Influence of Local Mechanical Parameters on Ultrasonic Wave Propagation in Large Forged Steel Ingots

In numerous applications, high strength forged steel requires uncompromising quality and mechanical properties to provide advanced performances. Ultrasonic testing are commonly used for material characterization to measure mechanical properties and for nondestructive testing to detect flaws or other features contained in metallic objects. In steel blocks of relatively small dimensions (at least two dimensions not exceeding a few centimeters), temperature and homogeneity are well controlled during the solidification. However, these parameters may become difficult to control during manufacturing of large objects. Forging and heat treatments are known to modify the microstructure and/or the grain size, therefore affecting elastic properties, and consequently, the ultrasonic inspection reliability. In this context, the relationship between ultrasonic group and phase velocity variations with local properties of a forged and heat-treated \(40{,}000~\mathrm {kg}\) bainitic steel block manufactured in an industrial setting was investigated. The block was cut into a \(20~\mathrm {mm}\) thick slice that was then divided into 875 parallelepiped samples. A subset was selected for ultrasonic measurements, metallurgical study, and chemical analysis. Ultrasonic phase velocity showed a strong correlation with grain size, whereas group velocity was shown to vary as a function of the Young’s modulus and the chemical composition. Tensile testing was performed to validate the Young’s modulus calculated from the ultrasonic group velocities.