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FP5

VESPISM Résumé de rapport

Project ID: G5RD-CT-2000-00315
Financé au titre de: FP5-GROWTH
Pays: United Kingdom

Appraisal of metallurgical theories

Physically-based models allow tests to be performed on the assumed theories: do they actually generate the correct microstructures, as are observed? These can address major issues, such as how does a particular morphology occur at all, or gradations: under what conditions will the growth change from one morphology to another?

The software has proved itself useful regarding prediction of cellular or dendritic morphology and the associated, characteristic length-scales. It has explained how under certain circumstances, the microsegregation peaks that were all aligned during solidification, can 'drift' and become out of phase during cooling in the solid-state. It is beginning to show insight into peritectic cracking susceptibility.

It has highlighted issues that need revised or new theories to explain them. The simulations, employing accepted assumptions/theory, produce fairly smooth kinetics for grain growth. The experimental results under Result 17748 show that in practice, grains grow with periods of stagnation punctuated by localised bursts of activity. Similarly, the simulations show that Strain Induced Boundary Migration should be the principal recrystallization mechanism: although this is the case in some steels, in typical formable sheet steels this mechanism is almost dormant, allowing alternative mechanisms to occur. The simulations are logical consequences of the assumed theories, so we have a problem with the theories.

Some progress has also been made concerning the formation of bainite. If the transformation is primarily diffusion driven, why do we get a morphology of 2-fold symmetry from a system of crystallographic/energetic 4-fold symmetry? Coupling with stress fields arising from the density difference between the parent austenite and the bainitic ferrite, it has been shown that a 2-fold symmetric forms can result. However, it did not prove possible to compare alternative theories of bainite formation within this project.

Informations connexes

Contact

Andrew HOWE, (Principal Scientist)
Tél.: +44-1709-825328
Fax: +44-1709-825337
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