The bainite transformation
«The mechanism of bainite formation has been the subject of numerous original research papers and reviews for almost a century hut without any signs of controversies being resol ved. For beginners and even for experts in related fields this wealth of info
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2-1 BainÎte structures The nature of bainite The common feature of the different bainite structures is that they ali contain dislocation-rich ferrite which frequently has a more or less acicular morphology. Two microstructures are generally distinguished, corresponding to upper bainite, which forms in a temperature range immediately below that for pearlite, and lower bainite, whose range of formation extends down to that for martensite. Upper bainite is comprised of lath bundles or sheaves, while lower bainite is in the form of individual plates. The ferrite in bainite structures is harder than normal ferrite due to its high dislocation content, with densities that range from about 10 15 to 10 14 m- 2 as the transformation temperature rises between 400 and 700°C. The microhardness ofbainite varies between 300 and 500 Hv It is now accepted that the bainite transformat ion occurs without redistribution of substitutional solute elements, the conditions of equilibrium at the interface corresponding to either the para-equilibrium (PE) or the no partitioning local equilibrium (NPLE) modes. For example, in a steel containing copper, no precipitates can be detected by conventional transmission electron microscopy in either the ferrite or cementite constituents of bainite formed at 350 0C. In contrast, copper particles appear during tempering treatments of several hours at and above 500 °C [Fou96]. This shows that there is no partitioning of copper during the upper bainite transformation itself. From a kinetic standpoint, the bainite transformation is not as rapid as that involving martensite. The rates of both nucleation and growth are controlled by carbon diffusion [Qui02]. In the temperature range concerned, growth can only occur at mobile incoherent
M. Durand-Charre, Microstructure of Steels and Cast Irons © Springer-Verlag Berlin Heidelberg 2004
THE MICROSTRUCTURE OF STEELS AND CAST IROHS interfaces and not at ones that are semi-coherent with the austenite and pinned by misfit dislocations (cf § 9-3). It involves a terrace and ledge mechanism. Two alternative interpretations are possible concern ing the rearrangement of the iron and substitutional solute atoms, which may occur either reconstructively, by diffusional exchanges restricted to the incoherent terrace edges, or displacively, as in the martensite transformation. What is dear is that it is the iron and slowly diffusing solutes that govern the change in crystal structure [Aar90J, [Bha92J, [Qui01].
The controversy The term bainite was first coined in 1934, in honour of Bain who reported this particular microstructure in 1933. The subject of the bainite transformation has remained a lively topic for debate ever since. The basic principles of the underlying mechanisms were announced by Zener as early as in 1 946 [Zen46]. The idea that growth is controlled by the diffusion of carbon was developed in the l%Os [Zac62J, but was not totally accepted at the time. Numerous studies revealed the extreme complexity of the bainite transformation. The subsequent use of more
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