How bainite is formed? Show with CCT diagram. Also explain how the grain size affected by cooling rate?
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 is no longer 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.
A continuous cooling transformation (CCT) phase diagram is often used when heat treating steel. These diagrams are used to represent which types of phase changes will occur in a material as it is cooled at different rates. These diagrams are often more useful than time-temperature-transformation diagrams because it is more convenient to cool materials at a certain rate (temperature-variable cooling), than to cool quickly and hold at a certain temperature (isothermal cooling).
When austenite is undercooled beyond a certain critical temperature, an acicular structure is seen to be formed which is known as bainite. It is normally formed at transformation temperatures in between 250-500 C
Bainitic structure is basically a mixture of ferrite and cementite, but it is different from pearlite in the sense that it doesn’t have an alternative lamellar layer structure. The structure is not very popular because it is very difficult to get since almost all austenite gets converted to pearlite before being transformed into bainite.
BAINITE FORMATION: The cooling rate should be optimum; not slow enough to get pearlite but not fast enough to get martensite. When austenite is undercooled below Bs temp, formation of regions with varying carbon concentration takes place. This difference in carbon concentration causes stress in the structure. The low carbon austenitic region transforms to ferrite by a diffusionless shear process. After that, precipitation of Fe3C may occur from the carbon enriched region and the carbon depleted region so formed again transforms into ferrite by the same mechanism. Thus, the bainitic transformation is based on both diffusional (redistribution of C atoms) as well as a diffusionless (shear process) mechanism.
Bainite is formed by the decomposition of austenite at a temperature which is below that which fine pearlite forms but which is above Ms ( Martensite start) temperature.
It is impossible to show bainite forming in CCT because bainite does not form in CCT.
In this cooling mechanism, there is less time available for austenite to transform into bainite because of the continuous cooling.
The grain size is affected by the cooling rate because the faster the cooling rate, the finer the grains. The slower the cooling rate, the coarser the grains.