Stronger steel in a flash

Materials World magazine
,
3 Jun 2011
bridge

A novel rapid heating and cooling process for producing advanced high strength steel (AHSS) microstructures could lead to lighter, stronger components for the transportation, heavy equipment and defence sectors.

A team of researchers at SFP Works, LLC and Ohio State University, USA, has developed a ‘flash processing’ technique, which is said to improve upon existing methods in a process that is said to take less than 10 seconds.

Lead Researcher Gary Cola explains that, ‘widely commercialised steel technology has not had evolutionary changes in generations. In other methods of steel heat-treating, higher strength comes at the expense of ductility.’

According to Cola, the patentpending process, ‘employs rapid heating to temperatures much higher than those traditionally used by industry, immediately followed by a hard quench to controllably create a mixed martensite and bainitic microstructure’.

The flash processing technique applies the induction/heat technology with a specialised coil design. This treatment entails austentising the material to well over 1,000ºC for only a few seconds and immediately quenching it in a water bath. Low intensity furnace tempering is only required if the steel’s carbon content is over 0.21%wt.

The resulting flash bainite has consistent ductility from 750MPa to more than 2,000MPa. A50 elongation ranges from seven to nine per cent, with total elongation reaching up to 11.5%, mentions Cola.

The team has processed dozens of different alloys to achieve high strength and ductility. Cola ascertains that, ‘plain carbon flash AISI1010 steel at 1,100MPa with eight per cent A50 elongation is the break even point compared to other commercially available advanced high strength steels. At higher strength, flash bainite maintains more ductility in leaner alloys’.

‘We conservatively estimate to need only 20% of the energy required by a typical Continuous Annealing Line used to manufacture advanced high strength steel,’ he says, adding that the technology has been tested by USA Army laboratories and nine other independent laboratories.

A 48-inch-wide pilot line is near completion, and full production with the team’s bespoke equipment could be underway by the end of 2011.

Cola outlines the effect this technology could have on industry. ‘Just imagine wherever you see steel or aluminium used and then think of the positive changes if just 25% of that material is replaced by thinner, lighter, stronger, less expensive flash bainite. Flash bainite has shown a higher specific strength than all of the cost effective, readily weldable metals we have been able to compare it against’, including several aluminium grades and titanium-64.

On future endeavours, Cola says, ‘about 99.9% of all work to date has been done with commercially available steel sheet, plate, and tubing. One aspect of future work will be to look into I-beams, angle iron and roll formed shapes. A more significant research focus will be the development of alloys tailored to flash processing and deployable in a wide range of applications’.

On the technology, Dr Richard Thackray, Tata Steel Lecturer at the University of Sheffield, UK, says, ‘The results obtained from flash processing of AHSS appear to be impressive, both in terms of the mechanical properties obtained and the potential for energy savings during the production route.

‘However, it will be interesting to see if there are any problems in scaling up to full production. The results from pilot studies will be key. I also think that a greater understanding of the microstructural evolution that occurs during this sort of heat treatment is necessary in order to fully optimise this process for certain applications, and also to be able to identify alloy compositions that might be suitable for processing in this way’.