Bainite :

The term bainite refers to a mixture of ferrite and carbide. This ferrite carbide mixture has a basic difference with respect to pearlite which is not a lamellar in nature. Hence higher magnification than those required for resolution of pearlite are required to resolved the bainite which is formed with in a specific temperature range.

Bainitic Transformation

● The upper limit, is just below the minimum temperature for pearlite transformation which is below the nose of the TTT curve for a steel. The lower limit of the temperature is above the temperature at which martensite formation starts.

● For eutectoid steel, bainite forms form the undercooled austenite in the temperature range of 200 to 500℃. Transformation of austenite to bainite occurs only below a definite temperature known as Bs temperature.

● Only few alloy steels exhibit the presence of sharp Bs temperature. For this steels two separate bays, one each for pearlite and bainite exist. Austenite can be transformed in to bainite either isothermally or by continuous cooling.

● Here shows the TTT curve for an alloy steel having separate pearlitic and bainitic bays. However, for carbon steels and most of the alloy steels, pearlitic and bainitic bays overlap and austenite transforms to pearlite or martensite depending on the cooling rate.

● In these steels, bainite cannot be produced by continuous cooling. It can be form by cooling austenite at a sufficiently high rate so that it's transformation to pearlite is suppressed. The austenite is then held in the bainitic range. Austenite to bainite transformation complete at a definite temperature which is denoted as Bf, the end temperature of bainitic transformation illustrate the isothermal transformation to bainite.
Formation of bainite
Formation of bainite by isothermal transformation of austenite

Mechanism of Bainitic Transformation :

Presence of incubation period for bainitic transformation => transformation diffusion controlled.
● The formation of bainite starts at a temperature at which the diffusion of iron atmos is very slow but the diffusion of carbon atmos is very important.

● The formation of bainite is always accompanied by surface distortion => that shear processes are also involved in bainitic transformation, which is both diffusion controlled and diffusionless processes.

● It exhibits both type of transformations that is when austenite undercooled below the Bs temperature, carbon atoms redistribute in the austenite by diffusion. Which leads to the formations of carbon concentration in austenite.

● The (Ms) start of martensitic transformation temperature for austenitic regions with low carbon content lies in between the Bs and Bf temperature. Here, low carbon austenitic regions transform to martensite by diffusionaless process. A portion of the carbon enriched austenitic regions dose not transform on cooling from holding temperature to room temperature.

● The degree of supersaturation increases with decrease in transformation region, carbon may precipitate out from the supersaturated ferrite. This happens when the bainitic transformation nation proceeds in the lower region of the transformation range.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.

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