What is Bainite?
Bainite is a microstructure that can be formed during heat treatment of carbon steel. It is formed by an isothermal transformation or continuous cooling of the steel. Named after the US-American metallurgist Edgar C. Bain, it is formed at temperatures and cooling rates that lie between those for martensite and pearlite formation. During the formation of bainite, different transformation mechanisms are possible, since folding processes in the crystal lattice and diffusion processes are coupled. This is a decisive difference to the formation of martensite. Bainite has no characteristic microstructure. This is due to the dependence between carbon content, cooling rate, alloying elements and the resulting formation temperature. Like perlite, bainite consists of the phases ferrite and cementite. However, the microstructure has a different shape, size and distribution. It can be divided into the main structural form of upper and lower bainite.
Definition
There are currently various definitions of bainite in the literature. Since these are linked to certain phenomena of phase transformation, it is not possible to decide unambiguously on the correctness of the individual definition. This circumstance can lead to great misunderstandings. These three definitions are common:
The microstructural definition
Bainite is seen in this definition as the non-lamellar product of ectoid decay, consisting of ferrite and carbide. The carbides are precipitated in the first formed ferrite. The precipitation can also occur at the interfaces of the ferrite. In time, the two product phases form one after the other under diffusion control.
The kinetic definition
The kinetic definition is based on the isothermal and kinetic time-temperature diagram. It is assumed that the curves at the beginning and the end of the bainite transformation are different compared to the transformation of pearlite. Thus, bainite has its own formation kinetics. The separation of pearlite and bainite transformation is done by a transformation inert region, whereby the expansion of the region is determined by the alloying elements. For some steels, bainite can be detected despite the absence of an inert transformation region. For this reason, this definition turns out to be unsuitable.
The surface relief definition
The surface relief definition is currently the most common in the literature. Here the relationship of the martensitic and the bainitic transformation is taken advantage of. The relationship is shown by a surface relief. Bainite can therefore be regarded as a plate-like phase, which is formed above the martensitic transformation. This is due to shearing from the austenite lattice. Responsible for this is a coordinated, but not thermally activated, atomic transfer across the moving phase boundary interface. The diffusion of interstitial atoms in the austenite before and after the shear determines the kinetics of the transformation.
Differences between the two structural forms
Bainite is basically divided into the structural forms “lower bainite” and “upper bainite”. The distinction is made according to the temperature at which it is formed. The mechanical properties of the bainite are influenced by the heat of formation, with toughness properties being particularly affected.
Lower bainite
In this case, the transformation temperature is just above the formation of martensite. The formation of ferrite leads to an enrichment of austenite and carbon. If further cooling takes place, the austenite area is transformed into ferrite, cementite and acicular bainite. A transformation to martensite can also take place. Lower bainite usually has not only lower high layer values, but also lower transition temperatures compared to upper bainite. This circumstance helps when using at low temperatures.
Upper bainite
If the formation takes place at temperatures below the range of pearlite formation, it is called upper bainite. Favourable diffusion conditions exist in this range. This allows the carbon to diffuse into the grain boundaries of the ferrite needles. This can then result in the formation of irregular and interrupted carbides. The distribution does not follow a pattern, giving the structure a granular appearance. For this reason, upper bainite is also called granular bainite. Compared to the lower bainite the range of the transition temperature is much higher. Upper bainite also achieves a higher impact energy value.
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