Bainite

Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C (depending on alloy content). First described by E. S. Davenport and Edgar Bain, it is one of the products that may form when austenite (the face centered cubic crystal structure of iron) is cooled past a temperature where it no longer is thermodynamically stable with respect to ferrite, cementite, or ferrite and cementite. Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite. Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C (depending on alloy content). First described by E. S. Davenport and Edgar Bain, it is one of the products that may form when austenite (the face centered cubic crystal structure of iron) is cooled past a temperature where it no longer is thermodynamically stable with respect to ferrite, cementite, or ferrite and cementite. 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 large density of dislocations in the ferrite present in bainite, and the fine size of the bainite platelets, makes this ferrite harder than it normally would be. The temperature range for transformation of austenite to bainite (125–550 °C) is between those for pearlite and martensite. In fact, there is no fundamental lower limit to the bainite-start temperature. 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 retard the formation of bainite, though carbon is the most effective in doing so. Aluminium or cobalt are exceptions in that they can accelerate the decomposition of austenite and raise the transformation temperature. The microstructures of martensite and bainite at first seem quite similar, consisting of thin plates which in low-alloy steels cluster together. This is a consequence of the two microstructures sharing many aspects of their transformation mechanisms. However, morphological differences do exist that require a transmission electron microscope to see. Under a light microscope, the microstructure of bainite appears darker than untempered martensite because the bainite has more substructure. The hardness of bainite can be between that of pearlite and untempered martensite in the same steel hardness. 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. 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. 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. On cooling, it tends to transform into a mixture of phases, ferrite and cementite, depending on the exact chemical composition. 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. This is because the diffusion of iron atoms becomes difficult below about 600 °C under typical processing conditions. 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. 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 sufficiently rapidly to avoid higher temperature transformations, can form martensite, without any diffusion of either iron or carbon, by the deformation of the austenite's face-centred crystal structure into a distorted body-centred tetragonal or body-centred cubic 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.