Centennial of the Diffusionless Paradigm of Bainite
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IGIN OF THE DIFFUSIONLESS GROWTH PARADIGM
IN 1910, an international committee was organized in order to consider ‘‘the nomenclature of the microscopic substances and structures of steel and cast iron.’’ Recommendations were presented in 1912,[1] and four kinds of transformation products were defined—pearlite, sorbite, troostite, and martensite—and it was stated that they all represent successive stages of the decomposition of austenite in the following series: austenite ! martensite ! troostite ! sorbite ! pearlite
This recommendation was not unreasonable because one did not distinguish between transformation products formed from austenite during cooling or by quenching and annealing and one had difficulties resolving fine two-phase microstructures. The kind of sorbite that formed during cooling now corresponds to bainite, and expressed with the present terminology, the 1912 recommendations thus contained the statement that bainite forms by an initial, martensitic process. It has been 100 years since the diffusionless growth mechanism was officially accepted as the first stage of bainite formation from austenite. It is interesting that the idea of diffusionless growth of bainite, although initiated by an inability to distinguish between different transformation products, later developed into a paradigm that strongly influenced the interpretation of research results for 100 years.
M. HILLERT, Professor Emeritus, and A. BORGENSTAM, Associate Professor, are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, 10044 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted January 25, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
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PROGRESS THROUGH THE ISOTHERMAL TECHNIQUE
It seems that the 1912 recommendations were well received and gained general acceptance, although there may have been some who doubted the indirect formation of the eutectoid microstructures, troostite, sorbite, and pearlite, in particular pearlite, which contains lamellar cementite. The doubts grew stronger as better microscopes and better experimental techniques became available. It became increasingly evident that all four transformation products could form directly from austenite during cooling. This development occurred mainly in France. In the following discussion, the term bainite will be used irrespective of what it was called at the time. As an example of the increasing understanding of the transformation of austenite, Hultgren in 1920,[2] using isothermal heat treatment interrupted by quenching, could publish micrographs of units of a microstructure he called ‘‘secondary ferrite’’ in a matrix of martensite, but today it would be regarded as upper bainite or maybe carbidefree bainite. He wrote that ‘‘the composition of the ferrite follows the extended equilibrium line,’’ i.e., the a/a + c phase boundary, and he illustrated this with a schematic Fe-C phase diagram. He thus proposed local equilibrium across a moving phase interface, and consequently, the growth of ferrite in an Fe-C alloy i
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