The bainite structure in steel

Bainite is a steel structure that resembles highly tempered martensite. It forms from austenite by quenching from the austenite region in a salt bath between 250-550 ° C and it is kept there for some time. It is called bainitic hardening or isothermal hardening. Bainite consists of ferrite and cementite. Many dislocations (stacking errors) in bainite make it harder than ferrite and perlite, but less hard but tougher than martensite. The bainitic structure forms the basis for modern structural steels with high strength, good toughness and weldability and reasonable weather resistance without heat treatment of the product afterwards, which became possible only after recent modernization of the steel rolling mills. Bainite and martensite do not appear in the iron-carbon diagram because they are not stable and only come about through heat treatment.

The IT or TTT diagram

The TTT diagram of a low-alloyed breeding steel. Cool from A3 (austenite area). At cooling rate V1 martensite is formed (below 200ºC). At V2, perlite (above 600ºC) is mixed with martensite. V3 produces bainite (below 500ºC) mixed with perlite and martensite / Source: Dumontierc, Wikimedia Commons (CC BY-SA-3.0)
The IT diagram (Isothermal transformation diagram) or the TTT diagram (Time Temperature Transformation diagram) provides insight into the formation of the microstructures during the heat treatment of a steel grade. Each composition (alloy) has its own TTT diagram. The construction of the diagram occurs by rapid cooling from the austenite region to the temperature Ts at which the structure S forms. During rapid cooling to a lower temperature, the formation of the equilibrium structure perlite, which is based on diffusion of iron and carbon atoms, is suppressed. There is no time for the transformation to perlite. After the rapid cooling, the temperature is kept constant (hence isothermal) until the transformation of structure S has taken place, in order to subsequently cool the sample to room temperature. The structure no longer changes during cooling to room temperature because the mobility of the atoms is too low for transformations. This procedure is repeated dozens of times for different types of steel at different temperatures, thus creating an IT or TTT diagram of a steel type. Drawing up the IT diagram for a steel type that is given its desired properties by heat treatment is time-consuming and therefore relatively expensive. But these diagrams should be present in the steel data sheet that is given its optimum properties by heat treatment.
The course of the perlite formation according to the S curve with horizontally the time and vertically the perlite formation. Perlite formation occurs via diffusion of atoms during the transformation time / Source: Populus, Wikimedia Commons (CC BY-SA-3.0)
Simplified IT or (TTT) diagram for 2 steels. Here you can clearly see the influence of alloying elements P is perlite; B is bainite; M is martensite / Source: Metallos, Wikimedia Commons (GFDL)
Continuous Cooling Transformation diagram (CCT), used for continuous cooling, which is often the case in practice. Depending on the cooling rate, mixing structures of Ferrite, perlite, bainite, martensite and residual austenite are created / Source: Slinky Puppet, Wikimedia Commons (CC BY-3.0)
Part of the iron-carbon diagram with the equilibrium structures ferrite, perlite and cementite. These structures are formed at high temperatures during cooling from the austenite. The formation takes time because they are created by diffusion of atoms. With rapid cooling, the transformation time is insufficient and hardening structures such as bainite and martensite are created / Source: Cdang, Wikimedia Commons (CC BY-SA-3.0)

The emergence of the perlite, bainite and martensite structure

The mechanical properties such as the hardness of the bainitic structure lie between those of perlite and martensite. The treatment of bainite cannot be seen in isolation from perlite and martensite, there will also be bainitic steel areas where perlite, martensite and residual austenite will differ due to local internal stresses and temperatures. The bainite structure has been requested because it has good mechanical properties. The properties of bainite can be compared to those of refined steel, but without additional heat treatment. The bainite structure is already realized during rolling in the steel plant.
Bainite is formed by rapid cooling while avoiding the perlite area. The bainite formation temperature is kept constant (isothermal cure) between 250 ° C and 550 ° C, depending on what is desired, high or low bainite. High bainite is heating between approximately 400ºC and 550 ° C and low bainite between approximately 250ºC and 400 ° C. The hardness of bainite varies from around Rc 40 for high bainite to around Rc 60 for low bainite. For comparison, the hardness of Martensiet is around Rc 65.

The emergence of perlite

According to the iron-carbon diagram, a low-carbon steel is made of austenite at around 900 ° C. Below about 723 ° C, austenite is not stable and a conversion takes place to ferrite and perlite. Perlite contains 0.8% carbon and consists of layers of ferrite (Fe) interspersed with layers of cementite (Fe3C).
When under-eutectic (<0.8% C) steel cools slowly, the formed structure of perlite and ferrite is stable and can be seen in the iron-carbon diagram. The transformation from austenite to perlite takes time because perlite grows due to diffusion of atoms. The diffusion becomes slower with decreasing temperature until it stops. The consequence of rapid cooling is that perlite formation soon stops while there is still (unstable) residual austenite that is, as it were, frozen and can no longer transform into ferrite and perlite due to diffusion of atoms. Restaustenite can partially turn into martensite at low temperatures

The origin of martensite

Austenite that cools quickly forms martensite by folding over the cubic planar centered lattice (KVR lattice) into a stretched (due to too many carbon atoms enclosed) cubic spatially centered lattice, the so-called tetragonal lattice. Martensite can only be formed at a low temperature if the voltage in the KVR grid is large enough to overcome the transformation threshold. The folding takes place at the Ms temperature (starting temperature of martensite formation). All carbon atoms in the original austenite lattice are frozen in the tetragonal lattice thereby stressed. The tension in this grid gives martensite its hardness.
Low bainite; Silicon steel 80Si10; Transform for 4 hours at 250ºC (1200x magnification) / Source: Luenibaer, Wikimedia Commons (CC BY-SA-3.0)
High bainite; Silicon steel 80Si10; Transform for 4 hours at 450ºC (1200x magnification) / Source: Luenibaer, Wikimedia Commons (CC BY-SA-3.0)

The emergence of high and low bainite

Unlike perlite, where ferrite and cementite grow together, bainite forms with carbon-saturated ferrite through diffusion of carbon and precipitation (precipitation) of cementite. There is low bainite, characterized by a needle-like "coagulation" structure that forms at temperatures between 250 and 400 ° C, close to the Martensite start temperature and high bainite that forms at higher temperatures between 400 and 550 ° C, close to the perlite area and characterized by a "bird-like" coagulation structure. The difference is due to the diffusion rate of carbon at the formation temperature of bainite. If the temperature is high, carbon rapidly diffuses from the ferrite into the carbon-rich residual austenite and forms cementite springs there (Fe3C) At low temperatures, carbon diffuses slowly and precipitates before it can grow as a 'feather'.

Difference between bainite and breeding structure

The bainite structure and the breeding structure resemble each other in terms of properties, but their formation mechanism differs, despite the fact that they form in the same temperature range. Breeding is the high tempering (low tempering is hardening) of martensite. Cooling (quenching) is done quickly to form martensite, which is subsequently annealed at processing temperature, a costly post-processing. Bainite is formed by rapid cooling of the sample from the austenite region to the temperature where according to the IT diagram bainite is formed and retaining this temperature during bainite formation, then cooling to room temperature. The bainite structure will not change during this last cooling because the mobility of the atoms is too low.

Summary theory of bainite formation

Carbon is an interstitial atom in iron, i.e. it is located between the larger iron atoms. Maximum solubility in ferrite (KRR lattice) is approximately 0.02% by weight at 727 ° C and practically zero at lower temperatures. The KRR lattice of ferrite has little storage space for carbon atoms. Maximum solubility of carbon in the KVR lattice of austenite is 2.14% by weight at 1147 ° C, the KVR lattice of austenite has a relatively large space between the iron atoms for storing carbon atoms.
The mechanical properties of bainite are related to the microstructure, i.e. how ferrite and cementite are distributed. The bainite transformation is effected by a shear transformation (flipping) of the austenitic KVR lattice, such as in the formation of martensite. Only carbon diffusion occurs during the formation of the Fe3C carbides (cementite) between the ferrite layers. The carbon diffuses from the ferrite and forms cementite next to the decarburized ferrite. The bainite structure has a higher strength and hardness than the perlite passed in the TTT diagram, but is less tough. It is less strong and hard like martensite but it is significantly tougher than martensite.

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