If steel is cooled in still air, the structure obtained is

Explanation:

TTT Diagram:

• The rate of phase transformation also depends upon the temperature.
• Temperature-time-transformation diagram (TTT), also known as isothermal transformation curves, are useful in planning heat treatments.
• The complete isothermal transformation diagram for an iron-carbon alloy of eutectoid composition A (austenite), B (Bainite), M (Martensite), P (Pearlite) is below:

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• Course pearlite:
  • The thickness ratio of ferrite and cementite layers in pearlite is 8:1.
  • At temperature just below eutectoid (727°c), relatively thick layers of both α ferrite and Fe₃C are produced this microstructure is called coarse pearlite.
  • The diffusion rate is relatively high
• Fine pearlite:
  • The rate of carbon diffusion decreases at lower temperatures and the layers become progressively thinner.
  • A thin layered structure produced in the vicinity of 540°c is termed as fine pearlite or sorbite.
  • If the steel is cooled at a faster rate, and even the finer structure of austenite decomposition is obtained which is called as troosite.
• Bainite:
  • ​Bainite is obtained at a higher cooling rate when alternating ferrite and cementite lamellae become thinner than that in fine pearlite.
  • For temperatures approximately between 300°c and 400°c, bainite forms a series of parallel needles of ferrite that re separated by elongated particles of cementite phase such as bainite is called upper bainite.
• Spherodite:
  • If pearlite and bainitic steel is heated to and kept at a temperature below eutectoid (727°c) for a sufficiently long period of time (18-24 hours).
  • a new microstructure is formed in which Fe₃C phase appears as sphere-like particles embedded in a continuous α -phase matrix. his microstructure is known as Spherodite.
• Martensite:
  • ​In the system, martensite is the hardest, strongest, and most brittle phase.
  • It is a metastable phase that results from a diffusion of the transformation of austenite when Fe-C alloys are rapidly cooled and quenched.