The Research On The Relationship Between Microstructure And Ultrasonic Testing Parameters Of GH706 Alloy

GH706 alloy is a kind of precipitation hardening model high temperature alloy,which is Fe-Ni-Cr based alloy.The properties of the alloy depends on the size and morphology of the grain as well as the morphology and distribution of precipitates in the metallographic structure.The conventional metallography will cause irreversible damage to parts.In this paper,the relationship between the metallographic structure of GH706 alloy and the ultrasonic signal was studied, and the ultrasonic nondestructive evaluation of the alloy was tried to achieve.The standard heat treatment of GH706 alloy is heating preservation one hour at980℃,air cooling and heating preservation three hours at 843℃,air cooling and heating preservation 8 hours at 720℃,and furnace cooling to 620℃ preservation 8 hours.In this paper, in order to obtain different metallographic structure,the stabilizing treatment time、aging treatment temperature and time were changed.The grain size of the sample was measured according to GB/T6394-2002 standard for determination of average grain size of metal.The hardness and nonlinear coefficient of the sample were measured to assess the aging state of the alloy,and the velocity and attenuation of ultrasonic longitudinal waves were measured by ultrasonic water-immersion detecting system.The relationship between metallographic structure and ultrasonic parameters was studied.The results of metallographic analysis show that:under the four condition that unheated、solid solution、stabilization and aging,the change of the grain size is not large,which represents that the GH706 alloy has excellent ability to inhibit grain growth in high temperature.After aging treatment, the average grain diameter variance increases,which represents that grain shape and the degree of inhomogeneity of the grain size is changed.The hardness does not change with the time of stabilization treatment.But as the prolonging of aging treatment time,the hardness slightly increase and decline subsequently which shows that the hardness is related to the mount ofη and? ? phases and the change of the coherent state with the matrix.After solid solution treatment,the ultrasonic longitudinal wave velocity increases and the velocity continues to increase after stabilization treatment,and decreases after aging treatment which indicated that with the solid solution atom dissolved in,the elastic modulus of GH706 alloy increases,and the elastic modulus of GH706 alloydecreases when the solid solution atom dissolved out.The ultrasonic attenuation is not related to the heat treatment state,but increases with the increase of average grain size,and there is a excellent correlation between the two parameters.Under the unheated、solid solution and stabilization treatment,the ultrasonic nonlinear coefficient is smaller.With the aging stage,nonlinear coefficient increases first and decreases then,which corresponding to the underage、the peak of aging and over aging.Under the standard heat treatment condition recommended by,the nonlinear coefficient is highest,characterized that precipitation strengthening effect is best under this condition.The results of the first bottom wave Fourier transform shows that,under unheated、solid solution and stabilization,the peak frequency and peak amplitude of the first bottom wave keep unchanged;After aging treatment, the peak frequency increases,and the peak amplitude decreases.According to the comprehensive analysis of the parameters mentioned above,the heat treatment state and the aging state of GH706 alloy can be judged.Results of the research lay the foundation for the nondestructive evaluation of the GH706 alloy,which provide theoretical and experimental data support for the interpretation of GH706 ultrasonic testing results.