Bainite

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Bainite in draw quality semi-killed steel

Bainite is a plate-like microstructure or phase morphology (not an equilibrium phase) that forms in steels at temperatures of 250–550 °C (depending on alloy content). First described by E. S. Davenport and Edgar Bain, it is one of the decomposition products that may form when austenite (the face centered cubic crystal structure of iron) is cooled past a critical temperature. This critical temperature is 1000K (727 °C, 1340 °F) in plain carbon steels. Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite.

A fine non-lamellar structure, bainite commonly consists of cementite and dislocation-rich ferrite. The high concentration of dislocations in the ferrite present in bainite makes this ferrite harder than it normally would be.

The temperature range for transformation to bainite (250–550 °C) is between those for pearlite and martensite. When formed during continuous cooling, the cooling rate to form bainite is more rapid than that required to form pearlite, but less rapid than is required to form martensite (in steels of the same composition). Most alloying elements will lower the temperature required for the maximum rate of formation of bainite, though carbon is the most effective in doing so.

The microstructures of martensite and bainite at first seem quite similar. This is a consequence of the two microstructures sharing many aspects of their transformation mechanisms. However, morphological differences do exist that require a TEM to see. Under a light microscope, the microstructure of bainite appears darker than martensite due to its low reflectivity.

Bainite is an intermediate of pearlite and martensite in terms of hardness. For this reason, the bainitic microstructure becomes useful in that no additional heat treatments are required after initial cooling to achieve a hardness value between that of pearlitic and martensitic steels.

History

In the 1920s 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). This microstructure was subsequently named bainite by Bain's colleagues at the United States Steel Corporation although it took some time for the name to be taken up by the scientific community with books as late as 1947 failing to mention bainite by name. Bain and Davenport also noted the existence of two distinct forms: 'upper-range' bainite which formed at higher temperatures and 'lower-range' bainite which formed near the martensite start temperature (these forms are now known as upper- and lower-bainite respectively). 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.

Formation

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Illustration of a continuous cooling transformation (cct) diagram for steel

At 900 °C a typical low-carbon steel is composed entirely of austenite, a high temperature phase of iron (the other being delta-ferrite at even higher temperatures). Below around 700 °C (727 °C in eutectic iron) the austenite is thermodynamically unstable and, under equilibrium conditions, it will undergo a eutectoid reaction and form 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. 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 the transformation will agree with the equilibrium predictions and pearlite will dominate the microstructure with some fraction of proeutectoid ferrite or cementite depending on the chemical composition. 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. While the interstitial carbon diffuses readily even at moderate temperatures the self-diffusion of iron becomes extremely slow at temperatures below 600 °C until, for all practical purposes, it stops. 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.

Austenite that is cooled very rapidly can form martensite, without any diffusion of either iron or carbon, by the shear of the austenite's face-centered crystal structure into a distorted body-centered tetragonal structure. 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. Further, it occurs without the diffusion of either substitutional or interstitial atoms and so martensite inherits the composition of the parent austenite.

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. In contrast to pearlite, where the ferrite and cementite grow cooperatively, bainite forms by the transformation of carbon-supersaturated ferrite with the subsequent diffusion of carbon and the precipitation of carbides. 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. There is some controversy over the specifics of bainite's transformation mechanism; both theories are represented below.

Displacive theory

One of the theories on the specific formation mechanism for bainite is that it occurs by a shear transformation, as in martensite. The transformation is said to cause a stress-relieving effect, which is confirmed by the orientation relationships present in bainitic microstructures. There are, however, similar stress-relief effects seen in transformations that are not considered to be martensitic in nature, but the term 'similar' does not imply identical. The relief associated with bainite is an invariant—plane strain with a large shear component. The only diffusion that occurs by this theory is during the formation of the carbide phase (usually cementite) between the ferrite plates.

Diffusive theory

The diffusive theory of bainite's transformation process is based on short range diffusion at the transformation front. Here, random and uncoordinated thermally activated atomic jumps control formation and the interface is then rebuilt by reconstructive diffusion. The mechanism is not able to explain the shape nor surface relief caused by the bainite transformation.

Morphology

Typically bainite manifiests as aggregates, termed sheaves, of ferrite plates (sub-units) separated by retained austenite, martensite or cementite. While the sub-units appear separate when viewed on a 2-dimensional section they are in fact interconnected in 3-dimensions and usually take on a lenticular plate or lath morphology. 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. Neural network models have indicated that this is not a direct effect of the temperature per se but rather a result of the temperature dependence of the driving force for the reaction and the strength of the austenite surrounding the plates. At higher temperatures, and hence lower undercooling, the reduced thermodynamic driving force causes a decrease in the nucleation rate which allows individual plates to grow larger before they physically impinge on each other. Further, the growth of the plates must be accommodated by plastic flow in the surrounding austenite which is difficult if the austenite is strong and resists the plate's growth.

Upper bainite

So-called "upper bainite" forms around 400–550 °C in sheaves. 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 0.03%, resulting in carbon-rich austenite around the laths.

The amount of ferrite that forms between the laths is based on the carbon content of the steel. For a low carbon steel, typically discontinuous "stringers" or small particles of cementite will be present between laths. For a higher carbon steel, the stringers become continuous along the length of the adjacent laths.

Lower bainite

Lower bainite forms between 250 and 400 °C and takes a more plate-like form than upper bainite. There are not nearly as many low angle boundaries between laths in lower bainite. In lower bainite, the habit plane in ferrite will also shift from <111> towards <110> as transformation temperature decreases. In lower bainite, cementite nucleates on the interface between ferrite and austenite.

Incomplete bainite transformation

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.

External links

• Online textbook devoted to bainite, from Cambridge University Press and the Institute of Materials, Minerals and Mining
• The Alloying Elements in Steel, by Edgar C. Bain
• Overview of Bainite in multiple languages
• Davenport and Bain's original article
• World's first bulk nanostructured metal