Brussels, 18 Nov 2002
The scientific truth behind the age-old blacksmith's secret that the strongest steel is formed by rapidly cooling it has been revealed by a joint Dutch, Danish and French project.
In an international study into the fine structure of steel, Technology Foundation STW researchers have analysed red hot steel with an x-ray microscope and discovered how, at a temperature of 900 degrees Celsius, numerous microscopic crystals suddenly developed in the steel.
The findings, published in the journal Science on 1 November describe how the microscopic crystals in steel are a measure of the metal's strength and determine the deformation characteristics. Steel with many small crystals is stronger than steel with a few large crystals. With these new findings, the steel industry can further refine and better manage the steel production process.
The research team directed an intense beam of x-rays onto a piece of steel heated to over 900 degrees Celsius. The temperature of the steel was then allowed to decrease by 5 degrees per minute. At 822 degrees Celsius the steel acquired a different crystalline structure. Whereas at high temperatures crystals with a diameter of 50 microns occur, the size of the crystals below 822 degrees Celsius is 10 to 40 microns. This was the first time that the formation of such steel crystals had been so clearly visualised.
The formation of new crystals seems to be much easier than materials researchers had previously thought. The energy required to make the conversion from the structure with large crystals to the 'cool' structure with many small crystals, is several orders of magnitude smaller than current models predict.
The research team contained material experts from Delft University of Technology in the Netherlands, The Risø National Laboratory in Denmark and the European Synchotron Radiation Facility (ESRF) in France.