Bainite

The fact that it can be produced during both isothermal or continuous cooling is a big advantage, because this facilitates the production of large components without excessive additions of alloying elements.

Unlike martensitic steels, alloys based on bainite often do not need further heat treatment after transformation in order to optimise strength and toughness.

[10] In the 1920s Davenport and Bain discovered a new steel microstructure that they provisionally called martensite-troostite, because it is intermediate between the known low-temperature martensite phase and what was then known as troostite (later 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.

[7] Above approximately 900 °C a typical low-carbon steel is composed entirely of austenite, a high-temperature phase of iron that has a cubic close-packed crystal structure.

If the steel is cooled slowly or isothermally transformed at elevated temperatures, the microstructure obtained will be closer to equilibrium,[13] 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 even though the reaction is incomplete and the remaining austenite is 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.

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.

This kind of deformation implies a disciplined motion of atoms (rather than a chaotic transfer associated with diffusion),[16] and is typical of all displacive transformations in steels, for example, martensite, bainite and Widmanstaetten ferrite.

In fact it is accepted by some that formation of Widmanstätten ferrite is controlled by carbon diffusion and do show a similar surface relief.

[25] Typically bainite manifests as aggregates, termed sheaves, of ferrite plates (sub-units) separated by retained austenite, martensite or cementite.

[27] 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.

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.

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.

[1] In the railway industry, bainite steel is commonly alloyed with vanadium to produce rails of very high strength, with good wear and rolling contact fatigue resistance.