Nondestructive Tertiary Mill-Scale Thickness Measurement on Commercial Hot-Rolled Steel Strip: Terahertz Time-of-Flight Tomography

Measuring mill-scale thickness in a nondestructive and noncontact fashion in a production environment remains a challenge, as knowledge of the thickness is central to processing of the steel strip. Terahertz time-of-flight tomography is demonstrated here to enable nondestructive and noncontact measurements of tertiary mill-scale thickness down to & AP;10 & mu;m on production hot-rolled steel, building on our recent work on thickness evaluation of laboratory-produced scale on steel coupons. X-ray diffraction (XRD) results show that the compositions of the mill-scale films manufactured under production conditions are slightly different from the laboratory-prepared steel coupons, resulting from the different compositions of phases present in the two types of samples. The mill-scale thickness is obtained after advanced signal processing of the raw data and shows good agreement with values obtained from destructive measurements. Compared to conventional techniques, such as scanning electron microscopy which is destructive, eddy current testing, which requires direct contact with the surface of samples investigated and is not suitable for large areas, or elevated temperatures, and acoustic inspection, whose spatial resolution is largely limited by its relatively long-wavelength, terahertz time-of-flight tomography may be implemented for in-line nondestructive thickness characterization and quality control of optically thin tertiary mill scale on steel products, even in a production environment.