Knock Knock
who’s there?
Thea
Thea who?
Thea later, alligator

I’m with you Mathew — bainite rocks!

Obviously they are not equilbrium phases, and cannot be represented on the equilbrium phase diagram. (or are they?)

We have to acknowledge that if we look in higher resolution at the pearlite phase we can see that it is an intimate mixture of ferrite and cementite. However it’s properties as a mixture are distinct. I don’t think we lose anything by thinking of pearlite as a phase of it’s own.

Similarly if we look as bainite we can see it is an intimate mixture of ferrite and austenite (in carbide free bainite) or ferrite and cementite.

We could justify that pearlite and bainite are represented on the equilibrium phase diagram in the ferrite + cementite phase region. Similarly any other microstructure that forms we could think of as a metastable phase. For example is high carbon martensite, is this a phase? Is it the same as BCC ferrite?

Thanks for question.

“What would Gibbs” say can become the new “what would Jesus say” or “what would Feynman say”.

If I had phrased the question which microstructure is the best I would run into the trouble of mixed microstructures, I guess I can avoid these problems by using the word constituent.

What would Gibbs say?

Unfortunately I can’t remember the reference for the above. I think there may be by a paper with David Edmonds of the University of Leeds as one of the authors.

Alternatives which might work in theory would be to try to refine as much as possible the austenite grain size or to try to allow the cementite to globalise, i.e. by show heating up to the austenitisation temperature, which would not be sufficient to completely dissolve the cementite.

I don’t believe that silicon will suppress the formation of cementite at high temperatures, it may in fact promote the formation of cementite (at high temperatures).

This can be shown by thermodynamic calculations.

Fast cooling from fully austenite may be one way to avoid cementite formation (or at least growth of large particles). However formation on g.b.s will be very fast.

Another way is very short austenitising heat treatments with rapid heating. This can be seen in the flash bainite process (http://www.bainitesteel.com/) you might find a paper by Suersh Babu that deals with this.

Sorry I don’t know the proper references about this I can only give a clue for the time being.

If you have luxury of changing the composition you could try shifting the eutectoid to higher carbon contents. Of other alloying to lower the cementite stability (this usually means removing alloying elements).