Casting the net – online inspection of steel

Materials World magazine
,
1 Aug 2008

A novel laser-electromagnetic acoustic transducer (EMAT) technique for detecting surface and internal defects in cast steel early in the manufacturing process is being explored. A pilot-scale experiment is running at international steelmaker Corus’ Teesside Technology Centre in Middlesborough, UK.

The technology involves firing laser pulses at the steel, which turns a small area of the surface into plasma, creating a thermal wave that gives rise to an ultrasonic wave. As the ultrasound travels along the surface and the internal profile of the steel, it is reflected back by defects such as corner cracking. A non-contact EMAT is used to detect the corresponding ultrasounds, enabling operators to use real time feedback control to streamline and optimise the casting process accordingly.

Project Manager and Corus Physicist Ian Baillie says, ‘The method has been used before for steel tubes by [the former] British Steel, but they closed down the factory. This is the first time it is being used for as-cast steel’.

The technique is said to ensure the quality of the material as it emerges from the caster and minimises scrap metal or components further downstream.

Baillie explains, ‘The manufacturing process involves moving steel in temperatures in excess of 800ºC. There are not many inspection techniques you can use because of that. Cameras are used to find surface defects, but during manufacturing you get steel oxide forming and it is difficult to tell the difference between steel oxide and the defects’.

Meanwhile, he says, conventional ultrasonics cannot be used to identify internal problems during casting. ‘Ultrasonics use a gel or water coupling, which would boil at these temperatures.’

The traditional offline approach for internal inspection involves taking sulphur prints of slices from the material once it has cooled down. ‘This is time-consuming and done three or four days after the steel is made,’ says Baillie.

The non-contact laser-EMAT probes could provide an alternative approach and have been installed at the continuous casting pilot plant at the Teesside Technology Centre. The first online measurements will be taken in December.

Next year, Baillie hopes to install the system on an industrial scale. ‘The reason for using the pilot plant is to cast different steel grades and create different defects for inspection so we have calibration. This will then be scaled up.’

Further information: Corus