Nondestructive Evaluation Of Microstructure Evolution Of Train Axel Steel Using Nonlinear Ultrasonic Method

Axle is the key component in the bogie of the train, and its performance endangers the safety of transportation directly. It’s important, therefore, to get an early understanding of the degradation of axle performance. The microstructure of the material affects its macroscopic mechanical properties, and the material microstructure also has close connection with the material nonlinearity. There is important application value to study and establish the relationship between the material nonlinear acoustic parameter and microstructure evolution, and to make a corresponding ultrasonic nondestructive evaluation on the degradation of materials.This paper studies the ultrasonic nonlinear parameter and microstructure evolution of axle steel specimens under tensile and fatigue loading by ultrasonic nondestructive testing and metallographic observation method. Relationship between ultrasonic nonlinear parameter and grain size is established based on the experimental results. Further analysis and discussion based on the theoretical model of dislocation and experimental data, and the relationship between attenuation parameter and micro grain size for the specimens under tensile and fatigue loading are also conducted in this paper. The research results based on above studies show that:(1) Under tensile loading, the relative nonlinear parameter and the average grain size increases with the incensement of plastic deformation at the point close to the necking point of the axle steel specimen, and the relative nonlinear parameter also increases with the incensement of average grain size. Variation of grain size could thus be characterized by the relative nonlinear parameters qualitatively. However, the experiment results of the relative nonlinear parameter and the grain size are different from at the far-necking point compared with that at near-necking point.(2) Under fatigue loading, the relative nonlinear parameter and the average grain size increase with the incensement of fatigue life at both near- and far-necking point of the axle steel specimen, and the relative nonlinear parameter increases with the incensement of average grain size. Compared with the experimental results of far-necking point, variation of the relative nonlinear parameter with grain size of the near-necking point is more related.(3) Under tensile loading, the relative attenuation parameter has a better response to the change of grain size at the near-necking point, while the situation is different at far-necking point. Under fatigue loading, the relative attenuation parameter cannot be an accurate evaluation on grain size at both near-a nd far-necking point of the axle steel specimen.(4) In experimental measurement, the relative nonlinear parameter increases with the increasement of fourth power of the pearlite radius, which coincides with the analysis and discussion based on the theoretical model of dislocation under the condition of the same dislocation density and stress. The relative nonlinear parameter and fourth power of the pearlite radius are linearly related.The results above show that the nonlinear parameter could be used to evaluate the evolution of the steel axle microstructure. Compared with the attenuation parameter, the nonlinear parameter can be used as a better indicator of the variation of the grain size. The research of this paper provides a solid basis for nondestructive evaluation of the material microstructure evolution caused by damage using ultrasonic nonlinear method.