The process has been debated for over a decade but now special X-ray tomography has revealed how crystals form within steel.
Steel, the combination of iron with up to two per cent carbon, is one of the materials featuring phase transitions in solids, said Dr.
Erik Offerman, material scientist at the 3mE faculty of the TU Delft. He recently published photos showing the phase change in a half millimetre-sized sample. The X-ray tomography was performed at the European Synchrotron Radiation Facility (ESRF) in Grenoble, and the article appeared in Scientific Reports (Nature publishing group) on August 3rd, 2016.
Why care about crystals in steel? Because they determine the material properties, which vary enormously. There is the basic ferrite which is well-deformable and has a reasonable strength. It is used for car bodies, bridges, and buildings. Then there is austenite, found instainless steel, that is resistant to aggressive chemicals. Or take martensite, a hardened type of steel that allows very little deformation. The wide spectrum of steel properties is the macroscopic effect of the internal crystal structure.
FurnaceFurnace
Furnace
To study the phase changes in steel, Offerman had a special furnace constructed which allows temperatures up to 1,500 degrees Celsius but still could have an X-ray machine at only seven millimetres distance. The sample is only a centimetre high and has a diameter of one millimetre. During the process, the furnace rotates in the intense X-ray beam to make 3D tomography possible. “Other than standard X-ray imaging”, said Offerman, “the synchrotron beam does not show differences in density, but in crystal orientation. Crystals make up the material like grains in a salt shaker.”
Phase changePhase change
Phase change
What the tomographic reconstruction shows are dozens of ferrite crystals slowly disappearing as the temperature rises from 830 to 850 degrees Celsius. At the same time, new austenite crystals are forming. The thousand-dollar question has always been: where do these new crystals form?
The theory was that ferrite crystals caught in between two specially orientated austenite crystals would be the first to transform into austenite. This is the so-called pill-box model developed by Dr. H.I. Aaronson of Carnegie Melon University (Pittsburgh, US). The two-sided orientation would bring down the activation energy and thus facilitate the transformation from one crystal form into another.
What the analysis of the phase changes showed was that not only ferrite crystals in between two austenite crystals transform. But also when there are one, three or four sides of contact. There goes the pill-box theory.
“The lower the activation energy, the easier the transformation goes”, explained Offerman. “The easier the change goes, the more nucleation cores there will be and the higher will be the strength and the deformability of the material.”
His conclusions go beyond basic science. Offerman hopes that better understanding of the nucleation process in steel will allow making stronger or more heat-resistant steel without the need for adding alloying metals like niobium. That will reduce the dependency on rare metals and improve reusability of steel.
–> H. Sharma, J. Sietsma & S.E. Offerman, Preferential nucleation during polymorphic transformations, Scientific Reports, 3 August 2016, DOI: 10.1038/srep30860
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