The microscopic crystals in steel are a measure of the metals 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.
In this joint Dutch-Danish-French project into the formation of the microstructure of steel, the research team directed an intense beam of x-rays onto a piece of steel heated to over 900 oC. That happened with a special x-ray microscope (ESRF) in the French city of Grenoble. The diffracted x-rays provide information about the internal structure of steel.
The research team allowed the temperature of the steel to decrease by 5oC per minute. At 822 oC 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 oC is 10 to 40 microns. This was the first time that the formation of such steel crystals had been so clearly visualised.
The mechanical properties of steel are largely determined by the rate at which atoms rearrange themselves. Rapid cooling results in many small crystals and thus strong steel. The traditional blacksmith already knew that steel could be strengthened by suddenly placing red-hot steel in cold water.
The formation of new crystals seems to be much easier than material 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, The Risø National Laboratory in Denmark and the European Synchotron Radiation Facility (ESRF) in France. The Dutch part of the study was financed by the Technology Foundation STW.
Further information can be obtained from Erik Offerman (Interfaculty Reactor Institute, Delft University of Technology), tel. +31 (0)15 2783673, fax +31 (0)15 2788303, e-mail s.e.offerman@iri.tudelft.nl, website http://www.tm.tudelft.nl/secties/mcm/people/offerman.html.