What is Bainite in Steel?
Bainite is a microstructure of steel that forms at temperatures between 250-550°C. It is named after the British metallurgist Edgar Bain, who discovered it in 1939 while studying the properties of steel.
The transformation from austenite decomposition into ferrite & cementite at a temperature above martensite formation but lower than the pearlite results in Bainite formation.
This unique temperature range allows the bainitic microstructure to have exceptional strength and toughness compared to other microstructures formed in steel.
Unlike other microstructures of steel, such as ferrite and pearlite, bainite is harder, stronger and tough.
Bainite Microstructure Characteristics
One of the key features of bainitic microstructures is the presence of retained austenite. This austenitic phase can provide additional toughness to the steel by acting as a transformation-induced plasticity (TRIP) agent.
Bainite consists of ferrite and cementite phases arranged in a specific crystallographic orientation. The resulting microstructure has excellent combinations of strength, toughness, and ductility compared to other steel microstructures like martensite or pearlite.
This unique microstructure is characterized by its fine and needle-like structure, which results from the transformation of austenite at low temperatures.
The characteristics of a bainitic microstructure are defined by its crystallographic structure, morphology, and composition. The crystallographic structure consists primarily of ferrite (alpha-iron, α-Fe) with nanometer-sized cementite (Fe3C) particles interspersed between the ferrite plates.
Bainite Microstructure Hardness
Typically, bainitic steel has a hardness range between 250-350 HB (Brinell Hardness), which is significantly higher than that of ferritic or pearlitic steels. However, with proper heat treatment techniques such as quenching or tempering, the hardness range can be adjusted to suit specific applications.
Types of Bainite Microstructure
There are three main types of bainite microstructures that can be formed depending on the cooling rate and temperature used during the transformation process as:
- Lower Bainite.
- Upper Bainite.
- Granular bainite.
