Bainite
Encyclopedia
Bainite is an acicular
microstructure (not a phase) that forms in steels at temperatures from approximately 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 of 727 °C (about 1340 °F). 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 simple 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.
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.
, the high temperature phase of iron. Below around 700 °C (723 °C in pure 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 kinetics
. This leads to the complexity of steel microstructures which are a 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.
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.
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.
Acicular (crystal habit)
Acicular, in mineralogy, refers to a crystal habit composed of a radiating mass of slender, needle-like crystals. Minerals with this habit tend to be fragile and complete, undamaged specimens can be uncommon.-Examples:...
microstructure (not a phase) that forms in steels at temperatures from approximately 250-550°C (depending on alloy content). First described by E. S. Davenport and Edgar Bain
Edgar Bain
Edgar C. Bain was an American metallurgist and member of the National Academy of Sciences, who worked for the US Steel Corporation of Pittsburgh, Pennsylvania. He worked on the alloying and heat treatment of steel; Bainite is named in his honor.He was born near LaRue, Ohio to Milton Henry and...
, it is one of the decomposition products that may form when austenite
Austenite
Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...
(the face centered cubic crystal structure of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
) is cooled past a critical temperature of 727 °C (about 1340 °F). Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite
Martensite
Martensite, named after the German metallurgist Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or...
.
A fine non-lamellar structure, bainite commonly consists of cementite
Cementite
Cementite, also known as iron carbide, is a chemical compound of iron and carbon, with the formula Fe3C . By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, though it is more...
and dislocation-rich ferrite
Ferrite
Ferrite may refer to:* Ferrite , iron or iron alloys with a body centred cubic crystal structure.* Ferrite , ferrimagnetic ceramic materials used in magnetic applications....
. 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
Pearlite
Pearlite is often said to be a two-phased, lamellar structure composed of alternating layers of alpha-ferrite and cementite that occurs in some steels and cast irons...
and martensite
Martensite
Martensite, named after the German metallurgist Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or...
. 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
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
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
Transmission electron microscopy
Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...
to see. Under a simple light microscope, the microstructure of bainite appears darker than martensite due to its low reflectivity
Reflectivity
In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
.
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 BainEdgar Bain
Edgar C. Bain was an American metallurgist and member of the National Academy of Sciences, who worked for the US Steel Corporation of Pittsburgh, Pennsylvania. He worked on the alloying and heat treatment of steel; Bainite is named in his honor.He was born near LaRue, Ohio to Milton Henry and...
discovered a new steel microstructure which they provisionally called martensite-troostite, due to it being intermediate between the already known low-temperature martensite
Martensite
Martensite, named after the German metallurgist Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or...
phase and what was then known as troostite (now fine-pearlite
Pearlite
Pearlite is often said to be a two-phased, lamellar structure composed of alternating layers of alpha-ferrite and cementite that occurs in some steels and cast irons...
). 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
Martensite
Martensite, named after the German metallurgist Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or...
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
At 900 °C a typical low-carbon steel is composed entirely of austeniteAustenite
Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...
, the high temperature phase of iron. Below around 700 °C (723 °C in pure iron) the austenite is thermodynamically unstable and, under equilibrium conditions, it will undergo a eutectoid reaction and form pearlite
Pearlite
Pearlite is often said to be a two-phased, lamellar structure composed of alternating layers of alpha-ferrite and cementite that occurs in some steels and cast irons...
- an interleaved mixture of ferrite
Ferrite (iron)
Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure. It is the component which gives steel and cast iron their magnetic properties, and is the classic example of a ferromagnetic material...
and cementite (Fe3C)
Cementite
Cementite, also known as iron carbide, is a chemical compound of iron and carbon, with the formula Fe3C . By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, though it is more...
. In addition to the thermodynamic considerations indicated by the phase diagram, the phase transformations in steel are heavily influenced by the kinetics
Chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...
. This leads to the complexity of steel microstructures which are a strongly influenced by the cooling rate. This can be illustrated by a continuous cooling transformation
Continuous cooling transformation
A continuous cooling transformation phase diagram is often used when heat treating steel. These diagrams are used to represent which types of phase changes will occur if a material at it is cooled at different rates...
(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
Martensite
Martensite, named after the German metallurgist Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or...
, 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
Tetragonal crystal system
In crystallography, the tetragonal crystal system is one of the 7 lattice point groups. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base and height .There are two tetragonal Bravais...
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
Neural network
The term neural network was traditionally used to refer to a network or circuit of biological neurons. The modern usage of the term often refers to artificial neural networks, which are composed of artificial neurons or nodes...
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 250-400°C and takes a more acicular 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 PressCambridge University PressCambridge University Press is the publishing business of the University of Cambridge. Granted letters patent by Henry VIII in 1534, it is the world's oldest publishing house, and the second largest university press in the world...
and the Institute of Materials, Minerals and MiningInstitute of Materials, Minerals and MiningThe Institute of Materials, Minerals and Mining is a major UK engineering institution whose activities encompass the whole materials cycle, from exploration and extraction, through characterisation, processing, forming, finishing and application, to product recycling and land reuse... - 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