Implementing an ultrasonic inspection system to find surface and internal defects in hot , moving steel using EMATs

There is a need to develop an automated, on-line inspection technique to examine the surface and internal quality (ie to find any defects present) in ‘as-cast’ steel products(1,2) during continuous casting operations. On-line implies that the inspection system would be installed on a continuous caster as the hot steel passes a fixed inspection point. Continuous casting is the process where liquid steel is poured continuously into a mould to form the shape of slabs, blooms or billets. The rolling process is the next stage in manufacturing and this is where as-cast products are rolled to different sizes and are worked in such a manner that they exhibit different micro/macro structural characteristics, leading to the different mechanical properties as required in the final product. Defects in ‘as-cast’ products can be classified into two main categories – surface and internal. Surface defects include cracks, blowholes and inclusions, whilst internal defects can also include cracks, porosity, inclusions and segregation. Typical casting defects can be seen in the schematic diagram of Figure 1. If continuous casting plant operators can be alerted, via an automated inspection facility, that the current cast of steel contains defects that are not permissible for the specific steel grade being cast, then steps can be taken to correct the problem in order to minimise any yield losses attributed to downgrading or scrapping a cast before downstream processing. The aim of current final product inspection is to ensure that no defective or ‘out-of-tolerance’ material is supplied to internal Corus Business Units or external customers as by this stage significant downstream processing has been incurred. A laser-EMAT system has been selected as one of the few viable inspection technologies capable of inspecting both the surface and bulk of the steel. The pulsed laser beam simultaneously generates surface and bulk waves, and on samples with thicknesses comparable to the ultrasonic frequencies examined, Lamb wave modes can also be generated. The laser-EMAT system is capable of surviving the hostile environment of the continuous casting process, where some of the problems involved in installing any measurement device can include: large vibrations of the manufacturing machinery, a hot, dirty environment caused by dust in the atmosphere and iron oxide scale that can clog instruments with mechanical parts, high temperatures of up to 1000°C, steel moving at speeds between 0.5 m/s and 5 m/s and depending on the location of the sensors, significant amounts of water and steam. Papers have been written that demonstrate laser-EMAT equipment being used to find and size defects on metal samples at room temperature(3,4). Previous work has also shown that ultrasonic measurements can be conducted at elevated temperatures(3-6). Some steel mills have conducted trials using EMAT-EMAT and laser-laser inspection techniques to find surface defects on steel, but this work has never been commercialised(7-9). Laser-EMAT technology has been used to find defects on hot, moving steel pipes and to measure the pipe wall thickness(10). No work has been reported to date on LASER-EMATS