Bainite self accomodating shear strains
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 (Fe C).
In addition to the thermodynamic considerations indicated by the phase diagram, the phase transformations in steel are heavily influenced by the chemical kinetics.
The transformation is said to cause a stress-relieving effect, which is confirmed by the orientation relationships present in bainitic microstructures.
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.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.Above 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).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).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).