I am working on the effect of heating rate during reaustenitisation on the parent austenite grain structure . It has been observed that by increasing the heating rate above a certain value a very fine normal parent austenite grains distribution changes to an abnormal grain structure ( mixture of large and fine grains). Different hypothesis has been taken to the account but no really reasonable reason has been found yet. Any idea about the reason or any similar observation?!
I could discuss that, however, I don't have any direct measurements concerning this matter.
Apparently grains that have favorable conditions of grain growth expand rapidly at a high heating rate.
Big grains have advantage over small grains in growing further.
The grains that nucleate early and have favorable conditions of grain growth expand rapidly, kinetics of grain growth of some grains overtake the kinetics of dense nucleation and impingement, when the heating rate is high enough?
Or even and dense nucleation, but the favorable grains grow rapidly with a lot of driving force? Could be, but just guessing. Any way, some grains do gain an advantage in grain growth compared to others.
One possible reason can be found in the following article
Article Insight into the induction hardening behavior of a new 0.40%...
So when quenching from austenite to martensite with a fast cooling rate, a high driving force for the phase transformation is present and diffusionless shear transformation takes place and when heating with a very high heating rate there is also a high driving force for the phase transformation and in this case massive phase transformation mechanism to austenite takes place and big grains form?
Niko Vähäkuopus
Yes, but it should be thermodynamically also possible (see Figs. 19 and 21)
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