the homogeneity of the forging GH738,providing technical support for the improvement and quality monitoring of the GH738 forging process,and has a high engineering application value.