Tempered Martensite To begin describing what bainite is it makes sense to start with martensite first. To form martensite we heat up the steel to high temperature to transform to a phase called austenite where we dissolve carbon in between the iron atoms see Austenitizing Part 1 , then quench the steel to lock in the carbon and form a hard phase called martensite see What Makes Quenched Steel so Hard? Following that we temper the martensite to allow some of the carbon out and increase the ductility of the martensite; the carbon comes out as very small carbides, a compound of iron and carbon see What Happens During Tempering? In the article on martensite formation I shared the following YouTube video to see the formation of the martensite laths: You can see that the laths grow almost instantaneously once they start forming nucleation. It is a rapid transition once a sufficiently low temperature is reached to drive the martensite formation. The two share similar features.
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History[ edit ] In the s Davenport and Bain discovered a new steel microstructure which they provisionally called martensite-troostite, due to it being intermediate between the already known low-temperature martensite phase and what was then known as troostite now fine- pearlite.
The early terminology was further confused by the overlap, in some alloys, of the lower-range of the pearlite reaction and the upper-range of the bainite with the additional possibility of proeutectoid ferrite. A steel of eutectoid composition will under equilibrium conditions transform into pearlite — an interleaved mixture of ferrite and cementite Fe3C. In addition to the thermodynamic considerations indicated by the phase diagram, the phase transformations in steel are heavily influenced by the chemical kinetics.
As a consequence, a complex array of microstructures occurs when the atomic mobility is limited. This leads to the complexity of steel microstructures which are strongly influenced by the cooling rate. This can be illustrated by a continuous cooling transformation CCT diagram which plots the time required to form a phase when a sample is cooled at a specific rate thus showing regions in time-temperature space from which the expected phase fractions can be deduced for a given thermal cycle.
If the steel is cooled slowly or isothermally transformed at elevated temperatures, the microstructure obtained will be closer to equilibrium,  containing for example of allotriomorphic ferrite, cementite and pearlite. However, the transformation from austenite to pearlite is a time-dependent reconstructive reaction which requires the large scale movement of the iron and carbon atoms.
As a consequence, a rapidly cooled steel may reach a temperature where pearlite can no longer form despite the reaction being incomplete and the remaining austenite being thermodynamically unstable. This non-equilibrium phase can only form at low temperatures, where the driving force for the reaction is sufficient to overcome the considerable lattice strain imposed by the transformation. The transformation is essentially time-independent with the phase fraction depending only the degree of cooling below the critical martensite start temperature.
Bainite occupies a region between these two process in a temperature range where iron self-diffusion is limited but there is insufficient driving force to form martensite. The bainite, like martensite, grows without diffusion but some of the carbon then partitions into any residual austenite, or precipitates as cementite. A further distinction is often made between so-called lower-bainite, which forms at temperatures closer to the martensite start temperature, and upper-bainite which forms at higher temperatures.
This distinction arises from the diffusion rates of carbon at the temperature at which the bainite is forming. If the temperature is high then the carbon will diffuse rapidly away from the newly formed ferrite and form carbides in the carbon-enriched residual austenite between the ferritic plates leaving them carbide-free. At low temperatures the carbon will diffuse more sluggishly and may precipitate before it can leave the bainitic ferrite. Displacive theory[ edit ] One of the theories on the specific formation mechanism for bainite is that it occurs by a shear transformation, as in martensite.
The crystal structure change is achieved by a deformation rather than by diffusion. The shape change associated with bainite is an invariant—plane strain with a large shear component. This kind of deformation implies a disciplined motion of atoms rather than a chaotic transfer associated with diffusion ,  and is typical of all displacive transformations in steels, for example, martensite, bainite and Widmanstaetten ferrite. There is a strain energy associated with such relief, that leads to the plate shape of the transformation product  Any diffusion is subsequent to the diffusionless transformation of austenite, for example the partitioning of carbon from supersaturated bainitic ferrite, or the precipitation of carbides; this is analogous to the tempering of martensite.
There are many features of bainite that are correctly predicted by this theory, including: the plate shape, which is a consequence of the minimisation of strain energy due to the shape deformation accompanying transformation. The transformation is a combination of deformation and crystal structure change, just like martensite.
Its growth rate thus depends on how rapidly carbon can diffuse from the growing ferrite into the austenite. A common misconception is that this mechanism excludes the possibility of coherent interfaces and a surface relief.
The sheaves themselves are wedge-shaped with the thicker end associated with the nucleation site. The thickness of the ferritic plates is found to increase with the transformation temperature. These sheaves contain several laths of ferrite that are approximately parallel to each other and which exhibit a Kurdjumov-Sachs relationship with the surrounding austenite, though this relationship degrades as the transformation temperature is lowered.
The ferrite in these sheaves has a carbon concentration below 0. For a low carbon steel, typically discontinuous "stringers" or small particles of cementite will be present between laths. For steel with a higher carbon content, the stringers become continuous along the length of the adjacent laths. There are not nearly as many low angle boundaries between laths in lower bainite. Incomplete transformation[ edit ] In the present context, "incomplete transformation" refers to the fact that in the absence of carbide precipitation, the bainite reaction stops well before the austenite reaches its equilibrium or paraequilibrium chemical composition.
It stops at the point where the free energies of austenite and ferrite of identical composition become the same, i. Early research on bainite found that at a given temperature only a certain volume fraction of the austenite would transform to bainite with the remainder decomposing to pearlite after an extended delay. This was the case despite the fact that a complete austenite to pearlite transformation could be achieved at higher temperatures where the austenite was more stable.
The fraction of bainite that could form increased as the temperature decreased. This was ultimately explained by accounting for the fact that when the bainitic ferrite formed the supersaturated carbon would be expelled to the surrounding austenite thus thermodynamically stabilising it against further transformation.
It forms at a higher temperature than martensite, and even the latter can autotemper. As a consequence, the growing plate of bainite is confronted by a forest of dislocations that eventually terminates its growth even before the plate has hit an austenite grain boundary. Plates of bainite can therefore be smaller than those of martensite in the same steel. The transformation then proceeds by a sub-unit mechanism involving the successive nucleation of new plates.
Bainite in Steels
In particular, the scale and extent of the structure is dependent directly on the fact that the atoms move in a disciplined fashion. This information can be exploited to develop unconventional alloys - for example, rail steels which do not rely on carbides for their properties, and the hardest ever bainite which can be manufactured in bulk form, without the need for rapid heat treatment or mechanical processing. It is shown in this paper that the theory of bainite can be used to create exciting alloys. Carbide-Free Bainitic Steels Conventional upper bainite consists of a non-lamellar mixture of bainitic ferrite plates with intervening particles of cementite.
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Bainite in steels