Measurement of quench depth in railroad wheels by diffuse ultrasonic backscatter

In this article, the measurement of quench depth in railroad wheels is demonstrated using a diffuse ultrasonic backscatter technique. A singly-scattered response (SSR) model that accounts for the gradation of lamellar spacing (duplex microstructure) within grains in the direction of ultrasonic propagation is developed based on the previous single-phase model. The effects of a graded microstructure on ultrasonic scattering are observed by comparing the spatial variance curve measured from the tread surface to that measured from the cross section. The experimental results show that the graded SSR model fits the spatial variance curve from the tread surface much better than the uniform SSR model. The spatial variance peaks increase in amplitude and shift to later times when the material path is increased (deeper focus). The experimental variance curve from the tread surface is then fit with the model in a least-squares sense such that the quench depth can be estimated. The diffuse ultrasonic backscatter technique can be used not only to distinguish microstructure changes, but also to quantify the quench depth, an outcome that may be applicable for quality control during manufacturing.