This document summarizes the microstructures of bainite, spheroidite, and martensite in eutectoid steel. Bainite forms as needles or plates between 215-540°C and consists of ferrite and cementite phases formed through diffusion. Spheroidite forms if pearlite or bainite is heated below the eutectoid temperature, resulting in a spheroidal cementite structure in ferrite. Martensite forms from a diffusionless transformation of austenite during quenching, resulting in a non-equilibrium body centered tetragonal structure appearing as plates or needles.

1. TTT DIAGRAM FOR EUTECTOID
STEEL(BAINITE, SPHERODITE,
MARTENSITE)
4PRESENTED BY
DARSHIT DESAI
EMAIL:
BUNNYDESAI1998@GMAIL.COM

2. BAINITE
• The microstructure of bainite consists of ferrite
and cementite phases, and thus diffusion
processes are involved in its formation.
• Bainite forms as needles or plates, depending on
the temperature of the transformation.
• The microstructure details of bainite are fine so
that their resolution is possible only using
electron microscopy can not in optical
microscope.

3. • Bainite forms at temperatures between about
215 and 540C (420 and 1000F).

5. Microstructure of Bainite

6. SPHEROIDITE
• If a steel alloy having either pearlitic or
bainitic microstructures is heated to, and left
at, a temperature below the eutectoid for a
sufficiently long period of time—for
• example,
at about 700C (1300F) for between 18 and
24h—yet another microstructure will form. It
is called spheroidite

7. Microstructure of Spheroidite
Cementite
Ferrite

8. MARTENSITE
• Martensite is formed when austenitized iron–
carbon alloys are rapidly cooled (or quenched)
to a relatively low temperature.
• Martensite is a nonequilibrium single-phase
structure that results from a diffusion less
transformation of austenite.
• Martensite is a nonequilibrium single-phase
structure that results from a diffusion less
transformation of austenite.

9. • Martensite have a body centered tetragonal
(BCT) crystal structure.
• Martensitic transformation does not involve
diffusion, it occurs almost instantaneously.
The Martensite grains nucleate and grow at a
very rapid rate Thus the martensitic
transformation rate is time independent.
• Martensite grains take on a plate like or
needle like microstructure.

11. Microstructure of Martensite
Return
austenite
Martensite needle

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