Pulsating steel - quicker and cleaner fabrication

Materials World magazine
,
4 Mar 2013

An electropulse-based technique could be a key to faster, greener fabrication of high-performance clean steel, say researchers at Imperial College London, UK. In this new method, a pulsed electric current is applied to liquid steel. The electropulse has a high current density and is applied for a very short time. Typically, each pulse lasts 1/50,000 of a second.

The main objective is to reduce the total oxide inclusion mass in liquid steels to ensure that chemistry and size distribution is controlled. Inclusions cause cracks and failure in the metal, so cleaning is paramount to the material’s performance.

Dr Rongshan Qin, a lead researcher on the work at Imperial College London, explains, ‘The damage is significant in low dimensional components and those steel parts working in high-speed environments. The bearing parts in heavy machinery and equipment require super-clean steels, so our aim was to try and improve mass fabrication methods.’

Electropulse technology has been applied in several aspects of metal processing, but it has never been used to clean steel. On developing the technique, he explains, ‘We used inhouse software to calculate the effect of electropulsing. Tests revealed that the electropulse expels low-conductive objects from a high conductive matrix. Inclusions are normally low conductive nonmetallic oxides.’

One existing problem is that conventional cleaning methods such as magnetic stirring or gas bubbling, cannot be used precisely or to remove small inclusions, and are largely based on either size or density discrepancies between inclusions and steels. Qin explains, ‘Often these methods are limited to removing inclusions larger than 20µm, so smaller inclusions remain in the steel. For example, if a steel knife with thickness of 20µm is made for the electronics industry, a 20µm inclusion will drop out of the knife and leave a hole. The reason is that these commonly used methods are size-dependent. With this process, large or small inclusions can be removed easily’.

The key to the method’s efficiency is the pulse. ‘It works in a very short period. Liquid steel needs to be contained where temperatures are higher than 1,200oC, and retaining the steel in such high temperature for a long time typically requires a lot of energy,’ he adds. ‘Here, the total electric energy is negligible. The electropulse affects the formation and motion of inclusions. This helps to minimise the size and amount of inclusions in the steel so a higher cleanliness of steel is achieved, over a shorter period.’

Qin says that this is the first trial to separate materials according to their electrical property differences, and hopes the process will also assist metal recycling. ‘We’re now looking to use this technology to separate copper and tin from steel scraps. So far, no other technology can achieve this,’ notes Qin.

The three-year project funded by EPSRC and TATA Steel is scheduled to end in October 2015. ‘We are still at the early stages, but we want to soon work with industry for a large-scale trial,’ adds Qin.