About Bainite

About the Author
My work currently involves developing weldable high strength bainitic steels for use as armour, before this I did various other things with steel and nickel alloys (mostly steel!). Some details of those things can be found here.

My name is Mathew Peet, I have a webpage at mathewpeet.org which has some more of my stuff.

About Bainite – potted history
Bainite is a microstructure of steel that forms at temperatures intermediate between the formation of pearlite and martensite in many steels. First discovered by Davenport and Bain who conducted a series of isothermal transformation experiments.

Historically there has been much discussion in the literature about the mechanism of transformation of bainite, with opinions varying from it being very similar to pearlite, to it being similar to martensite. Results of Ko and Cottrell showed that bainite exhibited a surface relief consistant with a martensitic transformation mechanism. Shortly after Matas and Hehemann introduced the concept of a sub-unit mechanism to explain the apparant inconsistancy between the slow speed of transformation and the high speed the interface would be expected to move.

Microstructures formed as a result of continuous cooling or by welding are often very complex and this leads to lots of scope for errors in identification of microstructures. This leads to more confusion over the appearance of the microstructure an an urge to classify constituents by appearance rather than transformation mechanism.

Generally bainitic ferrite forms austenite as plates before carbon is partitioned to the austenite. Carbide precipitation can also take place either within the ferrite or between the plates of ferrite due to supersaturation. This distinguishes upper-bainite and lower-bainite.

If transformation occurs without the formation of carbides the volume fraction of bainite forming will be limited by thermodynamics. Once the carbon in the austenite reaches a concentration were any new bainite that formed would have the same energy as the parent austenite. This is the ‘incomplete transformation’ and ‘T-zero concept’.

You can find more information on the phase transformations and complex properties group’s page about bainite.

About the blog
I thought It would be useful and fun for me to write this blog. Personally my aim is to develop better writing habits and to document some of the things I think about to render them useful. So far posts have included simple calculations, experimental results, discussion of what is happening in the world, and an attempt to model the revenue of James Bond movies using neural networks.

For the blog I would like to discuss about metallurgy and science and related topics, in the end everything depends on metallurgy so topics may become quite wide reaching at times. From time to time I may persuade other people to contribute to the blog too. Please feel welcome to the site and feel free to leave feedback on the posts, let me know what you want to read about.

Trootsite

This blog now has a companion blog http://troostite.wordpress.com/, who knows what will happen there.

Acicular Ferrite

This blog now has another sister blog http://acicularferrite.wordpress.com/, some content from the bainite blog is reblogged there. Maybe it will have it’s own content too.

Podcasts
Podcasts should be available by subscribing to this RSS Feed.

7 Responses

  1. I was just checking Wikipedia on Bainite and realized that if you change the language to Deutsch you will get twice as much information! apparently Germans know more1

    • It might just be that the German language needs twice as many words to describe a concept or a process.

    • I agree the german entry is much more comprehensive than the english one. It seems to cover a lot of relevant points. I think that figure/bild 1 can be a bit misleading though since it implies bainite is an equilibrium structure unless cooling rate is stated. I can just check rest of page using translation software. I am not sure why they dismiss kinetic definition of bainite, and I am not sure that surface relief definition the best way to decide what is bainite or not. Since Widmanstaetten ferrite and martensite also exhibit surface relief, you then need to see which transformation the surface relief is consistent with. I didn’t read the rest of the article but from the figures they seem to cover carbide precipitation (upper and lower bainite), orientation relationships and sub-unit mechanism. I don’t understand if the part about fatigue at the end is relevant or not, from the translation is seems just to be about steels in general.

  2. Can you recommend a recent reference(s) that provides a good summary of the current status of the shear vs. diffusion debate? Thanks. Great website.

  3. I think the best is “Bainite in Steels” by H.K.D.H. Bhadeshia.

    I have a thesis you can read at http://mathewpeet.org/thesis/

    There should be a literature review by Lucy Fielding available in the near future.

    Is there really still a debate about the nature of bainite?

  4. The Review by Lucy Fielding has now been published in Materials Science and Technology discussing the shear vs. diffusional bainite debate, it can also be downloaded from the phase transformations website. Bainite Literature Review

  5. Can someone provide me with the updated empirical formula to calculate Bainite start temperature and also the influence of various alloying elelments on bainite start temperature?

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