What is Bainite? How Bainite was named.

This article is related to my previous post What is Bainite?

This is an extract from ‘Pioneering in Steel Research’ by Edgar Bain, descibing how the name Bainite started to be used.

As late as 1933, we were still disturbed by a continuing conflict between the desire to use the classical names and definitions of the constituents of steel (especially of the aggregate or 2-phase constituents) and a need for greater precision and specificity. We wished, of course, to continue to reflect out admiration and respect for the great pioneering metallurgists in whose honour most of the constituents had been named. (I was always regretful, however, that the preceptive pioneer, Osmond, was not more worthily represented in the designations.) We were also not satisfied with our custom of speaking of the new intermediate transformation product (formed from austenite in the approximate range 500 to 200 C [930 to 390 F]) as troosite or martensite-troosite (too hastily chosen by Davenport and Bain in their first presentation of the new isothermal transformation product). Nor did we wish to use some larger chain of words to designate it.

These dissatisfactions have a bearing upon one of the most pleasant memories I have. One afternoon in 1934 (late summer, I believe), a group of my close associates in the laboratory sauntered into my office and presented to me a superb, large, neatly mounted micrograph at 1000X of a new, isothermally formed microstructure made from the first such series of specimens of isothermally transformed steel. It was designated in large capital letters BAINITE and below, “First noted by, and so named, in 1934 in honour of E. C. Bain by the research staff of the U. S. Steel Corporation”. The mounted micrograph was signed by Jose R. Vilella, John G. Zimmerman, E. S. Davenport, E. L. Roff and Robert H. Aborn. I scarcely add that this autographed micrograph is among my most cherished possessions.

The new name was soon adopted by other investigators of isothermal transformation structures, and Bain started to use the name after initially usually vaguely describing the microstructure, for example as ‘Dark-etching’ or as ‘martensite-trootsite’. (trootsite being an historical name for pearlite). There was some dissatisfaction with the then-current terms for the microstructures of steel, and Vilella, Guellich and Bain presented a paper on the naming of aggregate constituents of steel which they presented at the ASM convention in 1935. They suggested some changes in which the micromorphology of the microstructures rather than the possible conditions of their formation should serve as the basis for defining names, although they didn’t propose the name of Bainite, they did use it in their daily work.

An approach of naming microstructures is practical and can be useful for classification of the results systematic heat treatment, or process control work, avoiding any thought of the transformation mechanisms. If we want to develop an understanding of the microstructures and to have physically based models. Models which incorporate our understanding and allow us to predict microstuctures only a naming system based on transformation mechanisms is really going to be useful. Since we now know a fair amount about the mechanisms of the phase transformations a definition of microstructures based only upon appearance is no longer the most appropriate. It seems clear that Bainite transforms by a displacive mechanism, and also that bainite in high silicon transforms in a similar way despite the suppression of carbides.

Prof. Bhadeshia has outlined a useful classification of the decomposition products from austenite, this is available in his book “Bainite in Steels” which you can download here (see “The Transformations in Steel”).

Reconstructive Displacive
Allotriomorphic Ferrite Widmanstatten Ferrite Idiomorphic Ferrite Bainite and Acicular Ferrite Massive Ferrite Martensite Pearlite


  1. […] Bainite is a structure which can form in steels, named after Edgar Bain (photo below) who discovered (with E. C. Davenport) the structure around 1930. This microstructure generally forms as an aggregate of ferrite (the stable crystal structure of pure iron at room temperature) and cementite/ carbides (stoichmetric combinations of iron, other metallic elements and carbon). […]

  2. hi..
    im zureena from mechanical faculty in university technology malaysia. i would like ask you some question about TTT diagram. first i want to know who was construct TTT diagram. second how TTT diagram named. if you dont mind can you share with me which industry use TTT diagram..

  3. TTT means Time-Temperature-Transformation diagram. It was introduced by Edgar Bain who was the first to perform carefully controlled isothermal transformations. All metal producers should find these concepts useful, along with anyone who performs heat treatments. It could equally be applied to baking cakes, etc.

  4. It really depends on what microstructure and properties you want. If the application was designed to have a pearlitic shaft, and the steel used was designed to have a good combination of properties using a pearlitic core and a case hardened martensite periphery then that is probably what you should be trying to achieve. If you are at the design stage, trying to make a cost reduction, or improve the properties then think about it and have a try, I dont see any reason why the fine bainite should be bad, but of course it depends on what properties you need, and if the composition and heat treatment you want to use has sufficient toughness. Changing the microstructure without proper consideration of the application can have disastrous consequences so you should only do it if you are able to consider them all, and even better test it 🙂 This is why I didn’t really want to answer the question, there is no simple answer.

  5. Bainite is still being debated. This may be due to the fact that most steeels are continuously cooled not isothermally treated. The original
    “Bainite” designations were from the isothermal transformation diagram; namely upper and lower bainite. During continuous cooling the resulting microstructures can differ and may not have the distinction of upper and lower bainite. In fact, with lower carbon and sufficient alloy content (Mn, Si, Cr, etc.) there is a form of bainite called carbide-free bainite where retained austenite replaces the traditional iron carbide phase.

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