Acoustic control of the residual stresses in steel 5KhNM samples

Heat treatment (HT) is one of the main and most important stages of the technological cycle of manufacturing metal products. Residual stresses (RS) arising in metals upon their heat treatment significantly affect the service life of the products. The goal of the study is developing a technique for non-destructive acoustic testing of the residual stresses in steel samples. The method is based on the phenomenon of acoustoelasticity, i.e., the dependence of the acoustic characteristics on the parameters of the environment under study. The fields of residual stresses in rectangular samples made of die steel 5KhNM were formed using various HT modes, including cooling both in traditional media (water, oil, air) and using a water-air mixture (WAM). Control of the ratio of water-air parameters as well as the direction of the WAM flow provided the desired cooling rate and locality of the process. It is shown that the level of residual stresses decreases with the distance from the cooling plane, the microstructure and elastic characteristics of the material also change. The effect of cooling on the formation of temperature fields and the value of residual stresses was analyzed using computer simulation (CAE ANSYS program). The values of the characteristics of 5KhNM steel formed a set of input parameters in simulation. The simulation results proved the experimental data. Acoustic measurements carried out on the measuring and computing complex «ASTRON», provided determination of the velocities of elastic (longitudinal and transverse) waves and the elastic moduli of the material under study. The RS values obtained by the acoustic method were compared with the data obtained by the X-ray method. It is shown that the differences between the values measured by acoustic and X-ray methods do not exceed 10%. The results can be used in a production environment when measuring the residual stresses by the acoustic method in large-sized dies made of 5KhNM steel.

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